AM79C960KCW datasheet(43/127 Pages) AMD

43 / 127 page

P R E L I M I N A R Y

AMD

1-385

Am79C960

Twisted Pair Transceiver (T-MAU)

The T-MAU implements the Medium Attachment Unit

(MAU) functions for the Twisted Pair Medium, as speci-

fied by the supplement to IEEE 802.3 standard (Type

10BASE-T). The T-MAU provides twisted pair driver

and receiver circuits, including on-board transmit digital

predistortion and receiver squelch, and a number of ad-

ditional features including Link Status indication,

Automatic Twisted Pair Receive Polarity Detection/ Cor-

rection and indication, Receive Carrier Sense, Transmit

Active and Collision Present indication.

Twisted Pair Transmit Function

The differential driver circuitry in the TXD

± and TXP±

pins provides the necessary electrical driving capability

and the pre-distortion control for transmitting signals

over maximum length Twisted Pair cable, as specified

by the 10BASE-T supplement to the IEEE 802.3 Stan-

dard. The transmit function for data output meets the

propagation delays and jitter specified by the standard.

Twisted Pair Receive Function

The receiver complies with the receiver specifications of

the IEEE 802.3 10BASE-T Standard, including noise

immunity and received signal rejection criteria (‘Smart

Squelch’). Signals meeting this criteria appearing at the

RXD

± differential input pair are routed to the MENDEC.

The receiver function meets the propagation delays and

jitter requirements specified by the standard. The re-

ceiver squelch level drops to half its threshold value

after unsquelch to allow reception of minimum ampli-

tude signals and to offset carrier fade in the event of

worst case signal attenuation conditions.

Note that the 10BASE-T Standard defines the receive

input amplitude at the external Media Dependent Inter-

face (MDI). Filter and transformer loss are not specified.

The T-MAU receiver squelch levels are designed to ac-

count for a 1 dB insertion loss at 10 MHz for the type of

receive filters and transformers usually used.

Normal 10BASE-T compatible receive thresholds are

invoked when the LRT bit (CSR15, bit 9) is LOW. When

the LRT bit is set, the Low Receive Threshold option is

invoked, and the sensitivity of the T-MAU receiver is in-

creased. Increasing T-MAU sensitivity allows the use of

lines longer than the 100 m target distance of standard

10BASE-T (assuming typical 24 AWG cable). Increased

receiver sensitivity compensates for the increased sig-

nal attenuation caused by the additional cable distance.

However, making the receiver more sensitive means

that it is also more susceptible to extraneous noise, pri-

marily caused by coupling from co-resident services

(crosstalk). For this reason, end users may wish to in-

voke the Low Receive Threshold option on 4-pair cable

only. Multi-pair cables within the same outer sheath

have lower crosstalk attenuation, and may allow noise

emitted from adjacent pairs to couple into the receive

pair, and be of sufficient amplitude to falsely unsquelch

the T-MAU.

Link Test Function

The link test function is implemented as specified by

10BASE-T standard. During periods of transmit pair in-

activity, ’Link beat pulses’ will be periodically sent over

the twisted pair medium to constantly monitor medium

integrity.

When the link test function is enabled (DLNKTST bit in

CSR15 is cleared), the absence of link beat pulses and

receive data on the RXD

± pair will cause the TMAU to go

into the Link Fail state. In the Link Fail state, data trans-

mission, data reception, data loopback and the collision

detection functions are disabled and remain disabled

until valid data or greater than 5 consecutive link pulses

appear on the RXD

± pair. During Link Fail, the Link

Status (LNKST indicated by

LED0) signal is inactive.

When the link is identified as functional, the LNKST sig-

nal is asserted, and

LED0 output will be activated.

In order to inter-operate with systems which do not im-

plement Link Test, this function can be disabled by

setting the DLNKTST bit. With Link Test disabled, the

Data Driver, Receiver and Loopback functions as well

as Collision Detection remain enabled irrespective of

the presence or absence of data or link pulses on the

RXD

± pair. Link Test pulses continue to be sent regard-

less of the state of the DLNKTST bit.

Polarity Detection and Reversal

The T-MAU receive function includes the ability to invert

the polarity of the signals appearing at the RXD

± pair if

the polarity of the received signal is reversed (such as in

the case of a wiring error). This feature allows data pack-

ets received from a reverse wired RXD

± input pair to be

corrected in the T-MAU prior to transfer to the

MENDEC. The polarity detection function is activated

following reset or Link Fail, and will reverse the receive

polarity based on both the polarity of any previous link

beat pulses and the polarity of subsequent packets with

a valid End Transmit Delimiter (ETD).

When in the Link Fail state, the T-MAU will recognize

link beat pulses of either positive or negative polarity.

Exit from the Link Fail state occurs at the reception of 5–

6 consecutive link beat pulses of identical polarity. On

entry to the Link Pass state, the polarity of the last 5 link

beat pulses is used to determine the initial receive polar-

ity configuration and the receiver is reconfigured to

subsequently recognize only link beat pulses of the pre-

viously recognized polarity.

Positive link beat pulses are defined as transmitted sig-

nal with a positive amplitude greater than 585 mV

(LRT = HIGH) with a pulse width of 60 ns–200 ns. This

positive excursion may be followed by a negative excur-

sion. This definition is consistent with the expected

received signal at a correctly wired receiver, when a link

beat pulse, which fits the template of Figure 14-12 of the

10BASE-T Standard, is generated at a transmitter and

passed through 100 m of twisted pair cable.

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