
LAN8720A/LAN8720Ai
Small Footprint RMII 10/100 Ethernet 
Transceiver with HP Auto-MDIX Support
DatasheetPRODUCT FEATURES
Highlights

Single-Chip Ethernet Physical Layer Transceiver 
(PHY)
Comprehensive flexPWR® Technology 
— Flexible Power Management Architecture
— LVCMOS Variable I/O voltage range: +1.6V to +3.6V
— Integrated 1.2V regulator
HP Auto-MDIX support
Miniature 24-pin QFN lead-free RoHS compliant 
package (4 x 4 x 0.85mm height).

Target Applications

Set-Top Boxes
Networked Printers and Servers
Test Instrumentation
LAN on Motherboard
Embedded Telecom Applications
Video Record/Playback Systems
Cable Modems/Routers
DSL Modems/Routers
Digital Video Recorders
IP and Video Phones
Wireless Access Points
Digital Televisions
Digital Media Adaptors/Servers
Gaming Consoles
POE Applications (Refer to SMSC Application Note 17.18)

Key Benefits

High-Performance 10/100 Ethernet Transceiver
— Compliant with IEEE802.3/802.3u (Fast Ethernet)
— Compliant with ISO 802-3/IEEE 802.3 (10BASE-T)
— Loop-back modes
— Auto-negotiation
— Automatic polarity detection and correction
— Link status change wake-up detection
— Vendor specific register functions
— Supports the reduced pin count RMII interface
Power and I/Os
— Various low power modes
— Integrated power-on reset circuit
— Two status LED outputs
— Latch-Up Performance Exceeds 150mA per EIA/JESD 

78, Class II
— May be used with a single 3.3V supply
Additional Features
— Ability to use a low cost 25Mhz crystal for reduced BOM
Packaging
— 24-pin QFN (4x4 mm) Lead-Free RoHS Compliant 

package with RMII
Environmental
— Extended commercial temperature range 

(0°C to +85°C)
— Industrial temperature range version available 

(-40°C to +85°C)
SMSC LAN8720A/LAN8720Ai Revision 1.4 (08-23-12)

DATASHEET

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SUCCESS BY DESIGN
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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Order Numbers:
LAN8720A-CP-TR for 24-pin QFN lead-free RoHS compliant package (0 to +85°C temp)

LAN8720Ai-CP-TR for 24-pin QFN lead-free RoHS compliant package (-40 to +85°C temp)
Reel size is 4,000.

This product meets the halogen maximum concentration values per IEC61249-2-21

For RoHS compliance and environmental information, please visit www.smsc.com/rohs

Please contact your SMSC sales representative for additional documentation related to this product 
such as application notes, anomaly sheets, and design guidelines.

Revision 1.4 (08-23-12) 2 SMSC LAN8720A/LAN8720Ai

Copyright © 2012 SMSC or its subsidiaries. All rights reserved.
Circuit diagrams and other information relating to SMSC products are included as a means of illustrating typical applications. Consequently, complete information sufficient for
construction purposes is not necessarily given. Although the information has been checked and is believed to be accurate, no responsibility is assumed for inaccuracies. SMSC
reserves the right to make changes to specifications and product descriptions at any time without notice. Contact your local SMSC sales office to obtain the latest specifications
before placing your product order. The provision of this information does not convey to the purchaser of the described semiconductor devices any licenses under any patent
rights or other intellectual property rights of SMSC or others. All sales are expressly conditional on your agreement to the terms and conditions of the most recently dated
version of SMSC's standard Terms of Sale Agreement dated before the date of your order (the "Terms of Sale Agreement"). The product may contain design defects or errors
known as anomalies which may cause the product's functions to deviate from published specifications. Anomaly sheets are available upon request. SMSC products are not
designed, intended, authorized or warranted for use in any life support or other application where product failure could cause or contribute to personal injury or severe property
damage. Any and all such uses without prior written approval of an Officer of SMSC and further testing and/or modification will be fully at the risk of the customer. Copies of
this document or other SMSC literature, as well as the Terms of Sale Agreement, may be obtained by visiting SMSC’s website at http://www.smsc.com. SMSC is a registered
trademark of Standard Microsystems Corporation (“SMSC”). Product names and company names are the trademarks of their respective holders. 

The Microchip name and logo, and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.
SMSC DISCLAIMS AND EXCLUDES ANY AND ALL WARRANTIES, INCLUDING WITHOUT LIMITATION ANY AND ALL IMPLIED WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE, TITLE, AND AGAINST INFRINGEMENT AND THE LIKE, AND ANY AND ALL WARRANTIES ARISING FROM ANY COURSE
OF DEALING OR USAGE OF TRADE. IN NO EVENT SHALL SMSC BE LIABLE FOR ANY DIRECT, INCIDENTAL, INDIRECT, SPECIAL, PUNITIVE, OR CONSEQUENTIAL
DAMAGES; OR FOR LOST DATA, PROFITS, SAVINGS OR REVENUES OF ANY KIND; REGARDLESS OF THE FORM OF ACTION, WHETHER BASED ON CONTRACT;
TORT; NEGLIGENCE OF SMSC OR OTHERS; STRICT LIABILITY; BREACH OF WARRANTY; OR OTHERWISE; WHETHER OR NOT ANY REMEDY OF BUYER IS HELD
TO HAVE FAILED OF ITS ESSENTIAL PURPOSE, AND WHETHER OR NOT SMSC HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
DATASHEET

http://www.smsc.com/index.php?tid=219



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Table of Contents

Chapter 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1 General Terms and Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 General Description  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Chapter 2 Pin Description and Configuration  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 Pin Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Buffer Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Chapter 3 Functional Description  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 Transceiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.1 100BASE-TX Transmit  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.2 100BASE-TX Receive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.1.3 10BASE-T Transmit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1.4 10BASE-T Receive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2 Auto-negotiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.1 Parallel Detection  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.2 Restarting Auto-negotiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.3 Disabling Auto-negotiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.4 Half vs. Full Duplex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.3 HP Auto-MDIX Support. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.4 MAC Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.4.1 RMII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.5 Serial Management Interface (SMI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.6 Interrupt Management  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6.1 Primary Interrupt System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.6.2 Alternate Interrupt System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.7 Configuration Straps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.7.1 PHYAD[0]: PHY Address Configuration  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.7.2 MODE[2:0]: Mode Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.7.3 REGOFF: Internal +1.2V Regulator Configuration  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.7.4 nINTSEL: nINT/REFCLKO Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.8 Miscellaneous Functions  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.8.1 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.8.2 Variable Voltage I/O  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.8.3 Power-Down Modes  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.8.4 Isolate Mode  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.8.5 Resets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.8.6 Carrier Sense  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.8.7 Link Integrity Test  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.8.8 Loopback Operation  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.9 Application Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.9.1 Simplified System Level Application Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.9.2 Power Supply Diagram (1.2V Supplied by Internal Regulator) . . . . . . . . . . . . . . . . . . . . 43
3.9.3 Power Supply Diagram (1.2V Supplied by External Source). . . . . . . . . . . . . . . . . . . . . . 44
3.9.4 Twisted-Pair Interface Diagram (Single Power Supply). . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.9.5 Twisted-Pair Interface Diagram (Dual Power Supplies)  . . . . . . . . . . . . . . . . . . . . . . . . . 46

Chapter 4 Register Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1 Register Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 Control and Status Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.2.1 Basic Control Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

SMSC LAN8720A/LAN8720Ai 3 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.2 Basic Status Register  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.2.3 PHY Identifier 1 Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2.4 PHY Identifier 2 Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.5 Auto Negotiation Advertisement Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2.6 Auto Negotiation Link Partner Ability Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2.7 Auto Negotiation Expansion Register  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2.8 Mode Control/Status Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.2.9 Special Modes Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.10 Symbol Error Counter Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.2.11 Special Control/Status Indications Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.2.12 Interrupt Source Flag Register  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.2.13 Interrupt Mask Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2.14 PHY Special Control/Status Register  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Chapter 5 Operational Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.1 Absolute Maximum Ratings*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2 Operating Conditions** . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3 Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.3.1 REF_CLK In Mode  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3.2 REF_CLK Out Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5.4 DC Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.5 AC Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5.5.1 Equivalent Test Load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.5.2 Power Sequence Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.5.3 Power-On nRST & Configuration Strap Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.5.4 RMII Interface Timing  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.5.5 SMI Timing  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.6 Clock Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Chapter 6 Package Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Chapter 7 Datasheet Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Revision 1.4 (08-23-12) 4 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

SMSC LAN8720A/LAN8720Ai 5 Revision 1.4 (08-23-12)

DATASHEET

List of Figures
Figure 1.1 System Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
Figure 1.2 Architectural Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
Figure 2.1 24-QFN Pin Assignments (TOP VIEW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
Figure 3.1 100BASE-TX Transmit Data Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
Figure 3.2 100BASE-TX Receive Data Path. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
Figure 3.3 Relationship Between Received Data and Specific MII Signals  . . . . . . . . . . . . . . . . . . . . . .  21
Figure 3.4 Direct Cable Connection vs. Cross-over Cable Connection  . . . . . . . . . . . . . . . . . . . . . . . . .  26
Figure 3.5 MDIO Timing and Frame Structure - READ Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
Figure 3.6 MDIO Timing and Frame Structure - WRITE Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
Figure 3.7 External 50MHz clock sources the REF_CLK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
Figure 3.8 Sourcing REF_CLK from a 25MHz Crystal  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
Figure 3.9 Sourcing REF_CLK from External 25MHz Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
Figure 3.10 LED1/REGOFF Polarity Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
Figure 3.11 LED2/nINTSEL Polarity Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  38
Figure 3.12 Near-end Loopback Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  40
Figure 3.13 Far Loopback Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  41
Figure 3.14 Connector Loopback Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  41
Figure 3.15 Simplified System Level Application Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  42
Figure 3.16 Power Supply Diagram (1.2V Supplied by Internal Regulator)  . . . . . . . . . . . . . . . . . . . . . . .  43
Figure 3.17 Power Supply Diagram (1.2V Supplied by External Source) . . . . . . . . . . . . . . . . . . . . . . . . .  44
Figure 3.18 Twisted-Pair Interface Diagram (Single Power Supply) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
Figure 3.19 Twisted-Pair Interface Diagram (Dual Power Supplies). . . . . . . . . . . . . . . . . . . . . . . . . . . . .  46
Figure 5.1 Output Equivalent Test Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  68
Figure 5.2 Power Sequence Timing  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  69
Figure 5.3 Power-On nRST & Configuration Strap Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  70
Figure 5.4 RMII Timing (REF_CLK Out Mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  71
Figure 5.5 RMII Timing (REF_CLK In Mode)  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  72
Figure 5.6 SMI Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  73



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

Revision 1.4 (08-23-12) 6 SMSC LAN8720A/LAN8720Ai

DATASHEET

List of Tables
Table 2.1 RMII Signals  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 2.2 LED Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 2.3 Serial Management Interface (SMI) Pins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Table 2.4 Ethernet Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Table 2.5 Miscellaneous Pins  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 2.6 Analog Reference Pins  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 2.7 Power Pins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 2.8 24-QFN Package Pin Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Table 2.9 Buffer Types  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Table 3.1 4B/5B Code Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Table 3.2 Interrupt Management Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Table 3.3 Alternative Interrupt System Management Table  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Table 3.4 MODE[2:0] Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Table 3.5 Pin Names for Mode Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Table 3.6 nINTSEL Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Table 4.1 Register Bit Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Table 4.2 SMI Register Map  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Table 5.1 Device Only Current Consumption and Power Dissipation (REF_CLK In Mode) . . . . . . . . . . 64
Table 5.2 Device Only Current Consumption and Power Dissipation (REF_CLK Out Mode) . . . . . . . . . 65
Table 5.3 Non-Variable I/O Buffer Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Table 5.4 Variable I/O Buffer Characteristics  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Table 5.5 100BASE-TX Transceiver Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Table 5.6 10BASE-T Transceiver Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Table 5.7 Power Sequence Timing Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Table 5.8 Power-On nRST & Configuration Strap Timing Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Table 5.9 RMII Timing Values (REF_CLK Out Mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Table 5.10 RMII Timing Values (REF_CLK In Mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Table 5.11 RMII CLKIN (REF_CLK) Timing Values  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Table 5.12 SMI Timing Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Table 5.13 Crystal Specifications  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Table 7.1 Customer Revision History  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 1 Introduction

1.1   General Terms and Conventions
The following is list of the general terms used throughout this document:

1.2 General Description
The LAN8720A/LAN8720Ai is a low-power 10BASE-T/100BASE-TX physical layer (PHY) transceiver
with variable I/O voltage that is compliant with the IEEE 802.3-2005 standards. 

The LAN8720A/LAN8720Ai supports communication with an Ethernet MAC via a standard RMII
interface. It contains a full-duplex 10-BASE-T/100BASE-TX transceiver and supports 10Mbps
(10BASE-T) and 100Mbps (100BASE-TX) operation. The LAN8720A/LAN8720Ai implements auto-
negotiation to automatically determine the best possible speed and duplex mode of operation. HP
Auto-MDIX support allows the use of direct connect or cross-over LAN cables.

The LAN8720A/LAN8720Ai supports both IEEE 802.3-2005 compliant and vendor-specific register
functions. However, no register access is required for operation. The initial configuration may be
selected via the configuration pins as described in Section 3.7, "Configuration Straps," on page 31.
Register-selectable configuration options may be used to further define the functionality of the
transceiver.

Per IEEE 802.3-2005 standards, all digital interface pins are tolerant to 3.6V. The device can be
configured to operate on a single 3.3V supply utilizing an integrated 3.3V to 1.2V linear regulator. The
linear regulator may be optionally disabled, allowing usage of a high efficiency external regulator for
lower system power dissipation.

The LAN8720A/LAN8720Ai is available in both extended commercial and industrial temperature range
versions. A typical system application is shown in Figure 1.1.

BYTE 8-bits

FIFO First In First Out buffer; often used for elasticity buffer

MAC Media Access Controller

RMIITM Reduced Media Independent InterfaceTM 

N/A Not Applicable

X Indicates that a logic state is “don’t care” or undefined.

RESERVED Refers to a reserved bit field or address. Unless otherwise 
noted, reserved bits must always be zero for write 
operations. Unless otherwise noted, values are not 
guaranteed when reading reserved bits. Unless otherwise 
noted, do not read or write to reserved addresses.

SMI Serial Management Interface
SMSC LAN8720A/LAN8720Ai 7 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Figure 1.1 System Block Diagram

Figure 1.2 Architectural Overview

LAN8720A/
LAN8720Ai

10/100 
Ethernet

MAC
RMII

Mode LED

Transformer

Crystal or 
Clock 

Oscillator

MDI RJ45
R

M
II 

Lo
gi

c

Interrupt 
Generator

LEDs

PLL

Receiver

DSP System:
Clock

Data Recovery 
Equalizer

Squeltch 
& Filters

Analog-to-
Digital

10M RX 
Logic

100M RX 
Logic

100M PLL

10M PLL

Transmitter
10M 

Transmitter

100M 
Transmitter

10M TX 
Logic

100M TX 
Logic

Central Bias

PHY Address 
Latches

LAN8720A/LAN8720Ai

RBIAS

LED1

nINT

XTAL2

XTAL1/CLKIN

LED2

Management 
Control

Mode Control

Reset Control

MDIX 
Control

HP Auto-MDIX

RXP/RXN

TXP/TXN

TXD[0:1]

TXEN

RXD[0:1]

RXER

CRS_DV

MDC

MDIO

Auto-
Negotiation

RMIISEL

nRST

MODE[0:2]

SMI

PHYAD0
Revision 1.4 (08-23-12) 8 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 2 Pin Description and Configuration

Note: When a lower case “n” is used at the beginning of the signal name, it indicates that the signal
is active low. For example, nRST indicates that the reset signal is active low. 

Note: The buffer type for each signal is indicated in the BUFFER TYPE column. A description of the
buffer types is provided in Section 2.2.

Figure 2.1 24-QFN Pin Assignments (TOP VIEW)

VSS

NOTE: Exposed pad (VSS) on bottom of package must be connected to ground

SMSC
LAN8720A/LAN8720Ai

24 PIN QFN
(TOP VIEW)

MDIO

1 2 3 4 5 6

7

8

9

10

11

12

18 17 16 15 14 13

24

23

22

21

20

19

VD
DC

R

XT
AL

1/C
LK

IN

XT
AL

2

LE
D1

/R
EG

OF
F

LE
D2

/n
IN

TS
EL

VD
D2

A
TX

D1

TX
D0

TX
EN

nR
ST

nI
NT

/R
EF

CL
KO

MD
C

VDD1A

TXN

TXP

RXN

RXP

RBIAS

CRS_DV/MODE2

RXER/PHYAD0

VDDIO

RXD0/MODE0

RXD1/MODE1
SMSC LAN8720A/LAN8720Ai 9 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Table 2.1 RMII Signals

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1 Transmit Data 0
TXD0 VIS The MAC transmits data to the transceiver using 

this signal. 

1 Transmit Data 1
TXD1 VIS The MAC transmits data to the transceiver using 

this signal.

1 Transmit Enable
TXEN VIS

(PD)
Indicates that valid transmission data is present 
on TXD[1:0]. 

1

Receive 
Data 0

RXD0 VO8 Bit 0 of the 2 data bits that are sent by the 
transceiver on the receive path.

PHY 
Operating 
Mode 0 

Configuration 
Strap

MODE0 VIS
(PU)

Combined with MODE1 and MODE2, this 
configuration strap sets the default PHY mode. 

See Note 2.1 for more information on 
configuration straps. 
Note: Refer to Section 3.7.2, "MODE[2:0]: 

Mode Configuration," on page 31 for 
additional details.

1

Receive 
Data 1

RXD1 VO8 Bit 1 of the 2 data bits that are sent by the 
transceiver on the receive path.

PHY 
Operating 
Mode 1 

Configuration 
Strap

MODE1 VIS
(PU)

Combined with MODE0 and MODE2, this 
configuration strap sets the default PHY mode. 

See Note 2.1 for more information on 
configuration straps. 
Note: Refer to Section 3.7.2, "MODE[2:0]: 

Mode Configuration," on page 31 for 
additional details.

1

Receive Error RXER VO8 This signal is asserted to indicate that an error 
was detected somewhere in the frame presently 
being transferred from the transceiver. 

PHY Address 
0

Configuration 
Strap

PHYAD0 VIS
(PD)

This configuration strap sets the transceiver’s 
SMI address.

See Note 2.1 for more information on 
configuration straps. 
Note: Refer to Section 3.7.1, "PHYAD[0]: PHY 

Address Configuration," on page 31 for 
additional information.
Revision 1.4 (08-23-12) 10 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Note 2.1 Configuration strap values are latched on power-on reset and system reset. Configuration
straps are identified by an underlined symbol name. Signals that function as configuration
straps must be augmented with an external resistor when connected to a load. Refer to
Section 3.7, "Configuration Straps," on page 31 for additional information.

1

Carrier Sense 
/ Receive 
Data Valid

CRS_DV VO8 This signal is asserted to indicate the receive 
medium is non-idle. When a 10BASE-T packet is 
received, CRS_DV is asserted, but RXD[1:0] is 
held low until the SFD byte (10101011) is 
received. 
Note: Per the RMII standard, transmitted data 

is not looped back onto the receive data 
pins in 10BASE-T half-duplex mode.

PHY 
Operating 
Mode 2 

Configuration 
Strap

MODE2 VIS
(PU)

Combined with MODE0 and MODE1, this 
configuration strap sets the default PHY mode. 

See Note 2.1 for more information on 
configuration straps. 
Note: Refer to Section 3.7.2, "MODE[2:0]: 

Mode Configuration," on page 31 for 
additional details.

Table 2.2 LED Pins

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1

LED 1 LED1 O12 Link activity LED Indication. This pin is driven 
active when a valid link is detected and blinks 
when activity is detected.
Note: Refer to Section 3.8.1, "LEDs," on 

page 37 for additional LED information.

Regulator Off 
Configuration 

Strap

REGOFF IS
(PD)

This configuration strap is used to disable the 
internal 1.2V regulator. When the regulator is 
disabled, external 1.2V must be supplied to 
VDDCR.

When REGOFF is pulled high to VDD2A with 
an external resistor, the internal regulator is 
disabled. 
When REGOFF is floating or pulled low, the 
internal regulator is enabled (default).

See Note 2.2 for more information on 
configuration straps. 
Note: Refer to Section 3.7.3, "REGOFF: 

Internal +1.2V Regulator Configuration," 
on page 32 for additional details.

Table 2.1 RMII Signals (continued) 

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION
SMSC LAN8720A/LAN8720Ai 11 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Note 2.2 Configuration strap values are latched on power-on reset and system reset. Configuration
straps are identified by an underlined symbol name. Signals that function as configuration
straps must be augmented with an external resistor when connected to a load. Refer to
Section 3.7, "Configuration Straps," on page 31 for additional information.

1

LED 2 LED2 O12 Link Speed LED Indication. This pin is driven 
active when the operating speed is 100Mbps. It 
is inactive when the operating speed is 10Mbps 
or during line isolation.
Note: Refer to Section 3.8.1, "LEDs," on 

page 37 for additional LED information.

nINT/ 
REFCLKO 
Function 
Select 

Configuration 
Strap

nINTSEL IS
(PU)

This configuration strap selects the mode of the 
nINT/REFCLKO pin. 

When nINTSEL is floated or pulled to VDD2A, 
nINT is selected for operation on the 
nINT/REFCLKO pin (default).
When nINTSEL is pulled low to VSS, 
REFCLKO is selected for operation on the 
nINT/REFCLKO pin.

See Note 2.2 for more information on 
configuration straps. 
Note: Refer to See Section 3.8.1.2, "nINTSEL 

and LED2 Polarity Selection," on 
page 37 for additional information.

Table 2.3 Serial Management Interface (SMI) Pins

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1 SMI Data Input/Output
MDIO VIS/

VOD8
Serial Management Interface data input/output

1 SMI Clock MDC VIS Serial Management Interface clock

Table 2.4 Ethernet Pins

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1

Ethernet 
TX/RX 
Positive 

Channel 1

TXP AIO Transmit/Receive Positive Channel 1

1

Ethernet 
TX/RX 

Negative 
Channel 1

TXN AIO Transmit/Receive Negative Channel 1

Table 2.2 LED Pins (continued) 

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION
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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
1

Ethernet 
TX/RX 
Positive 

Channel 2

RXP AIO Transmit/Receive Positive Channel 2

1

Ethernet 
TX/RX 

Negative 
Channel 2

RXN AIO Transmit/Receive Negative Channel 2

Table 2.5 Miscellaneous Pins

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1

External 
Crystal 
Input

XTAL1 ICLK External crystal input

External 
Clock Input

CLKIN ICLK Single-ended clock oscillator input.
Note: When using a single ended clock 

oscillator, XTAL2 should be left 
unconnected.

1
External 
Crystal 
Output

XTAL2 OCLK External crystal output

1 External Reset
nRST VIS

(PU)
System reset. This signal is active low.

1

Interrupt 
Output

nINT VOD8
(PU)

Active low interrupt output. Place an external 
resistor pull-up to VDDIO.
Note: Refer to Section 3.6, "Interrupt 

Management," on page 29 for additional 
details on device interrupts.

Note: Refer to Section 3.8.1.2, "nINTSEL and 
LED2 Polarity Selection," on page 37 for 
details on how the nINTSEL 
configuration strap is used to determine 
the function of this pin.

Reference 
Clock Output

REFCLKO VO8 This optional 50MHz clock output is derived from 
the 25MHz crystal oscillator. REFCLKO is 
selectable via the nINTSEL configuration strap.
Note: Refer Section 3.7.4.2, "REF_CLK Out 

Mode," on page 34 for additional details.

Note: Refer to Section 3.8.1.2, "nINTSEL and 
LED2 Polarity Selection," on page 37 for 
details on how the nINTSEL 
configuration strap is used to determine 
the function of this pin.

Table 2.4 Ethernet Pins (continued) 

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION
SMSC LAN8720A/LAN8720Ai 13 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Table 2.6 Analog Reference Pins

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1

External 1% 
Bias Resistor 

Input

RBIAS AI This pin requires connection of a 12.1k ohm (1%) 
resistor to ground. 

Refer to the LAN8720A/LAN8720Ai reference 
schematic for connection information.
Note: The nominal voltage is 1.2V and the 

resistor will dissipate approximately 
1mW of power.

Table 2.7 Power Pins

NUM PINS NAME SYMBOL
BUFFER 

TYPE DESCRIPTION

1

+1.6V to 
+3.6V 

Variable I/O 
Power

VDDIO P +1.6V to +3.6V variable I/O power

Refer to the LAN8720A/LAN8720Ai reference 
schematic for connection information.

1

+1.2V Digital 
Core Power 

Supply

VDDCR P Supplied by the on-chip regulator unless 
configured for regulator off mode via the 
REGOFF configuration strap. 

Refer to the LAN8720A/LAN8720Ai reference 
schematic for connection information.
Note: 1 uF and 470 pF decoupling capacitors 

in parallel to ground should be used on 
this pin.

1

+3.3V 
Channel 1 

Analog Port 
Power

VDD1A P +3.3V Analog Port Power to Channel 1

Refer to the LAN8720A/LAN8720Ai reference 
schematic for connection information.

1

+3.3V 
Channel 2 

Analog Port 
Power

VDD2A P +3.3V Analog Port Power to Channel 2 and the 
internal regulator.

Refer to the LAN8720A/LAN8720Ai reference 
schematic for connection information.

1 Ground VSS P Common ground. This exposed pad must be connected to the ground plane with a via array.
Revision 1.4 (08-23-12) 14 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
2.1   Pin Assignments

Table 2.8  24-QFN Package Pin Assignments

PIN NUM PIN NAME PIN NUM PIN NAME

1 VDD2A 13 MDC

2 LED2/nINTSEL 14 nINT/REFCLKO

3 LED1/REGOFF 15 nRST

4 XTAL2 16 TXEN

5 XTAL1/CLKIN 17 TXD0

6 VDDCR 18 TXD1

7 RXD1/MODE1 19 VDD1A

8 RXD0/MODE0 20 TXN

9 VDDIO 21 TXP

10 RXER/PHYAD0 22 RXN

11 CRS_DV/MODE2 23 RXP

12 MDIO 24 RBIAS
SMSC LAN8720A/LAN8720Ai 15 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
2.2   Buffer Types

Note: The digital signals are not 5V tolerant. Refer to Section 5.1, "Absolute Maximum Ratings*," on
page 63 for additional buffer information.

Note 2.3 Sink and source capabilities are dependant on the VDDIO voltage. Refer to Section 5.1,
"Absolute Maximum Ratings*," on page 63 for additional information.

Table 2.9 Buffer Types

BUFFER TYPE DESCRIPTION

IS Schmitt-triggered input

O12 Output with 12mA sink and 12mA source

VIS Variable voltage Schmitt-triggered input

VO8 Variable voltage output with 8mA sink and 8mA source

VOD8 Variable voltage open-drain output with 8mA sink

PU 50uA (typical) internal pull-up. Unless otherwise noted in the pin description, internal pull-
ups are always enabled. 
Note: Internal pull-up resistors prevent unconnected inputs from floating. Do not rely on 

internal resistors to drive signals external to the device. When connected to a load 
that must be pulled high, an external resistor must be added.

PD 50uA (typical) internal pull-down. Unless otherwise noted in the pin description, internal 
pull-downs are always enabled.
Note: Internal pull-down resistors prevent unconnected inputs from floating. Do not rely 

on internal resistors to drive signals external to the device. When connected to a 
load that must be pulled low, an external resistor must be added.

AI Analog input

AIO Analog bi-directional

ICLK Crystal oscillator input pin

OCLK Crystal oscillator output pin

P Power pin
Revision 1.4 (08-23-12) 16 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 3 Functional Description

This chapter provides functional descriptions of the various device features. These features have been
categorized into the following sections:

Transceiver

Auto-negotiation

HP Auto-MDIX Support

MAC Interface

Serial Management Interface (SMI)

Interrupt Management

Configuration Straps

Miscellaneous Functions

Application Diagrams

3.1   Transceiver

3.1.1 100BASE-TX Transmit

The 100BASE-TX transmit data path is shown in Figure 3.1. Each major block is explained in the
following subsections.

3.1.1.1 100BASE-TX Transmit Data Across the RMII Interface

The MAC controller drives the transmit data onto the TXD bus and asserts TXEN to indicate valid data.
The data is latched by the transceiver’s RMII block on the rising edge of REF_CLK. The data is in the
form of 2-bit wide 50MHz data. 

Figure 3.1 100BASE-TX Transmit Data Path

MAC

Tx 
Driver

MLT-3 
Converter

NRZI 
Converter

4B/5B 
Encoder

CAT-5RJ45

25MHz by
5 bits

NRZI

MLT-3MLT-3

MLT-3

Scrambler 
and PISORMII

25MHz
by 4 bits

Ext Ref_CLK

PLL

RMII 50Mhz by 2 bits

MLT-3Magnetics

125 Mbps Serial
SMSC LAN8720A/LAN8720Ai 17 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.1.1.2 4B/5B Encoding

The transmit data passes from the RMII block to the 4B/5B encoder. This block encodes the data from
4-bit nibbles to 5-bit symbols (known as “code-groups”) according to Table 3.1. Each 4-bit data-nibble
is mapped to 16 of the 32 possible code-groups. The remaining 16 code-groups are either used for
control information or are not valid.

The first 16 code-groups are referred to by the hexadecimal values of their corresponding data nibbles,
0 through F. The remaining code-groups are given letter designations with slashes on either side. For
example, an IDLE code-group is /I/, a transmit error code-group is /H/, etc.

Table 3.1  4B/5B Code Table

CODE
GROUP SYM

RECEIVER
INTERPRETATION

TRANSMITTER
INTERPRETATION

11110 0 0 0000 DATA 0 0000 DATA

01001 1 1 0001 1 0001

10100 2 2 0010 2 0010

10101 3 3 0011 3 0011

01010 4 4 0100 4 0100

01011 5 5 0101 5 0101

01110 6 6 0110 6 0110

01111 7 7 0111 7 0111

10010 8 8 1000 8 1000

10011 9 9 1001 9 1001

10110 A A 1010 A 1010

10111 B B 1011 B 1011

11010 C C 1100 C 1100

11011 D D 1101 D 1101

11100 E E 1110 E 1110

11101 F F 1111 F 1111

11111 I IDLE Sent after /T/R until TXEN

11000 J First nibble of SSD, translated to “0101” 
following IDLE, else RXER 

Sent for rising TXEN

10001 K Second nibble of SSD, translated to 
“0101” following J, else RXER

Sent for rising TXEN

01101 T First nibble of ESD, causes de-assertion 
of CRS if followed by /R/, else assertion 
of RXER

Sent for falling TXEN

00111 R Second nibble of ESD, causes 
deassertion of CRS if following /T/, else 
assertion of RXER

Sent for falling TXEN

00100 H Transmit Error Symbol Sent for rising TXER

00110 V INVALID, RXER if during RXDV INVALID
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3.1.1.3 Scrambling

Repeated data patterns (especially the IDLE code-group) can have power spectral densities with large
narrow-band peaks. Scrambling the data helps eliminate these peaks and spread the signal power
more uniformly over the entire channel bandwidth. This uniform spectral density is required by FCC
regulations to prevent excessive EMI from being radiated by the physical wiring.

The seed for the scrambler is generated from the transceiver address, PHYAD, ensuring that in
multiple-transceiver applications, such as repeaters or switches, each transceiver will have its own
scrambler sequence.

The scrambler also performs the Parallel In Serial Out conversion (PISO) of the data.

3.1.1.4 NRZI and MLT-3 Encoding

The scrambler block passes the 5-bit wide parallel data to the NRZI converter where it becomes a
serial 125MHz NRZI data stream. The NRZI is encoded to MLT-3. MLT-3 is a tri-level code where a
change in the logic level represents a code bit “1” and the logic output remaining at the same level
represents a code bit “0”.

3.1.1.5 100M Transmit Driver

The MLT3 data is then passed to the analog transmitter, which drives the differential MLT-3 signal, on
outputs TXP and TXN, to the twisted pair media across a 1:1 ratio isolation transformer. The 10BASE-
T and 100BASE-TX signals pass through the same transformer so that common “magnetics” can be
used for both. The transmitter drives into the 100Ω impedance of the CAT-5 cable. Cable termination
and impedance matching require external components.

3.1.1.6 100M Phase Lock Loop (PLL)

The 100M PLL locks onto reference clock and generates the 125MHz clock used to drive the 125 MHz
logic and the 100BASE-TX transmitter.

11001 V INVALID, RXER if during RXDV INVALID

00000 V INVALID, RXER if during RXDV INVALID

00001 V INVALID, RXER if during RXDV INVALID

00010 V INVALID, RXER if during RXDV INVALID

00011 V INVALID, RXER if during RXDV INVALID

00101 V INVALID, RXER if during RXDV INVALID

01000 V INVALID, RXER if during RXDV INVALID

01100 V INVALID, RXER if during RXDV INVALID

10000 V INVALID, RXER if during RXDV INVALID

Table 3.1  4B/5B Code Table (continued) 

CODE
GROUP SYM

RECEIVER
INTERPRETATION

TRANSMITTER
INTERPRETATION
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3.1.2 100BASE-TX Receive

The 100BASE-TX receive data path is shown in Figure 3.2. Each major block is explained in the
following subsections.

3.1.2.1 100M Receive Input

The MLT-3 from the cable is fed into the transceiver (on inputs RXP and RXN) via a 1:1 ratio
transformer. The ADC samples the incoming differential signal at a rate of 125M samples per second.
Using a 64-level quanitizer, it generates 6 digital bits to represent each sample. The DSP adjusts the
gain of the ADC according to the observed signal levels such that the full dynamic range of the ADC
can be used.

3.1.2.2 Equalizer, Baseline Wander Correction and Clock and Data Recovery

The 6 bits from the ADC are fed into the DSP block. The equalizer in the DSP section compensates
for phase and amplitude distortion caused by the physical channel consisting of magnetics, connectors,
and CAT- 5 cable. The equalizer can restore the signal for any good-quality CAT-5 cable between 1m
and 150m.

