LAN89218 datasheet(67/148 Pages) MICROCHIP

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DS60001255B-page 67

LAN89218

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.

Writing register 4 bits [8:5] allows software control of the capabilities advertised by the PHY. Writing register 4 does

not automatically re-start auto-negotiation. Register 0, bit 9 must be set before the new abilities will be advertised.

Auto-negotiation can also be disabled via software by clearing register 0, bit 12.

The LAN89218 does not support “Next Page" capability.

4.7

Parallel Detection

If the LAN89218 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 bit 0 in register 6 is

cleared to indicate that the Link Partner is not capable of auto-negotiation. The Ethernet MAC has access to this

information via the management interface. If a fault occurs during parallel detection, bit 4 of register 6 is set.

Partner is not auto-negotiation capable, then register 5 is updated after completion of parallel detection to reflect the

speed capability of the Link Partner.

4.7.1

RE-STARTING AUTO-NEGOTIATION

Auto-negotiation can be re-started at any time by setting register 0, bit 9. Auto-negotiation will also re-start 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 writing to bit 9 of the control register, the LAN89218 will

respond by stopping all transmission/receiving operations. Once the break_link_timer is done, in the Auto-negotiation

state-machine (approximately 1200 ms) the auto-negotiation will re-start. The Link Partner will have also dropped the

link due to lack of a received signal, so it too will resume auto-negotiation.

4.7.2

DISABLING AUTO-NEGOTIATION

Auto-negotiation can be disabled by setting register 0, bit 12 to zero. The device will then force its speed of operation

to reflect the information in register 0, bit 13 (speed) and register 0, bit 8 (duplex). The speed and duplex bits in

4.7.3

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 internal carrier sense signal, CRS, responds to both transmit and

receive activity. In this mode, If data is received while the PHY is transmitting, a collision results.

In full-duplex mode, the PHY is able to transmit and receive data simultaneously. In this mode, the internal CRS

responds only to receive activity. The CSMA/CD protocol does not apply and collision detection is disabled.

Table 4-2 describes the behavior of the internal CRS bit under all receive/transmit conditions. The internal CRS signal

is used to trigger bit 10 (No Carrier) of the TX Status word (see Section 3.12.4). The CRS value, and subsequently

the No Carrier value, are invalid during any full-duplex transmission. Therefore, these signals cannot be used as a

verification method of transmitted packets when transmitting in 10/100 Mbps full-duplex modes.

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