AM79C901A datasheet(39/90 Pages) AMD

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Am79C901A

PR ELI M IN AR Y

Twisted Pair Interface Status

The Am79C901A device will power up in the Link Fail

state. The Auto-Negotiation algorithm will apply to

allow it to enter the Link Pass state.

In the Link Pass state, receive activity which passes the

pulse width/amplitude requirements of the RX± inputs

will cause the PCS Control block to assert the Carrier

Sense (CRS) signal at the MII interface. A collision

would cause the PCS Control block to assert Carrier

Sense (CRS) and Collision (COL) signals at the MII. In

the Link Fail state, this block would cause the PCS

Control block to deassert 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 MII and

allow the PCS Control block to assert or deassert 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 MII. If there is RX± activity, this block would

cause the PCS Control block to assert both COL and

CRS at the 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 MII.

Jabber Function

The Jabber function inhibits the 10BASE-T twisted pair

transmit function of the Am79C901A 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

MII and sets the Jabber Detect bit in Register 1 of the

active PHY. Once the internal transmit data stream

from the MENDEC stops, an unjab time of 250-750 ms

will elapse before this block causes the PCS Control

block to deassert 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 deassert the CRS signal

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 MII.

If there is RX± activity, this block will cause the PCS

Control block to assert both COL and CRS on the 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

ignored, but it will set the polarity to the correct state.

The reception of two consecutive packets will cause

the polarity to be locked, based on the polarity of the

End of Transmit Data (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 de-

termine the abilities of the devices sharing a link.

After exchanging abilities, the Am79C901A 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 resolution between exchanged abilities.

This exchange allows both devices at either end of

the link to take maximum advantage of their respec-

tive shared abilities.

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 information about the abilities of the

sending device. The receiver can accept and decode

an FLP burst to learn the abilities of the sending device.

The link pulses transmitted conform to the standard

10BASE-T template. The device can perform Auto-Ne-

gotiation with reverse polarity link pulses.

The Am79C901A 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 10.

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 Am79C901A PHY can auto-

matically negotiate with the network and yield the high-

est performance possible without software support.

Table 10.

Auto-Negotiation Capabilities

Network Speed

Physical Network Type

20 Mbps

10BASE-T, Full Duplex

10 Mbps

10BASE-T, Half Duplex

AM79C901A Datasheet(PDF) 39 Page - Advanced Micro Devices

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