If the DC content of the signal is such that the low-frequency components fall below the low frequency
pole of the isolation transformer, then the droop characteristics of the transformer will become
significant and Baseline Wander (BLW) on the received signal will result. To prevent corruption of the
received data, the transceiver corrects for BLW and can receive the ANSI X3.263-1995 FDDI TP-PMD
defined “killer packet” with no bit errors.

The 100M PLL generates multiple phases of the 125MHz clock. A multiplexer, controlled by the timing
unit of the DSP, selects the optimum phase for sampling the data. This is used as the received
recovered clock. This clock is used to extract the serial data from the received signal.

Figure 3.2 100BASE-TX Receive Data Path

MAC

A/D 
Converter

MLT-3 
Converter

NRZI 
Converter

4B/5B 
Decoder

Magnetics CAT-5RJ45

PLL

RMII 50Mhz by 2 bits
25MHz by

5 bits

NRZI

MLT-3MLT-3 MLT-3

6 bit Data

Descrambler 
and SIPO

125 Mbps Serial

DSP: Timing 
recovery, Equalizer 
and BLW Correction

MLT-3

RMII
25MHz

by 4 bits

Ext Ref_CLK
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3.1.2.3 NRZI and MLT-3 Decoding

The DSP generates the MLT-3 recovered levels that are fed to the MLT-3 converter. The MLT-3 is then
converted to an NRZI data stream. 

3.1.2.4 Descrambling

The descrambler performs an inverse function to the scrambler in the transmitter and also performs
the Serial In Parallel Out (SIPO) conversion of the data.

During reception of IDLE (/I/) symbols. the descrambler synchronizes its descrambler key to the
incoming stream. Once synchronization is achieved, the descrambler locks on this key and is able to
descramble incoming data.

Special logic in the descrambler ensures synchronization with the remote transceiver by searching for
IDLE symbols within a window of 4000 bytes (40us). This window ensures that a maximum packet size
of 1514 bytes, allowed by the IEEE 802.3 standard, can be received with no interference. If no IDLE-
symbols are detected within this time-period, receive operation is aborted and the descrambler re-starts
the synchronization process.

3.1.2.5 Alignment

The de-scrambled signal is then aligned into 5-bit code-groups by recognizing the /J/K/ Start-of-Stream
Delimiter (SSD) pair at the start of a packet. Once the code-word alignment is determined, it is stored
and utilized until the next start of frame.

3.1.2.6 5B/4B Decoding

The 5-bit code-groups are translated into 4-bit data nibbles according to the 4B/5B table. The
translated data is presented on the RXD[1:0] signal lines. The SSD, /J/K/, is translated to “0101 0101”
as the first 2 nibbles of the MAC preamble. Reception of the SSD causes the transceiver to assert the
receive data valid signal, indicating that valid data is available on the RXD bus. Successive valid code-
groups are translated to data nibbles. Reception of either the End of Stream Delimiter (ESD) consisting
of the /T/R/ symbols, or at least two /I/ symbols causes the transceiver to de-assert the carrier sense
and receive data valid signals.

Note: These symbols are not translated into data.

3.1.2.7 Receive Data Valid Signal

The Receive Data Valid signal (RXDV) indicates that recovered and decoded nibbles are being
presented on the RXD[1:0] outputs synchronous to RXCLK. RXDV becomes active after the /J/K/
delimiter has been recognized and RXD is aligned to nibble boundaries. It remains active until either
the /T/R/ delimiter is recognized or link test indicates failure or SIGDET becomes false.

RXDV is asserted when the first nibble of translated /J/K/ is ready for transfer over the Media
Independent Interface (MII mode). 

Figure 3.3 Relationship Between Received Data and Specific MII Signals

5 D5 data data data dataRXD

RX_DV

RX_CLK

5 D5 data data data dataCLEAR-TEXT 5J K

5 5 5

T R Idle
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3.1.2.8 Receiver Errors

During a frame, unexpected code-groups are considered receive errors. Expected code groups are the
DATA set (0 through F), and the /T/R/ (ESD) symbol pair. When a receive error occurs, the RXER
signal is asserted and arbitrary data is driven onto the RXD[1:0] lines. Should an error be detected
during the time that the /J/K/ delimiter is being decoded (bad SSD error), RXER is asserted true and
the value ‘1110’ is driven onto the RXD[1:0] lines. Note that the Valid Data signal is not yet asserted
when the bad SSD error occurs. 

3.1.2.9 100M Receive Data Across the RMII Interface

The 2-bit data nibbles are sent to the RMII block. These data nibbles are clocked to the controller at
a rate of 50MHz. The controller samples the data on the rising edge of XTAL1/CLKIN (REF_CLK). To
ensure that the setup and hold requirements are met, the nibbles are clocked out of the transceiver
on the falling edge of XTAL1/CLKIN (REF_CLK). 

3.1.3 10BASE-T Transmit

Data to be transmitted comes from the MAC layer controller. The 10BASE-T transmitter receives 4-bit
nibbles from the MII at a rate of 2.5MHz and converts them to a 10Mbps serial data stream. The data
stream is then Manchester-encoded and sent to the analog transmitter, which drives a signal onto the
twisted pair via the external magnetics.

The 10M transmitter uses the following blocks:

MII (digital)

TX 10M (digital)

10M Transmitter (analog)

10M PLL (analog)

3.1.3.1 10M Transmit Data Across the RMII Interface

The MAC controller drives the transmit data onto the TXD bus. TXD[1:0] shall transition synchronously
with respect to REF_CLK. When TXEN is asserted, TXD[1:0] are accepted for transmission by the
device. TXD[1:0] shall be “00” to indicate idle when TXEN is deasserted. Values of TXD[1:0] other than
“00” when TXEN is deasserted are reserved for out-of-band signalling (to be defined). Values other
than “00” on TXD[1:0] while TXEN is deasserted shall be ignored by the device.TXD[1:0] shall provide
valid data for each REF_CLK period while TXEN is asserted.

In order to comply with legacy 10BASE-T MAC/Controllers, in half-duplex mode the transceiver loops
back the transmitted data, on the receive path. This does not confuse the MAC/Controller since the
COL signal is not asserted during this time. The transceiver also supports the SQE (Heartbeat) signal. 

3.1.3.2 Manchester Encoding

The 4-bit wide data is sent to the 10M TX block. The nibbles are converted to a 10Mbps serial NRZI
data stream. The 10M PLL locks onto the external clock or internal oscillator and produces a 20MHz
clock. This is used to Manchester encode the NRZ data stream. When no data is being transmitted
(TXEN is low), the 10M TX block outputs Normal Link Pulses (NLPs) to maintain communications with
the remote link partner.

3.1.3.3 10M Transmit Drivers

The Manchester encoded data is sent to the analog transmitter where it is shaped and filtered before
being driven out as a differential signal across the TXP and TXN outputs.
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3.1.4 10BASE-T Receive

The 10BASE-T receiver gets the Manchester- encoded analog signal from the cable via the magnetics.
It recovers the receive clock from the signal and uses this clock to recover the NRZI data stream. This
10M serial data is converted to 4-bit data nibbles which are passed to the controller via MII at a rate
of 2.5MHz. 

This 10M receiver uses the following blocks:

Filter and SQUELCH (analog)

10M PLL (analog)

RX 10M (digital)

MII (digital)

3.1.4.1 10M Receive Input and Squelch

The Manchester signal from the cable is fed into the transceiver (on inputs RXP and RXN) via 1:1 ratio
magnetics. It is first filtered to reduce any out-of-band noise. It then passes through a SQUELCH
circuit. The SQUELCH is a set of amplitude and timing comparators that normally reject differential
voltage levels below 300mV and detect and recognize differential voltages above 585mV. 

3.1.4.2 Manchester Decoding

The output of the SQUELCH goes to the 10M RX block where it is validated as Manchester encoded
data. The polarity of the signal is also checked. If the polarity is reversed (local RXP is connected to
RXN of the remote partner and vice versa), the condition is identified and corrected. The reversed
condition is indicated by the XPOL bit of the Special Control/Status Indications Register. The 10M PLL
is locked onto the received Manchester signal, from which the 20MHz cock is generated. Using this
clock, the Manchester encoded data is extracted and converted to a 10MHz NRZI data stream. It is
then converted from serial to 4-bit wide parallel data.

The 10M RX block also detects valid 10Base-T IDLE signals - Normal Link Pulses (NLPs) - to maintain
the link. 

3.1.4.3 10M Receive Data Across the RMII Interface

The 2-bit data nibbles are sent to the RMII block. These data nibbles are valid on the rising edge of
the RMII REF_CLK.

3.1.4.4 Jabber Detection

Jabber is a condition in which a station transmits for a period of time longer than the maximum
permissible packet length, usually due to a fault condition, which results in holding the TXEN input for
a long period. Special logic is used to detect the jabber state and abort the transmission to the line
within 45ms. Once TXEN is deasserted, the logic resets the jabber condition.

As shown in Section 4.2.2, "Basic Status Register," on page 50, the Jabber Detect bit indicates that a
jabber condition was detected.

3.2   Auto-negotiation
The purpose of the auto-negotiation function is to automatically configure the transceiver to the
optimum link parameters based on the capabilities of its link partner. Auto-negotiation is a mechanism
for exchanging configuration information between two link-partners and automatically selecting the
highest performance mode of operation supported by both sides. Auto-negotiation is fully defined in
clause 28 of the IEEE 802.3 specification.

Once auto-negotiation has completed, information about the resolved link can be passed back to the
controller via the Serial Management Interface (SMI). The results of the negotiation process are
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reflected in the Speed Indication bits of the PHY Special Control/Status Register, as well as in the Auto
Negotiation Link Partner Ability Register. The auto-negotiation protocol is a purely physical layer
activity and proceeds independently of the MAC controller.

The advertised capabilities of the transceiver are stored in the Auto Negotiation Advertisement
Register. The default advertised by the transceiver is determined by user-defined on-chip signal
options.

The following blocks are activated during an Auto-negotiation session:

Auto-negotiation (digital)

100M ADC (analog)

100M PLL (analog)

100M equalizer/BLW/clock recovery (DSP)

10M SQUELCH (analog)

10M PLL (analog)

10M Transmitter (analog)

When enabled, auto-negotiation is started by the occurrence of one of the following events:

Hardware reset

Software reset

Power-down reset

Link status down

Setting the Restart Auto-Negotiate bit of the Basic Control Register

On detection of one of these events, the transceiver begins auto-negotiation by transmitting bursts of
Fast Link Pulses (FLP), which are bursts of link pulses from the 10M transmitter. They are shaped as
Normal Link Pulses and can pass uncorrupted down CAT-3 or CAT-5 cable. A Fast Link Pulse Burst
consists of up to 33 pulses. The 17 odd-numbered pulses, which are always present, frame the FLP
burst. The 16 even-numbered pulses, which may be present or absent, contain the data word being
transmitted. Presence of a data pulse represents a “1”, while absence represents a “0”. 

The data transmitted by an FLP burst is known as a “Link Code Word.” These are defined fully in IEEE
802.3 clause 28. In summary, the transceiver advertises 802.3 compliance in its selector field (the first
5 bits of the Link Code Word). It advertises its technology ability according to the bits set in the Auto
Negotiation Advertisement Register. 

There are 4 possible matches of the technology abilities. In the order of priority these are:

100M Full Duplex (Highest Priority) 

100M Half Duplex

10M Full Duplex 

10M Half Duplex (Lowest Priority)

If the full capabilities of the transceiver are advertised (100M, Full Duplex), and if the link partner is
capable of 10M and 100M, then auto-negotiation selects 100M as the highest performance mode. If
the link partner is capable of half and full duplex modes, then auto-negotiation selects full duplex as
the highest performance operation.

Once a capability match has been determined, the link code words are repeated with the acknowledge
bit set. Any difference in the main content of the link code words at this time will cause auto-negotiation
to re-start. Auto-negotiation will also re-start if not all of the required FLP bursts are received.

The capabilities advertised during auto-negotiation by the transceiver are initially determined by the
logic levels latched on the MODE[2:0] configuration straps after reset completes. These configuration
straps can also be used to disable auto-negotiation on power-up. Refer to Section 3.7.2, "MODE[2:0]:
Mode Configuration," on page 31 for additional information.
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Writing the bits 8 through 5 of the Auto Negotiation Advertisement Register allows software control of
the capabilities advertised by the transceiver. Writing the Auto Negotiation Advertisement Register
does not automatically re-start auto-negotiation. The Restart Auto-Negotiate bit of the Basic Control
Register must be set before the new abilities will be advertised. Auto-negotiation can also be disabled
via software by clearing the Auto-Negotiation Enable bit of the Basic Control Register.

Note: The device does not support “Next Page” capability.

3.2.1 Parallel Detection

If the LAN8720A/LAN8720Ai is connected to a device lacking the ability to auto-negotiate (i.e. no FLPs
are detected), it is able to determine the speed of the link based on either 100M MLT-3 symbols or
10M Normal Link Pulses. In this case the link is presumed to be half duplex per the IEEE standard.
This ability is known as “Parallel Detection.” This feature ensures interoperability with legacy link
partners. If a link is formed via parallel detection, then the Link Partner Auto-Negotiation Able bit of the
Auto Negotiation Expansion Register is cleared to indicate that the Link Partner is not capable of auto-
negotiation. The controller has access to this information via the management interface. If a fault
occurs during parallel detection, the Parallel Detection Fault bit of Link Partner Auto-Negotiation Able
is set.

Auto Negotiation Link Partner Ability Register is used to store the link partner ability information, which
is coded in the received FLPs. If the link partner is not auto-negotiation capable, then the Auto
Negotiation Link Partner Ability Register is updated after completion of parallel detection to reflect the
speed capability of the link partner.

3.2.2 Restarting Auto-negotiation

Auto-negotiation can be restarted at any time by setting the Restart Auto-Negotiate bit of the Basic
Control Register. Auto-negotiation will also restart if the link is broken at any time. A broken link is
caused by signal loss. This may occur because of a cable break, or because of an interruption in the
signal transmitted by the link partner. Auto-negotiation resumes in an attempt to determine the new
link configuration.

If the management entity re-starts auto-negotiation by setting the Restart Auto-Negotiate bit of the
Basic Control Register, the LAN8720A/LAN8720Ai will respond by stopping all transmission/receiving
operations. Once the break_link_timer is completed in the Auto-negotiation state-machine
(approximately 1200ms), auto-negotiation will re-start. In this case, the link partner will have also
dropped the link due to lack of a received signal, so it too will resume auto-negotiation.

3.2.3 Disabling Auto-negotiation

Auto-negotiation can be disabled by setting the Auto-Negotiation Enable bit of the Basic Control
Register to zero. The device will then force its speed of operation to reflect the information in the Basic
Control Register (Speed Select bit and Duplex Mode bit). These bits should be ignored when auto-
negotiation is enabled.

3.2.4 Half vs. Full Duplex

Half duplex operation relies on the CSMA/CD (Carrier Sense Multiple Access / Collision Detect)
protocol to handle network traffic and collisions. In this mode, the carrier sense signal, CRS, responds
to both transmit and receive activity. If data is received while the transceiver is transmitting, a collision
results. 

In full duplex mode, the transceiver is able to transmit and receive data simultaneously. In this mode,
CRS responds only to receive activity. The CSMA/CD protocol does not apply and collision detection
is disabled. 
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3.3   HP Auto-MDIX Support
HP Auto-MDIX facilitates the use of CAT-3 (10BASE-T) or CAT-5 (100BASE-T) media UTP
interconnect cable without consideration of interface wiring scheme. If a user plugs in either a direct
connect LAN cable, or a cross-over patch cable, as shown in Figure 3.4, the device’s Auto-MDIX
transceiver is capable of configuring the TXP/TXN and RXP/RXN pins for correct transceiver operation.

The internal logic of the device detects the TX and RX pins of the connecting device. Since the RX
and TX line pairs are interchangeable, special PCB design considerations are needed to accommodate
the symmetrical magnetics and termination of an Auto-MDIX design.

The Auto-MDIX function can be disabled via the AMDIXCTRL bit in the Special Control/Status
Indications Register.

Figure 3.4 Direct Cable Connection vs. Cross-over Cable Connection

1

2

3

4

5

6

7

8

TXP

TXN

RXP

Not Used

Not Used

RXN

Not Used

Not Used

1

2

3

4

5

6

7

8

TXP

TXN

RXP

Not Used

Not Used

RXN

Not Used

Not Used

Direct Connect Cable

RJ-45 8-pin straight-through 
for 10BASE-T/100BASE-TX 

signaling

1

2

3

4

5

6

7

8

TXP

TXN

RXP

Not Used

Not Used

RXN

Not Used

Not Used

1

2

3

4

5

6

7

8

TXP

TXN

RXP

Not Used

Not Used

RXN

Not Used

Not Used

Cross-Over Cable

RJ-45 8-pin cross-over for 
10BASE-T/100BASE-TX 

signaling
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3.4   MAC Interface

3.4.1 RMII 

The device supports the low pin count Reduced Media Independent Interface (RMII) intended for use
between Ethernet transceivers and switch ASICs. Under IEEE 802.3, an MII comprised of 16 pins for
data and control is defined. In devices incorporating many MACs or transceiver interfaces such as
switches, the number of pins can add significant cost as the port counts increase. RMII reduces this
pin count while retaining a management interface (MDIO/MDC) that is identical to MII. 

