LAN8742AI-CZ datasheet(23/124 Pages) MICROCHIP

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DS00001989A-page 23

LAN8742A/LAN8742Ai

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.

3.2.1

PARALLEL DETECTION

If the LAN8742A/LAN8742Ai 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 Reg-

ister, the LAN8742A/LAN8742Ai will respond by stopping all transmission/receiving operations. Once the break_link_-

timer is completed in the auto-negotiation state-machine (approximately 1250 ms), 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-negoti-

ation.

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 net-

work 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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