AM79C978VCSLASHW datasheet(99/261 Pages) AMD

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Am79C978

In the Link Pass state, receive activity which passes

the pulse width/amplitude requirements of the RX± in-

puts will cause the PCS Control block to assert Carrier

Sense (CRS) signal at the internal MII interface. A col-

lision would cause the PCS Control block to assert Car-

rier Sense (CRS) and Collision (COL) signals at the

internal MII. In the Link Fail state, this block would

cause the PCS Control block to de-assert Carrier

Sense (CRS) and Collision (COL).

In jabber detect mode, this block would cause the PCS

Control block to assert the COL signal at the internal

MII and allow the PCS Control block to assert or de-as-

sert the CRS pin to indicate the current state of the RX±

pair. If there is no receive activity on RX±, this block

would cause the PCS Control block to assert only the

COL pin at the internal MII. If there is RX± activity, this

block would cause the PCS Control block to assert

both COL and CRS at the internal MII.

Collision Detect Function

Simultaneous activity (presence of valid data signals)

from both the internal encoder transmit function and

the twisted pair RX± pins constitutes a collision,

thereby causing the PCS Control block to assert the

COL pin at the internal MII.

Jabber Function

The Jabber function inhibits the 10BASE-T twisted pair

transmit function of the Am79C978 device if the TX±

circuits are active for an excessive period (20-150 ms).

This prevents one port from disrupting the network due

to a stuck-on or faulty transmitter condition. If the max-

imum transmit time is exceeded, the data path through

the 10BASE-T transmitter circuitry is disabled (al-

though Link Test pulses will continue to be sent). The

PCS Control block also asserts the COL signal at the

internal MII and sets the Jabber Detect bit in Register 1

of the active PHY. Once the internal transmit data

stream from the Manchester Encoder/Decoder stops,

an unjab time of 250-750 ms will elapse before this

block causes the PCS Control block to de-assert the

COL indication and re-enable the transmit circuitry.

When jabber is detected, this block will cause the PCS

Control block to assert the COL signal and allow the

PCS Control block to assert or de-assert the CRS sig-

nal to indicate the current state of the RX± pair. If there

is no receive activity on RX±, this block causes the

PCS Control block to assert only the COL signal at the

internal MII. If there is RX± activity, this block will cause

the PCS Control block to assert both COL and CRS on

the internal MII.

Reverse Polarity Detect

The polarity for 10BASE-T signals is set by reception of

Normal Link Pulses (NLP) or packets. Polarity is

locked, however, by incoming packets only. The first

NLP received when trying to bring the link up will be ig-

nored, but it will set the polarity to the correct state. The

reception of two consecutive packets will cause the po-

larity to be locked, based on the polarity of the ETD. In

order to change the polarity once it has been locked,

the link must be brought down and back up again.

Auto-Negotiation

The object of the Auto-Negotiation function is to deter-

mine the abilities of the devices sharing a link. After ex-

changing abilities, the Am79C978 device and remote

link partner device acknowledge each other and make

a choice of which advertised abilities to support. The

Auto-Negotiation function facilitates an ordered resolu-

tion between exchanged abilities. This exchange al-

lows both devices at either end of the link to take

maximum advantage of their respective shared abili-

ties.

The Am79C978 device implements the transmit and

receive Auto-Negotiation algorithm as defined in IEEE

802.3u, Section 28. The Auto-Negotiation algorithm

uses a burst of link pulses called Fast Link Pulses

(FLPs). The burst of link pulses are spaced between 55

and 140 µs so as to be ignored by the standard

10BASE-T algorithm. The FLP burst conveys informa-

tion about the abilities of the sending device. The re-

ceiver can accept and decode an FLP burst to learn the

abilities of the sending device. The link pulses transmit-

ted conform to the standard 10BASE-T template. The

device can perform auto-negotiation with reverse po-

larity link pulses.

The Am79C978 device uses the Auto-Negotiation al-

gorithm to select the type connection to be established

according to the following priority: 10BASE-T full du-

plex, then 10BASE-T half-duplex. See Table 29.

The Auto-Negotiation algorithm is initiated by the fol-

lowing events: Auto-Negotiation enable bit is set, hard-

ware reset, soft reset, transition to link fail state (when

Auto-Negotiation enable bit is set), or Auto-Negotiation

restart bit is set. The result of the Auto-Negotiation pro-

cess can be read from the status register (Summary

Status Register, TBR24).

By default, the link partner must be at least 10BASE-T

half-duplex capable. The Am79C978 controller can au-

tomatically negotiate with the network and yield the

highest performance possible without software sup-

port. See the Network Port Manager section for more

details.

Table 29.

Auto-Negotiation Capabilities

Network Speed

Physical Network Type

20 Mbps

10BASE-T, Full Duplex

10 Mbps

10BASE-T, Half Duplex

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