The RMII interface has the following characteristics:

It is capable of supporting 10Mbps and 100Mbps data rates

A single clock reference is used for both transmit and receive

It provides independent 2-bit (di-bit) wide transmit and receive data paths

It uses LVCMOS signal levels, compatible with common digital CMOS ASIC processes

The RMII includes the following interface signals (1 optional):

transmit data - TXD[1:0]

transmit strobe - TXEN

receive data - RXD[1:0]

receive error - RXER (Optional) 

carrier sense - CRS_DV

Reference Clock - (RMII references usually define this signal as REF_CLK)

3.4.1.1 CRS_DV - Carrier Sense/Receive Data Valid

The CRS_DV is asserted by the device when the receive medium is non-idle. CRS_DV is asserted
asynchronously on detection of carrier due to the criteria relevant to the operating mode. In 10BASE-
T mode when squelch is passed, or in 100BASE-X mode when 2 non-contiguous zeroes in 10 bits are
detected, the carrier is said to be detected.

Loss of carrier shall result in the deassertion of CRS_DV synchronous to the cycle of REF_CLK which
presents the first di-bit of a nibble onto RXD[1:0] (i.e. CRS_DV is deasserted only on nibble
boundaries). If the device has additional bits to be presented on RXD[1:0] following the initial
deassertion of CRS_DV, then the device shall assert CRS_DV on cycles of REF_CLK which present
the second di-bit of each nibble and de-assert CRS_DV on cycles of REF_CLK which present the first
di-bit of a nibble. The result is, starting on nibble boundaries, CRS_DV toggles at 25 MHz in 100Mbps
mode and 2.5 MHz in 10Mbps mode when CRS ends before RXDV (i.e. the FIFO still has bits to
transfer when the carrier event ends). Therefore, the MAC can accurately recover RXDV and CRS.

During a false carrier event, CRS_DV shall remain asserted for the duration of carrier activity. The data
on RXD[1:0] is considered valid once CRS_DV is asserted. However, since the assertion of CRS_DV
is asynchronous relative to REF_CLK, the data on RXD[1:0] shall be “00” until proper receive signal
decoding takes place.

3.4.1.2 Reference Clock (REF_CLK)

The RMII REF_CLK is a continuous clock that provides the timing reference for CRS_DV, RXD[1:0],
TXEN, TXD[1:0] and RXER. The device uses REF_CLK as the network clock such that no buffering
is required on the transmit data path. However, on the receive data path, the receiver recovers the
clock from the incoming data stream, and the device uses elasticity buffering to accommodate for
differences between the recovered clock and the local REF_CLK.
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3.5   Serial Management Interface (SMI)
The Serial Management Interface is used to control the device and obtain its status. This interface
supports registers 0 through 6 as required by Clause 22 of the 802.3 standard, as well as “vendor-
specific” registers 16 to 31 allowed by the specification. Non-supported registers (such as 7 to 15) will
be read as hexadecimal “FFFF”. Device registers are detailed in Chapter 4, "Register Descriptions,"
on page 47.

At the system level, SMI provides 2 signals: MDIO and MDC. The MDC signal is an aperiodic clock
provided by the station management controller (SMC). MDIO is a bi-directional data SMI input/output
signal that receives serial data (commands) from the controller SMC and sends serial data (status) to
the SMC. The minimum time between edges of the MDC is 160 ns. There is no maximum time
between edges. The minimum cycle time (time between two consecutive rising or two consecutive
falling edges) is 400 ns. These modest timing requirements allow this interface to be easily driven by
the I/O port of a microcontroller.

The data on the MDIO line is latched on the rising edge of the MDC. The frame structure and timing
of the data is shown in Figure 3.5 and Figure 3.6. The timing relationships of the MDIO signals are
further described in Section 5.5.5, "SMI Timing," on page 73.

Figure 3.5 MDIO Timing and Frame Structure - READ Cycle

Figure 3.6 MDIO Timing and Frame Structure - WRITE Cycle

MDC

MDIO

Read Cycle
...

32 1's 0 1 1 0 A4 A3 A2 A1 A0 R4 R3 R2 R1 R0 D1...D15 D14 D0

Preamble Start ofFrame
OP 

Code PHY Address Register Address
Turn

Around
Data

Data From PhyData To Phy

MDC

MDIO ...32 1's 0 1 10 A4 A3 A2 A1 A0 R4 R3 R2 R1 R0

Write Cycle

D15 D14 D1 D0

...

DataPreamble Start ofFrame
OP 

Code PHY Address Register Address
Turn

Around

Data To Phy
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3.6   Interrupt Management
The device management interface supports an interrupt capability that is not a part of the IEEE 802.3
specification. This interrupt capability generates an active low asynchronous interrupt signal on the
nINT output whenever certain events are detected as setup by the Interrupt Mask Register.

The device’s interrupt system provides two modes, a Primary Interrupt mode and an Alternative
interrupt mode. Both systems will assert the nINT pin low when the corresponding mask bit is set.
These modes differ only in how they de-assert the nINT interrupt output. These modes are detailed in
the following subsections.

Note: The Primary interrupt mode is the default interrupt mode after a power-up or hard reset. The
Alternative interrupt mode requires setup after a power-up or hard reset.

3.6.1 Primary Interrupt System

The Primary interrupt system is the default interrupt mode (ALTINT bit of the Mode Control/Status
Register is “0”). The Primary interrupt system is always selected after power-up or hard reset. In this
mode, to set an interrupt, set the corresponding mask bit in the Interrupt Mask Register (see Table 3.2).
Then when the event to assert nINT is true, the nINT output will be asserted. When the corresponding
event to deassert nINT is true, then the nINT will be de-asserted.

Note 3.1 If the mask bit is enabled and nINT has been de-asserted while ENERGYON is still high,
nINT will assert for 256 ms, approximately one second after ENERGYON goes low when
the Cable is unplugged. To prevent an unexpected assertion of nINT, the ENERGYON
interrupt mask should always be cleared as part of the ENERGYON interrupt service
routine.

Table 3.2  Interrupt Management Table

MASK
INTERRUPT SOURCE 

FLAG INTERRUPT SOURCE
EVENT TO 

ASSERT nINT
EVENT TO

DE-ASSERT nINT

30.7 29.7 ENERGYON 17.1 ENERGYON Rising 17.1 
(Note 3.1)

Falling 17.1 or
Reading register 29

30.6 29.6 Auto-Negotiation 
complete

1.5 Auto-Negotiate 
Complete

Rising 1.5 Falling 1.5 or
Reading register 29

30.5 29.5 Remote Fault 
Detected

1.4 Remote Fault Rising 1.4 Falling 1.4, or 
Reading register 1 or 
Reading register 29

30.4 29.4 Link Down 1.2 Link Status Falling 1.2 Reading register 1 or
Reading register 29

30.3 29.3 Auto-Negotiation 
LP Acknowledge

5.14 Acknowledge Rising 5.14 Falling 5.14 or
Read register 29

30.2 29.2 Parallel Detection 
Fault

6.4 Parallel 
Detection Fault

Rising 6.4 Falling 6.4 or 
Reading register 6, or
Reading register 29 
or
Re-Auto Negotiate or
Link down

30.1 29.1 Auto-Negotiation 
Page Received

6.1 Page Received Rising 6.1 Falling of 6.1 or
Reading register 6, or
Reading register 29
Re-Auto Negotiate, or
Link Down.
SMSC LAN8720A/LAN8720Ai 29 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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Note: The ENERGYON bit in the Mode Control/Status Register is defaulted to a ‘1’ at the start of the
signal acquisition process, therefore the INT7 bit in the Interrupt Mask Register will also read
as a ‘1’ at power-up. If no signal is present, then both ENERGYON and INT7 will clear within
a few milliseconds.

3.6.2 Alternate Interrupt System

The Alternate interrupt system is enabled by setting the ALTINT bit of the Mode Control/Status Register
to “1”. In this mode, to set an interrupt, set the corresponding bit of the in the Mask Register 30, (see
Table 3.3). To Clear an interrupt, either clear the corresponding bit in the Interrupt Mask Register to
deassert the nINT output, or clear the interrupt source, and write a ‘1’ to the corresponding Interrupt
Source Flag. Writing a ‘1’ to the Interrupt Source Flag will cause the state machine to check the
Interrupt Source to determine if the Interrupt Source Flag should clear or stay as a ‘1’. If the Condition
to deassert is true, then the Interrupt Source Flag is cleared and nINT is also deasserted. If the
Condition to deassert is false, then the Interrupt Source Flag remains set, and the nINT remains
asserted.

For example, setting the INT7 bit in the Interrupt Mask Register will enable the ENERGYON interrupt.
After a cable is plugged in, the ENERGYON bit in the Mode Control/Status Register goes active and
nINT will be asserted low. To de-assert the nINT interrupt output, either clear the ENERGYON bit in
the Mode Control/Status Register by removing the cable and then writing a ‘1’ to the INT7 bit in the
Interrupt Mask Register, OR clear the INT7 mask (bit 7 of the Interrupt Mask Register).

Note: The ENERGYON bit in the Mode Control/Status Register is defaulted to a ‘1’ at the start of the
signal acquisition process, therefore the INT7 bit in the Interrupt Mask Register will also read
as a ‘1’ at power-up. If no signal is present, then both ENERGYON and INT7 will clear within
a few milliseconds.

Table 3.3  Alternative Interrupt System Management Table

MASK
INTERRUPT SOURCE 

FLAG INTERRUPT SOURCE
EVENT TO 

ASSERT nINT

CONDITION 
TO

DE-ASSERT

BIT TO 
CLEAR 

nINT

30.7 29.7 ENERGYON 17.1 ENERGYON Rising 17.1 17.1 low 29.7

30.6 29.6 Auto-Negotiation 
complete

1.5 Auto-Negotiate 
Complete

Rising 1.5 1.5 low 29.6

30.5 29.5 Remote Fault 
Detected

1.4 Remote Fault Rising 1.4 1.4 low 29.5

30.4 29.4 Link Down 1.2 Link Status Falling 1.2 1.2 high 29.4

30.3 29.3 Auto-Negotiation 
LP Acknowledge

5.14 Acknowledge Rising 5.14 5.14 low 29.3

30.2 29.2 Parallel 
Detection Fault

6.4 Parallel Detection 
Fault

Rising 6.4 6.4 low 29.2

30.1 29.1 Auto-Negotiation 
Page Received

6.1 Page Received Rising 6.1 6.1 low 29.1
Revision 1.4 (08-23-12) 30 SMSC LAN8720A/LAN8720Ai

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3.7   Configuration Straps
Configuration straps allow various features of the device to be automatically configured to user defined
values. Configuration straps are latched upon Power-On Reset (POR) and pin reset (nRST).
Configuration straps include internal resistors in order to prevent the signal from floating when
unconnected. If a particular configuration strap is connected to a load, an external pull-up or pull-down
resistor should be used to augment the internal resistor to ensure that it reaches the required voltage
level prior to latching. The internal resistor can also be overridden by the addition of an external
resistor.

Note: The system designer must guarantee that configuration strap pins meet the timing
requirements specified in Section 5.5.3, "Power-On nRST & Configuration Strap Timing," on
page 70. If configuration strap pins are not at the correct voltage level prior to being latched,
the device may capture incorrect strap values.

Note: When externally pulling configuration straps high, the strap should be tied to VDDIO, except
for REGOFF and nINTSEL which should be tied to VDD2A.

3.7.1 PHYAD[0]: PHY Address Configuration

The PHYAD0 bit is driven high or low to give each PHY a unique address. This address is latched into
an internal register at the end of a hardware reset (default = 0b). In a multi-PHY application (such as
a repeater), the controller is able to manage each PHY via the unique address. Each PHY checks
each management data frame for a matching address in the relevant bits. When a match is recognized,
the PHY responds to that particular frame. The PHY address is also used to seed the scrambler. In a
multi-PHY application, this ensures that the scramblers are out of synchronization and disperses the
electromagnetic radiation across the frequency spectrum.

The device’s SMI address may be configured using hardware configuration to either the value 0 or 1.
The user can configure the PHY address using Software Configuration if an address greater than 1 is
required. The PHY address can be written (after SMI communication at some address is established)
using the PHYAD bits of the Special Modes Register. The PHYAD0 hardware configuration strap is
multiplexed with the RXER pin.

3.7.2 MODE[2:0]: Mode Configuration

The MODE[2:0] configuration straps control the configuration of the 10/100 digital block. When the
nRST pin is deasserted, the register bit values are loaded according to the MODE[2:0] configuration
straps. The 10/100 digital block is then configured by the register bit values. When a soft reset occurs
via the Soft Reset bit of the Basic Control Register, the configuration of the 10/100 digital block is
controlled by the register bit values and the MODE[2:0] configuration straps have no affect.

The device’s mode may be configured using the hardware configuration straps as summarized in
Table 3.4. The user may configure the transceiver mode by writing the SMI registers.
SMSC LAN8720A/LAN8720Ai 31 Revision 1.4 (08-23-12)

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The MODE[2:0] hardware configuration pins are multiplexed with other signals as shown in Table 3.5.

3.7.3 REGOFF: Internal +1.2V Regulator Configuration

The incorporation of flexPWR technology provides the ability to disable the internal +1.2V regulator.
When the regulator is disabled, an external +1.2V must be supplied to the VDDCR pin. Disabling the
internal +1.2V regulator makes it possible to reduce total system power, since an external switching
regulator with greater efficiency (versus the internal linear regulator) can be used to provide +1.2V to
the transceiver circuitry.

Note: Because the REGOFF configuration strap shares functionality with the LED1 pin, proper
consideration must also be given to the LED polarity. Refer to Section 3.8.1.1, "REGOFF and
LED1 Polarity Selection," on page 37 for additional information on the relation between
REGOFF and the LED1 polarity. 

Table 3.4  MODE[2:0] Bus

MODE[2:0] MODE DEFINITIONS

DEFAULT REGISTER BIT VALUES

REGISTER 0 REGISTER 4

[13,12,10,8] [8,7,6,5]

000 10Base-T Half Duplex. Auto-negotiation disabled. 0000 N/A

001 10Base-T Full Duplex. Auto-negotiation disabled. 0001 N/A

010 100Base-TX Half Duplex. Auto-negotiation 
disabled.
CRS is active during Transmit & Receive.

1000 N/A

011 100Base-TX Full Duplex. Auto-negotiation disabled.
CRS is active during Receive.

1001 N/A

100 100Base-TX Half Duplex is advertised. Auto-
negotiation enabled.
CRS is active during Transmit & Receive.

1100 0100

101 Repeater mode. Auto-negotiation enabled. 
100Base-TX Half Duplex is advertised. 
CRS is active during Receive.

1100 0100

110 Power Down mode. In this mode the transceiver will 
wake-up in Power-Down mode. The transceiver 
cannot be used when the MODE[2:0] bits are set to 
this mode. To exit this mode, the MODE bits in 
Register 18.7:5(see Section 4.2.9, "Special Modes 
Register," on page 57) must be configured to some 
other value and a soft reset must be issued.

N/A N/A

111 All capable. Auto-negotiation enabled. X10X 1111

Table 3.5  Pin Names for Mode Bits

MODE BIT PIN NAME

MODE[0] RXD0/MODE0

MODE[1] RXD1/MODE1

MODE[2] CRS_DV/MODE2
Revision 1.4 (08-23-12) 32 SMSC LAN8720A/LAN8720Ai

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3.7.3.1 Disabling the Internal +1.2V Regulator

To disable the +1.2V internal regulator, a pull-up strapping resistor should be connected from the
REGOFF configuration strap to VDD2A. At power-on, after both VDDIO and VDD2A are within
specification, the transceiver will sample REGOFF to determine whether the internal regulator should
turn on. If the pin is sampled at a voltage greater than VIH, then the internal regulator is disabled and
the system must supply +1.2V to the VDDCR pin. The VDDIO voltage must be at least 80% of the
operating voltage level (1.44V when operating at 1.8V, 2.0V when operating at 2.5V, 2.64V when
operating at 3.3V) before voltage is applied to VDDCR. As described in Section 3.7.3.2, when
REGOFF is left floating or connected to VSS, the internal regulator is enabled and the system is not
required to supply +1.2V to the VDDCR pin.

3.7.3.2 Enabling the Internal +1.2V Regulator

The +1.2V for VDDCR is supplied by the on-chip regulator unless the transceiver is configured for the
regulator off mode using the REGOFF configuration strap as described in Section 3.7.3.1. By default,
the internal +1.2V regulator is enabled when REGOFF is floating (due to the internal pull-down
resistor). During power-on, if REGOFF is sampled below VIL, then the internal +1.2V regulator will turn
on and operate with power from the VDD2A pin.

3.7.4 nINTSEL: nINT/REFCLKO Configuration

The nINTSEL configuration strap is used to select between one of two available modes: REF_CLK In
Mode (nINT) and REF_CLK Out Mode. The configured mode determines the function of the
nINT/REFCLKO pin. The nINTSEL configuration strap is latched at POR and on the rising edge of the
nRST. By default, nINTSEL is configured for nINT mode via the internal pull-up resistor. 

