AM79C970 datasheet(58/168 Pages) AMD

58 / 168 page

P R E L I M I N A R Y

AMD

1-925

Am79C970

Collision Detection

A MAU detects the collision condition on the network

and generates a differential signal at the CI

± inputs. This

collision signal passes through an input stage which de-

tects signal levels and pulse duration. When the signal is

detected by the MENDEC it sets the ICLSN line HIGH.

The condition continues for approximately 1.5 bit times

after the last LOW-to-HIGH transition on CI

±.

Jitter Tolerance Definition

The MENDEC utilizes a clock capture circuit to align its

internal data strobe with an incoming bit stream. The

clock acquisition circuitry requires four valid bits with the

values 1010b. Clock is phase-locked to the negative

transition at the bit cell center of the second “0” in the

pattern.

Since data is strobed at 1/4 bit time, Manchester transi-

tions which shift from their nominal placement through

1/4 bit time will result in improperly decoded data. With

this as the criteria for an error, a definition of Jitter Han-

dling is:

The peak deviation approaching or crossing 1/4 bit

cell position from nominal input transition, for which

the MENDEC section will properly decode data.

Attachment Unit Interface (AUI)

The AUI is the PLS (Physical Layer Signaling) to PMA

(Physical Medium Attachment) interface which effec-

tively connects the DTE to a MAU. The differential inter-

face provided by the PCnet-PCI controller is fully

compliant to Section 7 of ISO 8802-3 (ANSI/IEEE

802.3).

After the PCnet-PCI controller initiates a transmission it

will expect to see data “looped-back” on the DI

± pair

(when the AUI port is selected). This will internally gen-

erate a “carrier sense”, indicating that the integrity of the

data path to and from the MAU is intact, and that the

MAU is operating correctly. This “carrier sense” signal

must be asserted within TBD bit times after the first

transmitted bit on DO

± (when using the AUI port). If “car-

rier sense” does not become active in response to the

data transmission, or becomes inactive before the end

of transmission, the loss of carrier (LCAR) error bit will

be set in the Transmit Descriptor Ring (TMD2, bit 27) af-

ter the frame has been transmitted.

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 ISO 8802-3 (IEEE/ANSI

802.3) standard (Type 10BASE-T). The T-MAU pro-

vides twisted pair driver and receiver circuits, including

on-board transmit digital predistortion and receiver

squelch and a number of additional features including

Link Status indication, Automatic Twisted-Pair Receive

Polarity Detection/Correction 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 ISO 8802-3 (IEEE/

ANSI 802.3) Standard. 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 ISO 8802-3 (IEEE/ANSI 802.3) 10BASE-T Stan-

dard, including noise immunity and received signal re-

jection 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 receiver squelch level drops to half its

threshold value after unsquelch to allow reception of

minimum amplitude signals and to offset carrier 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 specified.

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

count for a 1 dB insertion loss at 10 MHz, which is typical

for the type of receive filters/transformers employed.

Normal 10BASE-T compatible receive thresholds are

employed when the LRT bit (CSR15[9]) is LOW. When

the LRT bit is set (HIGH), the Low Receive Threshold

option is invoked, and the sensitivity of the T-MAU re-

ceiver is increased. This allows longer line lengths to be

employed, exceeding the 100 m target distance of nor-

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

increased receiver sensitivity compensates for the in-

creased 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, 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

AM79C970 Datasheet(PDF) 58 Page - Advanced Micro Devices

AM79C970 Datasheet(HTML) 58 Page - Advanced Micro Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL