AM79C970A datasheet(86/219 Pages) AMD

86 / 219 page

AMD

P R E L I M I N A R Y

Am79C970A

Multipair cables within the same outer sheath have

lower crosstalk attenuation, and may allow noise emit-

ted 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

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

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 T-MAU to

go into a 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 more than five consecutive link pulses

appear on the RXD

± pair. During Link Fail, the Link

Status signal is inactive. When the link is identified as

functional, the Link Status signal is asserted. The

LNKST 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 imple-

ment Link Test, this function can be disabled by setting

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 regardless 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 will reverse the re-

ceive polarity based on both the polarity of any previous

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 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

polarity 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 received signal

with a positive amplitude greater than 585 mV (LRT = 1)

with a pulse width of 60 ns–200 ns. This positive excur-

sion may be followed by a negative excursion. This defi-

nition 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 Stan-

dard 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 with

a pulse width of 60–200 ns. This negative excursion

may be followed by a positive excursion. This definition

is consistent with the expected received signal at a re-

verse wired receiver, when a link beat pulse which fits

the template of Figure 14-12 in the 10BASE-T Standard

is generated at a transmitter and passed through 100 m

of twisted pair cable.

The polarity detection/correction algorithm will remain

“armed” until two consecutive frames with valid ETD of

identical polarity are detected. When “armed”, the re-

ceiver is capable of changing the initial or previous po-

larity configuration based on the ETD polarity.

On receipt of the first frame with valid ETD following

H_RESET or Link Fail, the T-MAU will utilize the

inferred polarity information to configure its RXD

± input,

regardless of its previous state. On receipt of a second

frame with a valid ETD with correct polarity, the

detection/correction algorithm will “lock-in” the received

polarity. If the second (or subsequent) frame is not de-

tected as confirming the previous polarity decision, the

most recently detected ETD polarity will be used as the

default. Note that frames with invalid ETD have no effect

on updating the previous polarity decision. Once two

consecutive frames with valid ETD have been received,

the

T-MAU

will

disable

the

detection/correction

algorithm until either a Link Fail condition occurs or

H_RESET is activated.

During polarity reversal, an internal POL signal will be

active. During normal polarity conditions, this internal

POL signal is inactive. The state of this signal can be

read by software and/or displayed by LED when

enabled by the LED control bits in the Bus Configuration

Registers (BCR4 to BCR7).

Twisted Pair Interface Status

When the T-MAU is in Link Pass state, three signals

(XMT, RCV and COL) indicate whether the T-MAU is

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