The RMII REF_CLK is a continuous clock that provides the timing reference for CRS_DV, RXD[1:0],
TXEN, TXD[1:0] and RXER. The device uses REF_CLK as the network clock such that no buffering
is required on the transmit data path. However, on the receive data path, the receiver recovers the
clock from the incoming data stream. The device uses elasticity buffering to accommodate for
differences between the recovered clock and the local REF_CLK.

In REF_CLK In Mode, the 50MHz REF_CLK is driven on the XTAL1/CLKIN pin. This is the traditional
system configuration when using RMII, and is described in Section 3.7.4.1. When configured for
REF_CLK Out Mode, the device generates the 50MHz RMII REF_CLK and the nINT interrupt is not
available. REF_CLK Out Mode allows a low-cost 25MHz crystal to be used as the reference for
REF_CLK. This configuration may result in reduced system cost and is described in Section 3.7.4.2.

Note: Because the nINTSEL configuration strap shares functionality with the LED2 pin, proper
consideration must also be given to the LED polarity. Refer to Section 3.8.1.2, "nINTSEL and
LED2 Polarity Selection," on page 37 for additional information on the relation between
nINTSEL and the LED2 polarity. 

Table 3.6  nINTSEL Configuration

STRAP VALUE MODE REF_CLK DESCRIPTION

nINTSEL = 0 REF_CLK Out Mode nINT/REFCLKO is the source of REF_CLK. 

nINTSEL = 1 REF_CLK In Mode nINT/REFCLKO is an active low interrupt output. 
The REF_CLK is sourced externally and must be driven 
on the XTAL1/CLKIN pin.
SMSC LAN8720A/LAN8720Ai 33 Revision 1.4 (08-23-12)

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3.7.4.1 REF_CLK In Mode

In REF_CLK In Mode, the 50MHz REF_CLK is driven on the XTAL1/CLKIN pin. A 50MHz source for
REF_CLK must be available external to the device when using this mode. The clock is driven to both
the MAC and PHY as shown in Figure 3.7.

3.7.4.2 REF_CLK Out Mode

To reduce BOM cost, the device includes a feature to generate the RMII REF_CLK signal from a low-
cost, 25MHz fundamental crystal. This type of crystal is inexpensive in comparison to 3rd overtone
crystals that would normally be required for 50MHz. The MAC must be capable of operating with an
external clock to take advantage of this feature as shown in Figure 3.8. 

In order to optimize package size and cost, the REFCLKO pin is multiplexed with the nINT pin. In
REF_CLK Out mode, the nINT functionality is disabled to accommodate usage of REFCLKO as a
50MHz clock to the MAC.

Note: The REF_CLK Out Mode is not part of the RMII Specification. Timing in this mode is not
compliant with the RMII specification. To ensure proper system operation, a timing analysis of
the MAC and LAN8720 must be performed.

Figure 3.7 External 50MHz clock sources the REF_CLK

LAN8720A/LAN8720Ai
10/100 PHY

24-QFN
RMII

TXP

TXN

Mag RJ45

RXP

RXN

XTAL1/CLKIN

XTAL2

TXD[1:0]

2

RXD[1:0]

CRS_DV
2

RMII

LED[2:1]

2

Interface

MDIO
MDC
nINT

nRST

TXEN

MAC

Accepts external 
50MHz clock

50MHz 
Reference

Clock

All RMII signals are 
synchronous to the supplied 
clock

REF_CLK
RXER
Revision 1.4 (08-23-12) 34 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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In some system architectures, a 25MHz clock source is available. The device can be used to generate
the REF_CLK to the MAC as shown in Figure 3.9. It is important to note that in this specific example,
only a 25MHz clock can be used (clock cannot be 50MHz). Similar to the 25MHz crystal mode, the
nINT function is disabled.

Figure 3.8 Sourcing REF_CLK from a 25MHz Crystal

LAN8720A/LAN8720Ai
10/100 PHY

24-QFN
RMII

TXP

TXN

Mag RJ45

RXP

RXN

25MHz

XTAL1/CLKIN

XTAL2

LED[2:1]

2

Interface

nRST

TXD[1:0]

2

RXD[1:0]

CRS_DV
2

RMII MDIO
MDC

TXEN

REFCLKO

MAC

Capable of 
accepting 50MHz 

clock

Note: nINT not available in 
this configuration

RXERREF_CLK
SMSC LAN8720A/LAN8720Ai 35 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Figure 3.9 Sourcing REF_CLK from External 25MHz Source

LAN8720A/LAN8720Ai
10/100 PHY

24-QFN
RMII

TXP

TXN

Mag RJ45

RXP

RXN

XTAL1/CLKIN

XTAL2

TXD[1:0]

2

RXD[1:0]

CRS_DV
2

RMII

LED[2:1]

2

Interface

MDIO
MDC

nRST

TXEN

REFCLKO

MAC

Capable of 
accepting 50MHz 

clock

Note: nINT is not available in 
this configuration

RXER

25MHz
Clock

REF_CLK
Revision 1.4 (08-23-12) 36 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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3.8   Miscellaneous Functions

3.8.1 LEDs

Two LED signals are provided as a convenient means to determine the transceiver's mode of
operation. All LED signals are either active high or active low as described in Section 3.8.1.2,
"nINTSEL and LED2 Polarity Selection" and Section 3.8.1.1, "REGOFF and LED1 Polarity Selection,"
on page 37.

The LED1 output is driven active whenever the device detects a valid link, and blinks when CRS is
active (high) indicating activity.

The LED2 output is driven active when the operating speed is 100Mbps. This LED will go inactive
when the operating speed is 10Mbps or during line isolation.

Note: When pulling the LED1 and LED2 pins high, they must be tied to VDD2A, NOT VDDIO.

3.8.1.1 REGOFF and LED1 Polarity Selection

The REGOFF configuration strap is shared with the LED1 pin. The LED1 output will automatically
change polarity based on the presence of an external pull-up resistor. If the LED1 pin is pulled high to
VDD2A by an external pull-up resistor to select a logical high for REGOFF, then the LED1 output will
be active low. If the LED1 pin is pulled low by the internal pull-down resistor to select a logical low for
REGOFF, the LED1 output will then be an active high output. Figure 3.10 details the LED1 polarity for
each REGOFF configuration.

Note: Refer to Section 3.7.3, "REGOFF: Internal +1.2V Regulator Configuration," on page 32 for
additional information on the REGOFF configuration strap.

3.8.1.2 nINTSEL and LED2 Polarity Selection

The nINTSEL configuration strap is shared with the LED2 pin. The LED2 output will automatically
change polarity based on the presence of an external pull-down resistor. If the LED2 pin is pulled high
to VDD2A to select a logical high for nINTSEL, then the LED2 output will be active low. If the LED2

Figure 3.10 LED1/REGOFF Polarity Configuration

 

LED1/REGOFF

 ~270 ohms

REGOFF = 0 (Regulator ON)
LED output = Active High

~270 ohms

 
VDD2A

 REGOFF = 1 (Regulator OFF)
LED output = Active Low

LED1/REGOFF

10K 
SMSC LAN8720A/LAN8720Ai 37 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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pin is pulled low by an external pull-down resistor to select a logical low for nINTSEL, the LED2 output
will then be an active high output. Figure 3.11 details the LED2 polarity for each nINTSEL configuration. 

Note: Refer to Section 3.7.4, "nINTSEL: nINT/REFCLKO Configuration," on page 33 for additional
information on the nINTSEL configuration strap.

3.8.2 Variable Voltage I/O

The device’s digital I/O pins are variable voltage, allowing them to take advantage of low power savings
from shrinking technologies. These pins can operate from a low I/O voltage of +1.62V up to +3.6V.
The applied I/O voltage must maintain its value with a tolerance of ± 10%. Varying the voltage up or
down after the transceiver has completed power-on reset can cause errors in the transceiver operation.
Refer to Chapter 5, "Operational Characteristics," on page 63 for additional information.

Note: Input signals must not be driven high before power is applied to the device.

3.8.3 Power-Down Modes

There are two device power-down modes: General Power-Down Mode and Energy Detect Power-
Down Mode. These modes are described in the following subsections.

3.8.3.1 General Power-Down

This power-down mode is controlled via the Power Down bit of the Basic Control Register. In this
mode, the entire transceiver (except the management interface) is powered-down and remains in this
mode as long as the Power Down bit is “1”. When the Power Down bit is cleared, the transceiver
powers up and is automatically reset.

3.8.3.2 Energy Detect Power-Down

This power-down mode is activated by setting the EDPWRDOWN bit of the Mode Control/Status
Register. In this mode, when no energy is present on the line the transceiver is powered down (except
for the management interface, the SQUELCH circuit, and the ENERGYON logic). The ENERGYON
logic is used to detect the presence of valid energy from 100BASE-TX, 10BASE-T, or Auto-negotiation
signals.

In this mode, when the ENERGYON bit of the Mode Control/Status Register is low, the transceiver is
powered-down and nothing is transmitted. When energy is received via link pulses or packets, the
ENERGYON bit goes high and the transceiver powers-up. The device automatically resets into the

Figure 3.11 LED2/nINTSEL Polarity Configuration

 

 ~270 ohms

nINTSEL = 0
LED output = Active High

10K 

~270 ohms

 

VDD2A

 nINTSEL = 1
LED output = Active Low

LED2/nINTSEL

LED2/nINTSEL
Revision 1.4 (08-23-12) 38 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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state prior to power-down and asserts the nINT interrupt if the ENERGYON interrupt is enabled in the
Interrupt Mask Register. The first and possibly the second packet to activate ENERGYON may be lost. 

When the EDPWRDOWN bit of the Mode Control/Status Register is low, energy detect power-down is
disabled.

3.8.4 Isolate Mode

The device data paths may be electrically isolated from the RMII interface by setting the Isolate bit of
the Basic Control Register to “1”. In isolation mode, the transceiver does not respond to the TXD,
TXEN and TXER inputs, but does respond to management transactions.

Isolation provides a means for multiple transceivers to be connected to the same RMII interface without
contention. By default, the transceiver is not isolated (on power-up (Isolate=0).

3.8.5 Resets

The device provides two forms of reset: Hardware and Software. The device registers are reset by
both Hardware and Software resets. Select register bits, indicated as “NASR” in the register definitions,
are not cleared by a Software reset. The registers are not reset by the power-down modes described
in Section 3.8.3.

Note: For the first 16us after coming out of reset, the RMII interface will run at 2.5 MHz. After this
time, it will switch to 25 MHz if auto-negotiation is enabled.

3.8.5.1 Hardware Reset

A Hardware reset is asserted by driving the nRST input pin low. When driven, nRST should be held
low for the minimum time detailed in Section 5.5.3, "Power-On nRST & Configuration Strap Timing,"
on page 70 to ensure a proper transceiver reset. During a Hardware reset, an external clock must be
supplied to the XTAL1/CLKIN signal.

Note: A hardware reset (nRST assertion) is required following power-up. Refer to Section 5.5.3,
"Power-On nRST & Configuration Strap Timing," on page 70 for additional information.

3.8.5.2 Software Reset

A Software reset is activated by setting the Soft Reset bit of the Basic Control Register to “1”. All
registers bits, except those indicated as “NASR” in the register definitions, are cleared by a Software
reset. The Soft Reset bit is self-clearing. Per the IEEE 802.3u standard, clause 22 (22.2.4.1.1) the reset
process will be completed within 0.5s from the setting of this bit.

3.8.6 Carrier Sense 

The carrier sense (CRS) is output on the CRS_DV pin. CRS is a signal defined by the MII specification
in the IEEE 802.3u standard. The device asserts CRS based only on receive activity whenever the
transceiver is either in repeater mode or full-duplex mode. Otherwise the transceiver asserts CRS
based on either transmit or receive activity.

The carrier sense logic uses the encoded, unscrambled data to determine carrier activity status. It
activates carrier sense with the detection of 2 non-contiguous zeros within any 10 bit span. Carrier
sense terminates if a span of 10 consecutive ones is detected before a /J/K/ Start-of Stream Delimiter
pair. If an SSD pair is detected, carrier sense is asserted until either /T/R/ End–of-Stream Delimiter
pair or a pair of IDLE symbols is detected. Carrier is negated after the /T/ symbol or the first IDLE. If
/T/ is not followed by /R/, then carrier is maintained. Carrier is treated similarly for IDLE followed by
some non-IDLE symbol.
SMSC LAN8720A/LAN8720Ai 39 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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3.8.7 Link Integrity Test

The device performs the link integrity test as outlined in the IEEE 802.3u (Clause 24-15) Link Monitor
state diagram. The link status is multiplexed with the 10Mbps link status to form the Link Status bit in
the Basic Status Register and to drive the LINK LED (LED1).

The DSP indicates a valid MLT-3 waveform present on the RXP and RXN signals as defined by the
ANSI X3.263 TP-PMD standard, to the Link Monitor state-machine, using the internal DATA_VALID
signal. When DATA_VALID is asserted, the control logic moves into a Link-Ready state and waits for
an enable from the auto-negotiation block. When received, the Link-Up state is entered, and the
Transmit and Receive logic blocks become active. Should auto-negotiation be disabled, the link
integrity logic moves immediately to the Link-Up state when the DATA_VALID is asserted.

To allow the line to stabilize, the link integrity logic will wait a minimum of 330 μsec from the time
DATA_VALID is asserted until the Link-Ready state is entered. Should the DATA_VALID input be
negated at any time, this logic will immediately negate the Link signal and enter the Link-Down state.

When the 10/100 digital block is in 10BASE-T mode, the link status is derived from the 10BASE-T
receiver logic.

3.8.8 Loopback Operation

The device may be configured for near-end loopback and far loopback. These loopback modes are
detailed in the following subsections.

3.8.8.1 Near-end Loopback

Near-end loopback mode sends the digital transmit data back out the receive data signals for testing
purposes, as indicated by the blue arrows in Figure 3.12. The near-end loopback mode is enabled by
setting the Loopback bit of the Basic Control Register to “1”. A large percentage of the digital circuitry
is operational in near-end loopback mode because data is routed through the PCS and PMA layers
into the PMD sublayer before it is looped back. The transmitters are powered down regardless of the
state of TXEN.

3.8.8.2 Far Loopback

Far loopback is a special test mode for MDI (analog) loopback as indicated by the blue arrows in
Figure 3.14. The far loopback mode is enabled by setting the FARLOOPBACK bit of the Mode
Control/Status Register to “1”. In this mode, data that is received from the link partner on the MDI is
looped back out to the link partner. The digital interface signals on the local MAC interface are isolated.

Figure 3.12 Near-end Loopback Block Diagram

SMSC
Ethernet Transceiver

10/100
Ethernet

MAC

CAT-5XFMR

Digital
RXD

TXD

Analog
RX

TXX

X

Revision 1.4 (08-23-12) 40 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.8.8.3 Connector Loopback

The device maintains reliable transmission over very short cables, and can be tested in a connector
loopback as shown in Figure 3.14. An RJ45 loopback cable can be used to route the transmit signals
an the output of the transformer back to the receiver inputs, and this loopback will work at both 10 and
100.

3.9   Application Diagrams
This section provides typical application diagrams for the following:

Simplified System Level Application Diagram

Power Supply Diagram (1.2V Supplied by Internal Regulator)

Power Supply Diagram (1.2V Supplied by External Source)

Twisted-Pair Interface Diagram (Single Power Supply)

Twisted-Pair Interface Diagram (Dual Power Supplies)

Figure 3.13 Far Loopback Block Diagram

Figure 3.14 Connector Loopback Block Diagram

Far-end system

SMSC
Ethernet Transceiver

10/100
Ethernet

MAC

CAT-5XFMR

Digital
RXD

TXD

Analog
RX

TX
Link 

Partner
X

X

SMSC
Ethernet Transceiver

10/100
Ethernet

MAC
XFMR

Digital
RXD

TXD

Analog
RX

TX
1
2
3
4
5
6
7
8

RJ45 Loopback Cable.
Created by connecting pin 1 to pin 3 
and connecting pin 2 to pin 6.
SMSC LAN8720A/LAN8720Ai 41 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.9.1 Simplified System Level Application Diagram

Figure 3.15 Simplified System Level Application Diagram

LAN8720A/LAN8720Ai
10/100 PHY

24-QFN
RMII

TXP

TXN

Mag RJ45

RXP

RXN

25MHz

XTAL1/CLKIN

XTAL2

TXD[1:0]

2

RXD[1:0]

RXER
2

RMII

LED[2:1]

2

Interface

MDIO
MDC
nINT

nRST

TXEN
Revision 1.4 (08-23-12) 42 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.9.2 Power Supply Diagram (1.2V Supplied by Internal Regulator)

Figure 3.16 Power Supply Diagram (1.2V Supplied by Internal Regulator)

LAN8720A/LAN8720Ai
24-QFN

RBIAS

VSS 12.1k

VDD2A

CBYPASS

CBYPASS

VDD1AVDDIO

CBYPASSCF

VDDDIO
Supply

1.8 - 3.3V

Power
Supply

3.3V

VDDCR

 

 

LED1/
REGOFF

~270 Ohm

Core Logic

Internal 
Regulator

Ch.2 3.3V 
Circuitry

INOUT

Ch.1 3.3V 
Circuitry

470 pF1 uF
SMSC LAN8720A/LAN8720Ai 43 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.9.3 Power Supply Diagram (1.2V Supplied by External Source)

Figure 3.17 Power Supply Diagram (1.2V Supplied by External Source)

LAN8720A/LAN8720Ai
24-QFN

RBIAS

VSS 12.1k

VDD2A

CBYPASS

CBYPASS

VDD1AVDDIO

CBYPASSCF

VDDDIO
Supply

1.8 - 3.3V

Power
Supply

3.3V

VDDCR

LED1/
REGOFF

Core Logic

Internal 
Regulator

(Disabled)

Ch.2 3.3V 
Circuitry

INOUT

Ch.1 3.3V 
Circuitry

VDDCR
Supply

1.2V

 
~270 Ohm

10k

470 pF1 uF
Revision 1.4 (08-23-12) 44 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.9.4 Twisted-Pair Interface Diagram (Single Power Supply)

Figure 3.18 Twisted-Pair Interface Diagram (Single Power Supply)

Magnetics

LAN8720A/LAN8720Ai
24-QFN

VDD2A
CBYPASS

TXP

TXN

Power 
Supply

3.3V

1
2
3
4
5
6
7
8

1000 pF
3 kV

RJ45

75

75
RXP

RXN

CBYPASS

VDD1A
CBYPASS

49.9 Ohm Resistors

Ferrite 
bead
SMSC LAN8720A/LAN8720Ai 45 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
3.9.5 Twisted-Pair Interface Diagram (Dual Power Supplies)

Figure 3.19 Twisted-Pair Interface Diagram (Dual Power Supplies)

Magnetics

LAN8720A/LAN8720Ai
24-QFN

VDD2A
CBYPASS

TXP

TXN

Power
Supply

3.3V

1
2
3
4
5
6
7
8

1000 pF
3 kV

RJ45

75

75
RXP

RXN

CBYPASS

VDD1A
CBYPASS

49.9 Ohm Resistors Power
Supply

2.5V - 3.3V
Revision 1.4 (08-23-12) 46 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 4 Register Descriptions

This chapter describes the various control and status registers (CSR’s). All registers follow the IEEE
802.3 (clause 22.2.4) management register set. All functionality and bit definitions comply with these
standards. The IEEE 802.3 specified register index (in decimal) is included with each register definition,
allowing for addressing of these registers via the Serial Management Interface (SMI) protocol.

