LAN8720AI datasheet(25/79 Pages) SMSC

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

Datasheet

SMSC LAN8720A/LAN8720Ai

Revision 1.4 (08-23-12)

DATASHEET

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

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

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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