AM79C971VCW datasheet(73/265 Pages) AMD

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Am79C971

ANSI 802.3) Standard. The transmit function for data

output meets the propagation delays and jitter speci-

fied by the standard.

Twisted Pair Receive Function

The receiver complies with the receiver specifications

of the ISO 8802-3 (IEEE/ANSI 802.3) 10BASE-T Stan-

dard, including noise immunity and received signal re-

jection criteria (Smart Squelch). Signals meeting these

criteria appearing at the RXD

± differential input pair are

routed to the MENDEC. The receiver function meets

the propagation delays and jitter requirements speci-

fied by the standard. The receiver squelch level drops

to half its threshold value after unsquelch to allow re-

ception of minimum amplitude signals and to offset car-

rier fade in the event of worst case signal attenuation

and crosstalk noise 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 speci-

fied. The T-MAU receiver squelch levels are defined to

account for a 1-dB insertion loss at 10 MHz, which is

typical for the type of receive filters/transformers em-

ployed.

Normal 10BASE-T compatible receive thresholds are

employed when the LRT bit (CSR15, bit 9) is cleared to

0. When the LRT bit is set to 1, the Low Receive

Threshold option is invoked, and the sensitivity of the T-

MAU receiver is increased. This allows longer line

lengths to be employed, exceeding the 100-meter (m)

target distance of normal 10BASE-T (assuming typical

24 AWG cable). The increased receiver sensitivity

compensates for the increased signal 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, it is recommended that

when using the Low Receive Threshold option that the

service should be installed 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, being of

sufficient amplitude to falsely unsquelch the T-MAU.

Link Test Function

The Link Test Function is implemented as specified by

the 10BASE-T standard. During periods of transmit

pair inactivity, link beat pulses will be periodically sent

over the twisted pair medium to constantly monitor me-

dium 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 T-MAU to

go into a Link Fail state. In the Link Fail state, data

transmission, data reception, data loopback and the

collision detection functions are disabled and remain

disabled until valid data or more than five consecutive

link pulses appear on the RXD

± pair. During Link Fail,

the Link Status signal is inactive. When the link is iden-

tified as functional, the Link Status signal is asserted.

The LED0 pin displays the Link Status signal by default.

The T-MAU will power up in the Link Fail state and the

normal algorithm will apply to allow it to enter the Link

Pass state. If T-MAU is selected using the PORTSEL

bits in CSR15, the T-MAU will be forced into the Link

Fail state when moving from AUI to T-MAU selection.

Transmission attempts during Link Fail state will pro-

duce no network activity and will produce LCAR and

CERR error indications.

In order to interoperate with systems which do not im-

plement Link Test, this function can be disabled by set-

ting the DLNKTST bit in CSR15. 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

frames 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 H_RESET or Link Fail, and it will reverse the

receive polarity based on both the polarity of any previ-

ous link beat pulses and the polarity of subsequent

frames 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 is made due to the recep-

tion of 5 to 6 consecutive link beat pulses of identical

polarity. On entry to the Link Pass state, the polarity of

the last five link beat pulses is used to determine the

initial receive polarity configuration, and the receiver is

reconfigured to subsequently recognize only link beat

pulses of the previously recognized polarity.

Positive link beat pulses are defined as received signal

with a positive amplitude greater than 585 mV (LRT =

1) with a pulse width of 60 ns to 200 ns. This positive

excursion may be followed by a negative excursion.

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.

Negative link beat pulses are defined as received sig-

nals with a negative amplitude greater than 585 mV

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