4.1   Register Nomenclature
Table 4.1 describes the register bit attribute notation used throughout this document.

Many of these register bit notations can be combined. Some examples of this are shown below:

R/W: Can be written. Will return current setting on a read.

R/WAC: Will return current setting on a read. Writing anything clears the bit.

Table 4.1  Register Bit Types

REGISTER BIT TYPE 
NOTATION REGISTER BIT DESCRIPTION

R Read: A register or bit with this attribute can be read.

W Read: A register or bit with this attribute can be written.

RO Read only: Read only. Writes have no effect.

WO Write only: If a register or bit is write-only, reads will return unspecified data.

WC Write One to Clear: writing a one clears the value. Writing a zero has no effect

WAC Write Anything to Clear: writing anything clears the value.

RC Read to Clear: Contents is cleared after the read. Writes have no effect.

LL Latch Low: Clear on read of register.

LH Latch High: Clear on read of register.

SC Self-Clearing: Contents are self-cleared after the being set. Writes of zero have no 
effect. Contents can be read.

SS Self-Setting: Contents are self-setting after being cleared. Writes of one have no 
effect. Contents can be read.

RO/LH Read Only, Latch High: Bits with this attribute will stay high until the bit is read. After 
it is read, the bit will either remain high if the high condition remains, or will go low if 
the high condition has been removed. If the bit has not been read, the bit will remain 
high regardless of a change to the high condition. This mode is used in some Ethernet 
PHY registers.

NASR Not Affected by Software Reset. The state of NASR bits do not change on assertion 
of a software reset.

RESERVED Reserved Field: Reserved fields must be written with zeros to ensure future 
compatibility. The value of reserved bits is not guaranteed on a read.
SMSC LAN8720A/LAN8720Ai 47 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2   Control and Status Registers
Table 4.2 provides a list of supported registers. Register details, including bit definitions, are provided
in the proceeding subsections.

Table 4.2  SMI Register Map

REGISTER INDEX
(DECIMAL) REGISTER NAME GROUP

0 Basic Control Register Basic

1 Basic Status Register Basic

2 PHY Identifier 1 Extended

3 PHY Identifier 2 Extended

4 Auto-Negotiation Advertisement Register Extended

5 Auto-Negotiation Link Partner Ability Register Extended

6 Auto-Negotiation Expansion Register Extended

17 Mode Control/Status Register Vendor-specific

18 Special Modes Vendor-specific

26 Symbol Error Counter Register Vendor-specific

27 Control / Status Indication Register Vendor-specific

29 Interrupt Source Register Vendor-specific

30 Interrupt Mask Register Vendor-specific

31 PHY Special Control/Status Register Vendor-specific
Revision 1.4 (08-23-12) 48 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.1 Basic Control Register

Note 4.1 The default value of this bit is determined by the MODE[2:0] configuration straps. Refer to
Section 3.7.2, "MODE[2:0]: Mode Configuration," on page 31 for additional information.

Index (In Decimal): 0 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15 Soft Reset
1 = software reset. Bit is self-clearing. When setting this bit do not set other 
bits in this register. The configuration (as described in Section 3.7.2, 
"MODE[2:0]: Mode Configuration," on page 31) is set from the register bit 
values, and not from the mode pins.

R/W
SC

0b

14 Loopback
0 = normal operation
1 = loopback mode

R/W 0b

13 Speed Select
0 = 10Mbps
1 = 100Mbps
Note: Ignored if Auto-negotiation is enabled (0.12 = 1).

R/W Note 4.1

12 Auto-Negotiation Enable
0 = disable auto-negotiate process
1 = enable auto-negotiate process (overrides 0.13 and 0.8)

R/W Note 4.1

11 Power Down
0 = normal operation 
1 = General power down mode
Note: The Auto-Negotiation Enable must be cleared before setting the 

Power Down.

R/W 0b

10 Isolate
0 = normal operation
1 = electrical isolation of PHY from the RMII

R/W 0b

9 Restart Auto-Negotiate
0 = normal operation
1 = restart auto-negotiate process
Note: Bit is self-clearing.

R/W
SC

0b

8 Duplex Mode
0 = half duplex
1 = full duplex
Note: Ignored if Auto-Negotiation is enabled (0.12 = 1).

R/W Note 4.1

7:0 RESERVED RO -
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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.2 Basic Status Register

Index (In Decimal): 1 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15 100BASE-T4
0 = no T4 ability
1 = T4 able

RO 0b

14 100BASE-TX Full Duplex
0 = no TX full duplex ability
1 = TX with full duplex

RO 1b

13 100BASE-TX Half Duplex
0 = no TX half duplex ability
1 = TX with half duplex

RO 1b

12 10BASE-T Full Duplex
0 = no 10Mbps with full duplex ability
1 = 10Mbps with full duplex

RO 1b

11 10BASE-T Half Duplex
0 = no 10Mbps with half duplex ability
1 = 10Mbps with half duplex

RO 1b

10 100BASE-T2 Full Duplex
0 = PHY not able to perform full duplex 100BASE-T2
1 = PHY able to perform full duplex 100BASE-T2

RO 0b

9 100BASE-T2 Half Duplex
0 = PHY not able to perform half duplex 100BASE-T2
1 = PHY able to perform half duplex 100BASE-T2

RO 0b

8 Extended Status
0 = no extended status information in register 15
1 = extended status information in register 15

RO 0b

7:6 RESERVED RO -

5 Auto-Negotiate Complete
0 = auto-negotiate process not completed
1 = auto-negotiate process completed

RO 0b

4 Remote Fault
1 = remote fault condition detected
0 = no remote fault

RO/LH 0b

3 Auto-Negotiate Ability
0 = unable to perform auto-negotiation function
1 = able to perform auto-negotiation function

RO 1b

2 Link Status
0 = link is down
1 = link is up

RO/LL 0b

1 Jabber Detect
0 = no jabber condition detected
1 = jabber condition detected

RO/LH 0b

0 Extended Capabilities
0 = does not support extended capabilities registers
1 = supports extended capabilities registers

RO 1b
Revision 1.4 (08-23-12) 50 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.3 PHY Identifier 1 Register

Index (In Decimal): 2 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:0 PHY ID Number
Assigned to the 3rd through 18th bits of the Organizationally Unique 
Identifier (OUI), respectively.

R/W 0007h
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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.4 PHY Identifier 2 Register

Note 4.2 The default value of this field will vary dependant on the silicon revision number. 

Index (In Decimal): 3 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:10 PHY ID Number
Assigned to the 19th through 24th bits of the OUI.

R/W 110000b

9:4 Model Number
Six-bit manufacturer’s model number. 

R/W 001111b

3:0 Revision Number
Four-bit manufacturer’s revision number. 

R/W Note 4.2
Revision 1.4 (08-23-12) 52 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.5 Auto Negotiation Advertisement Register

Note 4.3 The default value of this bit is determined by the MODE[2:0] configuration straps. Refer to
Section 3.7.2, "MODE[2:0]: Mode Configuration," on page 31 for additional information.

Index (In Decimal): 4 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:14 RESERVED RO -

13 Remote Fault
0 = no remote fault
1 = remote fault detected

R/W 0b

12 RESERVED RO -

11:10 Pause Operation
00 = No PAUSE 
01 = Symmetric PAUSE
10 = Asymmetric PAUSE toward link partner
11 = Advertise support for both Symmetric PAUSE and Asymmetric PAUSE 
toward local device
Note: When both Symmetric PAUSE and Asymmetric PAUSE are set, the 

device will only be configured to, at most, one of the two settings 
upon auto-negotiation completion.

R/W 00b

9 RESERVED RO -

8 100BASE-TX Full Duplex
0 = no TX full duplex ability
1 = TX with full duplex

R/W Note 4.3

7 100BASE-TX
0 = no TX ability
1 = TX able

R/W 1b

6 10BASE-T Full Duplex
0 = no 10Mbps with full duplex ability
1 = 10Mbps with full duplex

R/W Note 4.3

5 10BASE-T
0 = no 10Mbps ability
1 = 10Mbps able

R/W Note 4.3

4:0 Selector Field
00001 = IEEE 802.3 

R/W 00001b
SMSC LAN8720A/LAN8720Ai 53 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.6 Auto Negotiation Link Partner Ability Register

Index (In Decimal): 5 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15 Next Page
0 = no next page ability
1 = next page capable
Note: This device does not support next page ability.

RO 0b

14 Acknowledge
0 = link code word not yet received
1 = link code word received from partner

RO 0b

13 Remote Fault
0 = no remote fault
1 = remote fault detected

RO 0b

12:11 RESERVED RO -

10 Pause Operation
0 = No PAUSE supported by partner station
1 = PAUSE supported by partner station

RO 0b

9 100BASE-T4
0 = no T4 ability
1 = T4 able
Note: This device does not support T4 ability.

RO 0b

8 100BASE-TX Full Duplex
0 = no TX full duplex ability
1 = TX with full duplex

RO 0b

7 100BASE-TX
0 = no TX ability
1 = TX able

RO 0b

6 10BASE-T Full Duplex
0 = no 10Mbps with full duplex ability
1 = 10Mbps with full duplex

RO 0b

5 10BASE-T
0 = no 10Mbps ability
1 = 10Mbps able

RO 0b

4:0 Selector Field
00001 = IEEE 802.3 

RO 00001b
Revision 1.4 (08-23-12) 54 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.7 Auto Negotiation Expansion Register

Index (In Decimal): 6 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:5 RESERVED RO -

4 Parallel Detection Fault
0 = no fault detected by parallel detection logic
1 = fault detected by parallel detection logic

RO/LH 0b

3 Link Partner Next Page Able
0 = link partner does not have next page ability
1 = link partner has next page ability

RO 0b

2 Next Page Able
0 = local device does not have next page ability
1 = local device has next page ability

RO 0b

1 Page Received
0 = new page not yet received
1 = new page received

RO/LH 0b

0 Link Partner Auto-Negotiation Able
0 = link partner does not have auto-negotiation ability
1 = link partner has auto-negotiation ability

RO 0b
SMSC LAN8720A/LAN8720Ai 55 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.8 Mode Control/Status Register

Index (In Decimal): 17 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:14 RESERVED RO -

13 EDPWRDOWN
Enable the Energy Detect Power-Down mode:
0 = Energy Detect Power-Down is disabled
1 = Energy Detect Power-Down is enabled

R/W 0b

12:10 RESERVED RO -

9 FARLOOPBACK
Enables far loopback mode (i.e., all the received packets are sent back 
simultaneously (in 100BASE-TX only)). This mode works even if the Isolate 
bit (0.10) is set. 

0 = Far loopback mode is disabled
1 = Far loopback mode is enabled

Refer to Section 3.8.8.2, "Far Loopback," on page 40 for additional 
information. 

R/W 0b

8:7 RESERVED RO -

6 ALTINT
Alternate Interrupt Mode:
0 = Primary interrupt system enabled (Default)
1 = Alternate interrupt system enabled
Refer to Section 3.6, "Interrupt Management," on page 29 for additional 
information.

R/W 0b

5:2 RESERVED RO -

1 ENERGYON
Indicates whether energy is detected. This bit transitions to “0” if no valid 
energy is detected within 256ms. It is reset to “1” by a hardware reset and 
is unaffected by a software reset. Refer to Section 3.8.3.2, "Energy Detect 
Power-Down," on page 38 for additional information.

RO 1b

0 RESERVED R/W 0b
Revision 1.4 (08-23-12) 56 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.9 Special Modes Register

Note 4.4 The default value of this field is determined by the MODE[2:0] configuration straps. Refer
to Section 3.7.2, "MODE[2:0]: Mode Configuration," on page 31 for additional information.

Note 4.5 The default value of this field is determined by the PHYAD[0] configuration strap. Refer to
Section 3.7.1, "PHYAD[0]: PHY Address Configuration," on page 31 for additional
information.

Index (In Decimal): 18 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15 RESERVED RO -

14 RESERVED 
Write as 1, ignore on read.

R/W
NASR

1b

13:8 RESERVED RO -

7:5 MODE
Transceiver mode of operation. Refer to Section 3.7.2, "MODE[2:0]: Mode 
Configuration," on page 31 for additional details.

R/W
NASR

Note 4.4

4:0 PHYAD
PHY Address. The PHY Address is used for the SMI address and for 
initialization of the Cipher (Scrambler) key. Refer to Section 3.7.1, 
"PHYAD[0]: PHY Address Configuration," on page 31 for additional details.

R/W
NASR

Note 4.5
SMSC LAN8720A/LAN8720Ai 57 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.10 Symbol Error Counter Register

Index (In Decimal): 26 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:0 SYM_ERR_CNT
The symbol error counter increments whenever an invalid code symbol is 
received (including IDLE symbols) in 100BASE-TX mode. The counter is 
incremented only once per packet, even when the received packet contains 
more than one symbol error. This counter increments up to 65,536 (216) and 
rolls over to 0 after reaching the maximum value. 
Note: This register is cleared on reset, but is not cleared by reading the 

register. This register does not increment in 10BASE-T mode.

RO 0000h
Revision 1.4 (08-23-12) 58 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.11 Special Control/Status Indications Register

Index (In Decimal): 27 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15 AMDIXCTRL
HP Auto-MDIX control:
0 = Enable Auto-MDIX
1 = Disable Auto-MDIX (use 27.13 to control channel)

R/W 0b

14 RESERVED RO -

13 CH_SELECT
Manual channel select:
0 = MDI (TX transmits, RX receives)
1 = MDIX (TX receives, RX transmits)

R/W 0b

12 RESERVED RO -

11 SQEOFF
Disable the SQE test (Heartbeat):
0 = SQE test is enabled
1 = SQE test is disabled

R/W
NASR

0b

10:5 RESERVED RO -

4 XPOL
Polarity state of the 10BASE-T:
0 = Normal polarity
1 = Reversed polarity

RO 0b

3:0 RESERVED RO -
SMSC LAN8720A/LAN8720Ai 59 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.12 Interrupt Source Flag Register

Index (In Decimal): 29 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:8 RESERVED RO -

7 INT7
0 = not source of interrupt
1 = ENERGYON generated

RO/LH 0b

6 INT6
0 = not source of interrupt
1 = Auto-Negotiation complete

RO/LH 0b

5 INT5
0 = not source of interrupt
1 = Remote Fault Detected

RO/LH 0b

4 INT4
0 = not source of interrupt
1 = Link Down (link status negated)

RO/LH 0b

3 INT3
0 = not source of interrupt
1 = Auto-Negotiation LP Acknowledge

RO/LH 0b

2 INT2
0 = not source of interrupt
1 = Parallel Detection Fault

RO/LH 0b

1 INT1
0 = not source of interrupt
1 = Auto-Negotiation Page Received

RO/LH 0b

0 RESERVED RO 0b
Revision 1.4 (08-23-12) 60 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.13 Interrupt Mask Register

Index (In Decimal): 30 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:8 RESERVED RO -

7:1 Mask Bits
0 = interrupt source is masked
1 = interrupt source is enabled
Note: Refer to Section 4.2.12, "Interrupt Source Flag Register," on 

page 60 for details on the corresponding interrupt definitions.

R/W 0000000b

0 RESERVED RO -
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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
4.2.14 PHY Special Control/Status Register

Index (In Decimal): 31 Size: 16 bits

BITS DESCRIPTION TYPE DEFAULT

15:13 RESERVED RO -

12 Autodone
Auto-negotiation done indication:
0 = Auto-negotiation is not done or disabled (or not active)
1 = Auto-negotiation is done

RO 0b

11:5 RESERVED - Write as 0000010b, ignore on read. R/W 0000010b

4:2 Speed Indication
HCDSPEED value:
001 = 10BASE-T half-duplex
101 = 10BASE-T full-duplex
010 = 100BASE-TX half-duplex
110 = 100BASE-TX full-duplex

RO XXX

1:0 RESERVED RO -
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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 5 Operational Characteristics

5.1   Absolute Maximum Ratings*
Supply Voltage (VDDIO, VDD1A, VDD2A) (Note 5.1) . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to +3.6V

Digital Core Supply Voltage (VDDCR) (Note 5.1)  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to +1.5V

Ethernet Magnetics Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.5V to +3.6V

Positive voltage on signal pins, with respect to ground (Note 5.2) . . . . . . . . . . . . . . . . . . . . . . . . . . +6V 

Negative voltage on signal pins, with respect to ground (Note 5.3) . . . . . . . . . . . . . . . . . . . . . . . . -0.5V

Positive voltage on XTAL1/CLKIN, with respect to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +4.6V

Positive voltage on XTAL2, with respect to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +2.5V

Ambient Operating Temperature in Still Air (TA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Note 5.40

Storage Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-55oC to +150oC

Junction to Ambient (θJA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59.8oC/W

Junction to Case (θJC)  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12.6oC/W

Lead Temperature Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Refer to JEDEC Spec. J-STD-020

HBM ESD Performance per JEDEC JESD22-A114. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Class 3A

IEC61000-4-2 Contact Discharge ESD Performance (Note 5.5) . . . . . . . . . . . . . . . . . . . . . . . . . .+/-8kV

IEC61000-4-2 Air-Gap Discharge ESD Performance (Note 5.5) . . . . . . . . . . . . . . . . . . . . . . . . .+/-15kV

Latch-up Performance per EIA/JESD 78 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+/-150mA

Note 5.1 When powering this device from laboratory or system power supplies, it is important that
the absolute maximum ratings not be exceeded or device failure can result. Some power
supplies exhibit voltage spikes on their outputs when AC power is switched on or off. In
addition, voltage transients on the AC power line may appear on the DC output. If this
possibility exists, it is suggested that a clamp circuit be used.

Note 5.2 This rating does not apply to the following pins: XTAL1/CLKIN, XTAL2, RBIAS.

Note 5.3 This rating does not apply to the following pins: RBIAS.

Note 5.4 0oC to +85oC for extended commercial version, -40oC to +85oC for industrial version.

Note 5.5 Performed by independent 3rd party test facility.

*Stresses exceeding those listed in this section could cause permanent damage to the device. This is
a stress rating only. Exposure to absolute maximum rating conditions for extended periods may affect
device reliability. Functional operation of the device at any condition exceeding those indicated in
Section 5.2, "Operating Conditions**", Section 5.1, "Absolute Maximum Ratings*", or any other
applicable section of this specification is not implied. Note, device signals are NOT 5 volt tolerant
unless specified otherwise.
SMSC LAN8720A/LAN8720Ai 63 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
5.2   Operating Conditions**
Supply Voltage (VDDIO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +1.62V to +3.6V

Analog Port Supply Voltage (VDD1A, VDD2A)  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +3.0V to +3.6V

Digital Core Supply Voltage (VDDCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +1.14V to +1.26V

Ethernet Magnetics Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +2.25V to +3.6V

Ambient Operating Temperature in Still Air (TA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Note 5.4

**Proper operation of the device is guaranteed only within the ranges specified in this section. After
the device has completed power-up, VDDIO and the magnetics power supply must maintain their
voltage level with +/-10%. Varying the voltage greater than +/-10% after the device has completed
power-up can cause errors in device operation.

Note: Do not drive input signals without power supplied to the device.

5.3   Power Consumption
This section details the device power measurements taken over various operating conditions. Unless
otherwise noted, all measurements were taken with power supplies at nominal values (VDDIO, VDD1A,
VDD2A = 3.3V, VDDCR = 1.2V). See Section 3.8.3, "Power-Down Modes," on page 38 for a
description of the power down modes. For more information on the REF_CLK modes, see Section
3.7.4, "nINTSEL: nINT/REFCLKO Configuration," on page 33.

5.3.1 REF_CLK In Mode

Table 5.1  Device Only Current Consumption and Power Dissipation (REF_CLK In Mode)

POWER PIN GROUP

VDDA3.3 
POWER 

PINS(mA)

VDDCR
POWER
PIN(mA)

VDDIO 
POWER 
PIN(mA)

TOTAL 
CURRENT 

(mA)

TOTAL 
POWER 

(mW)

100BASE-TX /W TRAFFIC

Max 28 21 0.6 49 159

Typical 26 19 0.5 45 148

Min 23 18 0.3 41 96
Note 5.6

10BASE-T /W TRAFFIC

Max 9.7 13 0.6 24 77

Typical 8.9 12 0.5 22 70

Min 8.3 12 0.3 20 42
Note 5.6

ENERGY DETECT POWER 
DOWN

Max 4.2 3.0 0.2 7.4 25

Typical 4.1 1.9 0.2 6.2 21

Min 3.9 1.9 0 5.8 16
Note 5.6

GENERAL POWER DOWN

Max 0.4 2.8 0.2 3.4 11.2

Typical 0.3 1.8 0.2 2.3 7.6

Min 0.3 1.7 0 2 3.0
Note 5.6
Revision 1.4 (08-23-12) 64 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Note: The current at VDDCR is either supplied by the internal regulator from current entering at
VDD2A, or from an external 1.2V supply when the internal regulator is disabled.

Note: Current measurements do not include power applied to the magnetics or the optional external
LEDs. The Ethernet component current is typically 41mA in 100BASE-TX mode and 100mA in
10BASE-T mode, independent of the 2.5V or 3.3V supply rail of the transformer.

Note 5.6 Calculated with full flexPWR features activated: VDDIO=1.8V & internal regulator disabled.

5.3.2 REF_CLK Out Mode
.

Note: The current at VDDCR is either supplied by the internal regulator from current entering at
VDD2A, or from an external 1.2V supply when the internal regulator is disabled.

Note: Current measurements do not include power applied to the magnetics or the optional external
LEDs. The Ethernet component current is typically 41mA in 100BASE-TX mode and 100mA in
10BASE-T mode, independent of the 2.5V or 3.3V supply rail of the transformer.

Note 5.7 Calculated with full flexPWR features activated: VDDIO=1.8V & internal regulator disabled.

Table 5.2  Device Only Current Consumption and Power Dissipation (REF_CLK Out Mode)

POWER PIN GROUP

VDDA3.3 
POWER 

PINS(MA)

VDDCR
POWER
PIN(MA)

VDDIO 
POWER 
PIN(MA)

TOTAL 
CURRENT 

(MA)

TOTAL 
POWER 

(MW)

100BASE-T /W TRAFFIC

Max 28 20 6.3 54 179

Typical 26 19 5.8 50 164

Min 22 15 2.9 39 93
Note 5.7

10BASE-T /W TRAFFIC

Max 9.9 13 6.4 30 96

Typical 8.8 12 5.6 26 85

Min 7.1 10 3.0 20 41
Note 5.7

ENERGY DETECT POWER 
DOWN

Max 4.5 2.7 0.3 7.5 25

Typical 4.0 1.5 0.2 5.7 19

Min 3.9 1.2 0 5.1 15
Note 5.7

GENERAL POWER DOWN

Max 0.4 2.5 0.2 3.1 10.2

Typical 0.4 1.3 0.2 1.9 6.3

Min 0.4 1.0 0 1.4 2.5
Note 5.7
SMSC LAN8720A/LAN8720Ai 65 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
5.4   DC Specifications
Table 5.3 details the non-variable I/O buffer characteristics. These buffer types do not support variable
voltage operation. Table 5.4 details the variable voltage I/O buffer characteristics. Typical values are
provided for 1.8V, 2.5V, and 3.3V VDDIO cases.

Note 5.8 This specification applies to all inputs and tri-stated bi-directional pins. Internal pull-down
and pull-up resistors add +/- 50uA per-pin (typical).

Note 5.9 XTAL1/CLKIN can optionally be driven from a 25MHz single-ended clock oscillator.

Table 5.3 Non-Variable I/O Buffer Characteristics

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

IS Type Input Buffer

Low Input Level

High Input Level

Negative-Going Threshold

Positive-Going Threshold

Schmitt Trigger Hysteresis 
(VIHT - VILT)

Input Leakage
(VIN = VSS or VDDIO)

Input Capacitance

VILI

VIHI

VILT

VIHT

VHYS

IIH

CIN

-0.3

1.01

1.39

336

-10

1.19

1.59

399

3.6

1.39

1.79

459

10

2

V

V

V

V

mV

uA

pF

Schmitt trigger

Schmitt trigger

Note 5.8

O12 Type Buffers

Low Output Level

High Output Level

VOL

VOH VDD2A - 0.4

0.4 V

V

IOL = 12mA

IOH = -12mA
ICLK Type Buffer 
(XTAL1 Input)

Low Input Level

High Input Level

VILI

VIHI

-0.3

1.4

0.35

VDD2A + 0.4

V

V

Note 5.9
Revision 1.4 (08-23-12) 66 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Note 5.10 This specification applies to all inputs and tri-stated bi-directional pins. Internal pull-down
and pull-up resistors add +/- 50uA per-pin (typical).

Note 5.11 Measured at line side of transformer, line replaced by 100Ω (+/- 1%) resistor.

Note 5.12 Offset from 16nS pulse width at 50% of pulse peak.

Note 5.13 Measured differentially.

Table 5.4 Variable I/O Buffer Characteristics

PARAMETER SYMBOL MIN
1.8V 
TYP

2.5V 
TYP

3.3V
TYP MAX UNITS NOTES

VIS Type Input Buffer

Low Input Level

High Input Level

Neg-Going Threshold

Pos-Going Threshold

Schmitt Trigger 
Hysteresis (VIHT - VILT)

Input Leakage
(VIN = VSS or VDDIO)

Input Capacitance

VILI

VIHI

VILT

VIHT

VHYS

IIH

CIN

-0.3

0.64

0.81

102

-10

0.83

0.99

158

1.15

1.29

136

1.41

1.65

138

3.6

1.76

1.90

288

10

2

V

V

V

V

mV

uA

pF

Schmitt trigger

Schmitt trigger

Note 5.10

VO8 Type Buffers

Low Output Level

High Output Level

VOL

VOH VDDIO - 0.4

0.4 V

V

IOL = 8mA

IOH = -8mA
VOD8 Type Buffer

Low Output Level VOL 0.4 V IOL = 8mA

Table 5.5 100BASE-TX Transceiver Characteristics

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Peak Differential Output Voltage High VPPH 950 - 1050 mVpk Note 5.11

Peak Differential Output Voltage Low VPPL -950 - -1050 mVpk Note 5.11

Signal Amplitude Symmetry VSS 98 - 102 % Note 5.11

Signal Rise and Fall Time TRF 3.0 - 5.0 nS Note 5.11

Rise and Fall Symmetry TRFS - - 0.5 nS Note 5.11

Duty Cycle Distortion DCD 35 50 65 % Note 5.12

Overshoot and Undershoot VOS - - 5 %

Jitter 1.4 nS Note 5.13
SMSC LAN8720A/LAN8720Ai 67 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Note 5.14 Min/max voltages guaranteed as measured with 100Ω resistive load.

5.5   AC Specifications
This section details the various AC timing specifications of the device. 

Note: The SMI timing adheres to the IEEE 802.3 specification. Refer to the IEEE 802.3 specification
for additional timing information.

Note: The RMII timing adheres to the RMII Consortium RMII Specification R1.2.

5.5.1 Equivalent Test Load

Output timing specifications assume a 25pF equivalent test load, unless otherwise noted, as illustrated
in Figure 5.1 below.

Table 5.6 10BASE-T Transceiver Characteristics

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Transmitter Peak Differential Output Voltage VOUT 2.2 2.5 2.8 V Note 5.14

Receiver Differential Squelch Threshold VDS 300 420 585 mV

Figure 5.1 Output Equivalent Test Load

25 pF

OUTPUT
Revision 1.4 (08-23-12) 68 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
5.5.2 Power Sequence Timing

This diagram illustrates the device power sequencing requirements. The VDDIO, VDD1A, VDD2A and
magnetics power supplies can turn on in any order provided they all reach operational levels within
the specified time period tpon. Device power supplies can turn off in any order provided they all reach
0 volts within the specified time period poff. 

Note: When the internal regulator is disabled, a power-up sequencing relationship exists between
VDDCR and the 3.3V power supply. For additional information refer to Section 3.7.3,
"REGOFF: Internal +1.2V Regulator Configuration," on page 32.

Figure 5.2 Power Sequence Timing

Table 5.7  Power Sequence Timing Values

SYMBOL DESCRIPTION MIN TYP MAX UNITS

tpon Power supply turn on time 50 mS

tpoff Power supply turn off time 500 mS

VDDIO

Magnetics 
Power

tpon tpoff

VDD1A, 
VDD2A
SMSC LAN8720A/LAN8720Ai 69 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
5.5.3 Power-On nRST & Configuration Strap Timing

This diagram illustrates the nRST reset and configuration strap timing requirements in relation to
power-on. A hardware reset (nRST assertion) is required following power-up. For proper operation,
nRST must be asserted for no less than trstia. The nRST pin can be asserted at any time, but must
not be deasserted before tpurstd after all external power supplies have reached 80% of their nominal
operating levels. In order for valid configuration strap values to be read at power-up, the tcss and tcsh
timing constraints must be followed. Refer to Section 3.8.5, "Resets," on page 39 for additional
information.

Note: nRST deassertion must be monotonic.

Note: Device configuration straps are latched as a result of nRST assertion. Refer to Section 3.7,
"Configuration Straps," on page 31 for details. Configuration straps must only be pulled high or
low and must not be driven as inputs.

Note 5.15 20 clock cycles for 25MHz, or 40 clock cycles for 50MHz.

Figure 5.3 Power-On nRST & Configuration Strap Timing

Table 5.8  Power-On nRST & Configuration Strap Timing Values

SYMBOL DESCRIPTION MIN TYP MAX UNITS

tpurstd External power supplies at 80% to nRST deassertion 25 mS

tpurstv External power supplies at 80% to nRST valid 0 nS

trstia nRST input assertion time 100 μS

tcss Configuration strap pins setup to nRST deassertion 200 nS

tcsh Configuration strap pins hold after nRST deassertion 1 nS

totaa Output tri-state after nRST assertion 50 nS

todad Output drive after nRST deassertion 2 800
(Note 5.15)

nS

tcss

nRST

Configuration Strap 
Pins Input

trstia

tcsh

Configuration Strap 
Pins Output Drive

todad

All External 
Power Supplies tpurstd

80%

tpurstv

totaa
Revision 1.4 (08-23-12) 70 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
5.5.4 RMII Interface Timing

5.5.4.1 RMII Timing (REF_CLK Out Mode)

The 50MHz REF_CLK OUT timing applies to the case when nINTSEL is pulled-low. In this mode, a
25MHz crystal or clock oscillator must be input on the XTAL1/CLKIN and XTAL2 pins. For more
information on REF_CLK Out Mode, see Section 3.7.4.2, "REF_CLK Out Mode," on page 34.

Note 5.16 Timing was designed for system load between 10 pf and 25 pf.

Figure 5.4 RMII Timing (REF_CLK Out Mode) 

Table 5.9 RMII Timing Values (REF_CLK Out Mode)

SYMBOL DESCRIPTION MIN MAX UNITS NOTES

tclkp REFCLKO period 20 ns

tclkh REFCLKO high time tclkp*0.4 tclkp*0.6 ns

tclkl REFCLKO low time tclkp*0.4 tclkp*0.6 ns

toval RXD[1:0], RXER, CRS_DV output valid from 
rising edge of REFCLKO

5.0 ns Note 5.16

tohold RXD[1:0], RXER, CRS_DV output hold from 
rising edge of REFCLKO

1.4 ns Note 5.16

tsu TXD[1:0], TXEN setup time to rising edge of 
REFCLKO

7.0 ns Note 5.16

tihold TXD[1:0], TXEN input hold time after rising edge 
of REFCLKO

2.0 ns Note 5.16

REFCLKO

RXD[1:0], 
RXER

CRS_DV

tclkh tclkl
tclkp

toval toholdtoval

tovaltohold

tsu

TXD[1:0]

TXEN

tihold tsu tihold tihold

tsutihold
SMSC LAN8720A/LAN8720Ai 71 Revision 1.4 (08-23-12)

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Revision 1.4 (08-23-12) 72 SMSC LAN8720A/LAN8720Ai

5.5.4.2 RMII Timing (REF_CLK In Mode)

The 50MHz REF_CLK IN timing applies to the case when nINTSEL is floated or pulled-high. In this
mode, a 50MHz clock must be input on the CLKIN pin. For more information on REF_CLK In Mode,
see Section 3.7.4.1, "REF_CLK In Mode," on page 34.

