AM79C978A datasheet(72/256 Pages) AMD

72 / 256 page

Am79C978A

has internal filtering and does not require external filter

modules or common mode chokes.

Signals appearing at the RX± differential input pair are

routed to the internal decoder. The receiver function

meets the propagation delays and jitter requirements

specified by the 10BASE-T standard. The receiver

squelch level drops to half its threshold value after un-

squelch to allow reception of minimum amplitude sig-

nals and to mitigate carrier fade in the event of worst

case signal attenuation and crosstalk noise conditions.

Figure 37.

10BASE-T Transmit and Receive Data

Paths

Twisted Pair Interface Status

The Am79C978A 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± in-

puts will cause the PCS Control block to assert Carrier

Sense (CRS) signal at the internal MII interface. A col-

lision would cause the PCS Control block to assert Car-

rier Sense (CRS) and Collision (COL) signals at the

internal MII. In the Link Fail state, this block would

cause the PCS Control block to de-assert 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 internal

MII and allow the PCS Control block to assert or de-as-

sert 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 internal MII. If there is RX± activity, this

block would cause the PCS Control block to assert

both COL and CRS at the internal 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 internal MII.

Jabber Function

The Jabber function inhibits the 10BASE-T twisted pair

transmit function of the Am79C978A 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

internal 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 de-assert 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 de-assert the CRS sig-

nal 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

internal MII. If there is RX± activity, this block will cause

the PCS Control block to assert both COL and CRS on

the internal 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 ig-

nored, but it will set the polarity to the correct state. The

reception of two consecutive packets will cause the po-

larity to be locked, based on the polarity of the 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 deter-

mine the abilities of the devices sharing a link. After

exchanging abilities, the Am79C978A device and re-

mote link partner device acknowledge each other and

make a choice of which advertised abilities to support.

The Auto-Negotiation function facilitates an ordered

r e s o lu tio n be twee n ex cha nge d ab il iti e s. T h is

Clock

Data

Manchester

Encoder

Clock

Data

Manchester

Decoder

Squelch

Circuit

RX Driver

RX±

TX±

TX Driver

22399A-40

AM79C978A Datasheet(PDF) 72 Page - Advanced Micro Devices

AM79C978A Datasheet(HTML) 72 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