Note 5.17 Timing was designed for system load between 10 pf and 25 pf.

Figure 5.5 RMII Timing (REF_CLK In Mode) 

Table 5.10 RMII Timing Values (REF_CLK In Mode)

SYMBOL DESCRIPTION MIN MAX UNITS NOTES

tclkp CLKIN period 20 ns

tclkh CLKIN high time tclkp*0.35 tclkp*0.65 ns

tclkl CLKIN low time tclkp*0.35 tclkp*0.65 ns

toval RXD[1:0], RXER, CRS_DV output valid from 
rising edge of CLKIN

14.0 ns Note 5.17

tohold RXD[1:0], RXER, CRS_DV output hold from 
rising edge of CLKIN

3.0 ns Note 5.17

tsu TXD[1:0], TXEN setup time to rising edge of 
CLKIN

4.0 ns Note 5.17

tihold TXD[1:0], TXEN input hold time after rising edge 
of CLKIN

1.5 ns Note 5.17

CLKIN
(REF_CLK)

RXD[1:0], 
RXER

CRS_DV

tclkh tclkl
tclkp

toval toholdtoval

tovaltohold

tsu

TXD[1:0]

TXEN

tihold tsu tihold tihold

tsutihold
DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
5.5.4.3 RMII CLKIN Requirements

5.5.5 SMI Timing
This section specifies the SMI timing of the device. Please refer to Section 3.5, "Serial Management
Interface (SMI)," on page 28 for additional details.

Table 5.11  RMII CLKIN (REF_CLK) Timing Values

PARAMETER MIN TYP MAX UNITS NOTES

CLKIN frequency 50 MHz

CLKIN Frequency Drift ± 50 ppm

CLKIN Duty Cycle 40 60 %

CLKIN Jitter 150 psec p-p – not RMS

Figure 5.6 SMI Timing

Table 5.12 SMI Timing Values

SYMBOL DESCRIPTION MIN MAX UNITS NOTES

tclkp MDC period 400 ns

tclkh MDC high time 160 (80%) ns

tclkl MDC low time 160 (80%) ns

tval
MDIO (read from PHY) output valid from rising 
edge of MDC

300 ns

tohold
MDIO (read from PHY) output hold from rising 
edge of MDC

0 ns

tsu
MDIO (write to PHY) setup time to rising edge 
of MDC

10 ns

tihold
MDIO (write to PHY) input hold time after rising 
edge of MDC

10 ns

MDC

MDIO

tclkh tclkl
tclkp

tohold

MDIO

tsu tihold
(Data-Out)

(Data-In)

tohold

tval
SMSC LAN8720A/LAN8720Ai 73 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

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5.6   Clock Circuit
The device can accept either a 25MHz crystal or a 25MHz single-ended clock oscillator (±50ppm)
input. If the single-ended clock oscillator method is implemented, XTAL2 should be left unconnected
and XTAL1/CLKIN should be driven with a nominal 0-3.3V clock signal. See Table 5.13 for the
recommended crystal specifications.

Note 5.18 The maximum allowable values for Frequency Tolerance and Frequency Stability are
application dependant. Since any particular application must meet the IEEE ±50 PPM Total
PPM Budget, the combination of these two values must be approximately ±45 PPM
(allowing for aging).

Note 5.19 Frequency Deviation Over Time is also referred to as Aging.

Note 5.20 The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as 
±100 PPM.

Note 5.21 0oC for extended commercial version, -40oC for industrial version.

Note 5.22 This number includes the pad, the bond wire and the lead frame. PCB capacitance is not
included in this value. The XTAL1/CLKIN pin, XTAL2 pin and PCB capacitance values are
required to accurately calculate the value of the two external load capacitors. The total load
capacitance must be equivalent to what the crystal expects to see in the circuit so that the
crystal oscillator will operate at 25.000 MHz.

Table 5.13 Crystal Specifications

PARAMETER SYMBOL MIN NOM MAX UNITS NOTES

Crystal Cut AT, typ

Crystal Oscillation Mode Fundamental Mode

Crystal Calibration Mode Parallel Resonant Mode

Frequency Ffund - 25.000 - MHz

Frequency Tolerance @ 25oC Ftol - - ±50 PPM Note 5.18

Frequency Stability Over Temp Ftemp - - ±50 PPM Note 5.18

Frequency Deviation Over Time Fage - +/-3 to 5 - PPM Note 5.19

Total Allowable PPM Budget - - ±50 PPM Note 5.20

Shunt Capacitance CO - 7 typ - pF

Load Capacitance CL - 20 typ - pF

Drive Level PW 300 - - uW

Equivalent Series Resistance R1 - - 30 Ohm

Operating Temperature Range Note 5.21 - +85 oC

XTAL1/CLKIN Pin Capacitance - 3 typ - pF Note 5.22

XTAL2 Pin Capacitance - 3 typ - pF Note 5.22
Revision 1.4 (08-23-12) 74 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 6 Package Outline

Notes:
1. All dimensions are in millimeters unless otherwise noted.
2. Dimension “b” applies to plated terminals and is measured between 0.15 and 0.30 mm from the terminal tip.
3. The pin 1 identifier may vary, but is always located within the zone indicated.

MIN NOMINAL MAX REMARKS
A 0.70 0.85 1.00 Overall Package Height

A1 0 0.02 0.05 Standoff
A2 - - 0.90 Mold Cap Thickness
D/E 3.90 4.00 4.10 X/Y Body Size

 D1/E1 3.55 3.75 3.95 X/Y Mold Cap Size
D2/E2 2.40 2.50 2.60 X/Y Exposed Pad Size

L 0.30 0.40 0.50 Terminal Length
b 0.18 0.25 0.30 Terminal Width
k 0.25 - - Terminal to Exposed Pad Clearance
e 0.50 BSC Terminal Pitch
SMSC LAN8720A/LAN8720Ai 75 Revision 1.4 (08-23-12)

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Revision 1.4 (08-23-12) 76 SMSC LAN8720A/LAN8720Ai

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Note: Standard reel size is 4000 pieces per reel.
SMSC LAN8720A/LAN8720Ai 77 Revision 1.4 (08-23-12)

DATASHEET

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Chapter 7 Datasheet Revision History

Table 7.1  Customer Revision History

REVISION LEVEL & DATE SECTION/FIGURE/ENTRY CORRECTION

Rev. 1.4
(08-23-12)

Section 4.2.2, "Basic Status 
Register," on page 50

Updated definitions of bits 10:8.

Section 4.2.11, "Special 
Control/Status Indications 
Register," on page 59

Updated bit 11 definition.

Rev. 1.3
(03-12-12)

Company disclaimer on 
page 2

Removed company address and phone numbers.

Rev. 1.3
(04-20-11)

Table 5.9, “RMII Timing 
Values (REF_CLK Out 
Mode),” on page 71

Updated toval maximum value from 10.0ns to 
5.0ns.

Table 2.7, “Power Pins,” on 
page 14

Updated VDDCR pin note to include requirement 
of 1uF and 470pF decoupling capacitors in parallel 
to ground on the VDDCR pin.

Figure 3.16 Power Supply 
Diagram (1.2V Supplied by 
Internal Regulator) on 
page 43 and Figure 3.16 
Power Supply Diagram 
(1.2V Supplied by Internal 
Regulator) on page 43

Updated diagrams to include 1uF and 470pF 
decoupling capacitors on the VDDCR pin.

Rev. 1.2 (11-10-10) Section 5.5.4, "RMII 
Interface Timing," on 
page 71

Updated diagrams and tables to include RXER.

Section 3.7.4.2, "REF_CLK 
Out Mode," on page 34

Added timing note in Section 3.7.4.2, "REF_CLK 
Out Mode"

Section 5.5.4, "RMII 
Interface Timing," on 
page 71

Corrected signal names on RMII timing diagrams 
and tables. Updated Table 5.9 toval, tohold, tsu, and 
tihold with 10 ns, 1.4 ns, 7.0 ns, and 2.0 ns, 
respectively.

Table 5.8, “Power-On nRST 
& Configuration Strap 
Timing Values,” on page 70

Updated todad description: “Output drive after 
nRST deassertion”

Rev. 1.1 (04-09-10) Section 5.1, "Absolute 
Maximum Ratings*"

Modified “HBM ESD Performance by adding “per 
JEDEC JESD22-A114” and changed “+/-5kV” to 
“Class 3A”

Section 5.3, "Power 
Consumption," on page 64

Corrected typo in the current consumption table 
row title: “100BASE-TX /W TRAFFIC”

Section 5.3, "Power 
Consumption," on page 64

Corrected typo in note regarding Ethernet 
component current: 
“The Ethernet component current is typically 41mA 
in 100BASE-TX mode and 100mA in 10BASE-T 
mode, independent of the 2.5V or 3.3V supply rail 
of the transformer.”
Revision 1.4 (08-23-12) 78 SMSC LAN8720A/LAN8720Ai

DATASHEET



Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet
Table 5.3, “Non-Variable I/O 
Buffer Characteristics,” on 
page 66

Corrected O12 VOH minimum value to “VDD2A - 
0.4”
Corrected ICLK VILI maximum value to “0.35”
Corrected ICLK VIHI maximum value to “VDD2A + 
0.4”

Section 5.2, "Operating 
Conditions**," on page 64

Added note: “Do not drive input signals without 
power supplied to the device.”

Section 5.1, "Absolute 
Maximum Ratings*," on 
page 63

Corrected IEC61000-4-2 Contact Discharge ESD 
Performance to +/-8kV.

Section 4.2.4, "PHY 
Identifier 2 Register," on 
page 52

Corrected Model Number default value to 
“001111b”.

Section 3.8.8.2, "Far 
Loopback," on page 40

Added far loopback description.

Section 4.2.8, "Mode 
Control/Status Register," on 
page 56

Added FARLOOPBACK (bit 9) description.

Rev. 1.0 (12-09-09) Document reworked for clarity and consistency with other SMSC documentation.

Rev. 1.0 (04-15-09) Initial Release

Table 7.1  Customer Revision History (continued) 

REVISION LEVEL & DATE SECTION/FIGURE/ENTRY CORRECTION
SMSC LAN8720A/LAN8720Ai 79 Revision 1.4 (08-23-12)

DATASHEET


	Chapter 1 Introduction
	1.1 General Terms and Conventions
	1.2 General Description
	Figure 1.1 System Block Diagram
	Figure 1.2 Architectural Overview


	Chapter 2 Pin Description and Configuration
	Figure 2.1 24-QFN Pin Assignments (TOP VIEW)
	Table 2.1 RMII Signals
	Table 2.2 LED Pins
	Table 2.3 Serial Management Interface (SMI) Pins
	Table 2.4 Ethernet Pins
	Table 2.5 Miscellaneous Pins
	Table 2.6 Analog Reference Pins
	Table 2.7 Power Pins
	2.1 Pin Assignments
	Table 2.8 24-QFN Package Pin Assignments

	2.2 Buffer Types
	Table 2.9 Buffer Types


	Chapter 3 Functional Description
	3.1 Transceiver
	3.1.1 100BASE-TX Transmit
	Figure 3.1 100BASE-TX Transmit Data Path
	Table 3.1 4B/5B Code Table

	3.1.2 100BASE-TX Receive
	Figure 3.2 100BASE-TX Receive Data Path
	Figure 3.3 Relationship Between Received Data and Specific MII Signals

	3.1.3 10BASE-T Transmit
	3.1.4 10BASE-T Receive

	3.2 Auto-negotiation
	3.2.1 Parallel Detection
	3.2.2 Restarting Auto-negotiation
	3.2.3 Disabling Auto-negotiation
	3.2.4 Half vs. Full Duplex

	3.3 HP Auto-MDIX Support
	Figure 3.4 Direct Cable Connection vs. Cross-over Cable Connection

	3.4 MAC Interface
	3.4.1 RMII

	3.5 Serial Management Interface (SMI)
	Figure 3.5 MDIO Timing and Frame Structure - READ Cycle
	Figure 3.6 MDIO Timing and Frame Structure - WRITE Cycle

	3.6 Interrupt Management
	3.6.1 Primary Interrupt System
	Table 3.2 Interrupt Management Table

	3.6.2 Alternate Interrupt System
	Table 3.3 Alternative Interrupt System Management Table


	3.7 Configuration Straps
	3.7.1 PHYAD[0]: PHY Address Configuration
	3.7.2 MODE[2:0]: Mode Configuration
	Table 3.4 MODE[2:0] Bus
	Table 3.5 Pin Names for Mode Bits

	3.7.3 REGOFF: Internal +1.2V Regulator Configuration
	3.7.4 nINTSEL: nINT/REFCLKO Configuration
	Table 3.6 nINTSEL Configuration
	Figure 3.7 External 50MHz clock sources the REF_CLK
	Figure 3.8 Sourcing REF_CLK from a 25MHz Crystal
	Figure 3.9 Sourcing REF_CLK from External 25MHz Source


	3.8 Miscellaneous Functions
	3.8.1 LEDs
	Figure 3.10 LED1/REGOFF Polarity Configuration
	Figure 3.11 LED2/nINTSEL Polarity Configuration

	3.8.2 Variable Voltage I/O
	3.8.3 Power-Down Modes
	3.8.4 Isolate Mode
	3.8.5 Resets
	3.8.6 Carrier Sense
	3.8.7 Link Integrity Test
	3.8.8 Loopback Operation
	Figure 3.12 Near-end Loopback Block Diagram
	Figure 3.13 Far Loopback Block Diagram
	Figure 3.14 Connector Loopback Block Diagram


	3.9 Application Diagrams
	3.9.1 Simplified System Level Application Diagram
	Figure 3.15 Simplified System Level Application Diagram

	3.9.2 Power Supply Diagram (1.2V Supplied by Internal Regulator)
	Figure 3.16 Power Supply Diagram (1.2V Supplied by Internal Regulator)

	3.9.3 Power Supply Diagram (1.2V Supplied by External Source)
	Figure 3.17 Power Supply Diagram (1.2V Supplied by External Source)

	3.9.4 Twisted-Pair Interface Diagram (Single Power Supply)
	Figure 3.18 Twisted-Pair Interface Diagram (Single Power Supply)

	3.9.5 Twisted-Pair Interface Diagram (Dual Power Supplies)
	Figure 3.19 Twisted-Pair Interface Diagram (Dual Power Supplies)



	Chapter 4 Register Descriptions
	4.1 Register Nomenclature
	Table 4.1 Register Bit Types

	4.2 Control and Status Registers
	Table 4.2 SMI Register Map
	4.2.1 Basic Control Register
	4.2.2 Basic Status Register
	4.2.3 PHY Identifier 1 Register
	4.2.4 PHY Identifier 2 Register
	4.2.5 Auto Negotiation Advertisement Register
	4.2.6 Auto Negotiation Link Partner Ability Register
	4.2.7 Auto Negotiation Expansion Register
	4.2.8 Mode Control/Status Register
	4.2.9 Special Modes Register
	4.2.10 Symbol Error Counter Register
	4.2.11 Special Control/Status Indications Register
	4.2.12 Interrupt Source Flag Register
	4.2.13 Interrupt Mask Register
	4.2.14 PHY Special Control/Status Register


	Chapter 5 Operational Characteristics
	5.1 Absolute Maximum Ratings*
	5.2 Operating Conditions**
	5.3 Power Consumption
	5.3.1 REF_CLK In Mode
	Table 5.1 Device Only Current Consumption and Power Dissipation (REF_CLK In Mode)

	5.3.2 REF_CLK Out Mode
	Table 5.2 Device Only Current Consumption and Power Dissipation (REF_CLK Out Mode)


	5.4 DC Specifications
	Table 5.3 Non-Variable I/O Buffer Characteristics
	Table 5.4 Variable I/O Buffer Characteristics
	Table 5.5 100BASE-TX Transceiver Characteristics
	Table 5.6 10BASE-T Transceiver Characteristics

	5.5 AC Specifications
	5.5.1 Equivalent Test Load
	Figure 5.1 Output Equivalent Test Load

	5.5.2 Power Sequence Timing
	Figure 5.2 Power Sequence Timing
	Table 5.7 Power Sequence Timing Values

	5.5.3 Power-On nRST & Configuration Strap Timing
	Figure 5.3 Power-On nRST & Configuration Strap Timing
	Table 5.8 Power-On nRST & Configuration Strap Timing Values

	5.5.4 RMII Interface Timing
	Figure 5.4 RMII Timing (REF_CLK Out Mode)
	Table 5.9 RMII Timing Values (REF_CLK Out Mode)
	Figure 5.5 RMII Timing (REF_CLK In Mode)
	Table 5.10 RMII Timing Values (REF_CLK In Mode)
	Table 5.11 RMII CLKIN (REF_CLK) Timing Values

	5.5.5 SMI Timing
	Figure 5.6 SMI Timing
	Table 5.12 SMI Timing Values


	5.6 Clock Circuit
	Table 5.13 Crystal Specifications


	Chapter 6 Package Outline
	Chapter 7 Datasheet Revision History
	Table 7.1 Customer Revision History
