AM79C970A datasheet(87/219 Pages) AMD

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P R E L I M I N A R Y

AMD

Am79C970A

transmitting, receiving, or in a collision state with both

functions active simultaneously. These signals are in-

ternal signals that can be programmed to appear on any

of the LED output pins. Programming is done by writing

to BCR4 to BCR7.

In the Link Fail state, XMT, RCV and COL are inactive.

Collision Detection Function

Activity on both twisted pair signals RXD

± and TXD± at

the same time constitutes a collision, thereby causing

the internal COL signal to be activated. COL will remain

active until one of the two colliding signals changes from

active to idle. However, transmission attempt in Link Fail

state results in LCAR and CERR indication. COL stays

active for 2 bit times at the end of a collision.

Signal Quality Error Test Function

The Signal Quality Error (SQE) test function (also called

Heartbeat) is disabled when the 10BASE-T port

is selected.

Jabber Function

The Jabber function prevents the twisted pair transmit

function of the T-MAU TXD

± from being active for an ex-

cessive period of time (20 ms to 150 ms). This prevents

any one node from disrupting the network due to a

“stuck-on” or faulty transmitter. If this maximum transmit

time is exceeded, the T-MAU transmitter circuitry is

disabled, the JAB bit is set (CSR4, bit 1) and the COL

signal is asserted. Once the transmit data stream is re-

moved, the T-MAU waits an “unjab” time of 250 ms to

750 ms before it deasserts COL and re-enables the

transmit circuitry.

Power Down

The T-MAU circuitry can be made to go into a power

savings mode. The T-MAU will go into the power down

mode when H_RESET is active, when coma mode is ac-

tive, or when the T-MAU is not selected. Refer to the

section “Power Savings Modes” for descriptions of the

various power down modes.

Any of the three conditions listed above resets the inter-

nal logic of the T-MAU and places the device into power

down mode. In this mode, the Twisted Pair driver pins

(TXD

±, TXP±) are driven LOW, and the internal T-MAU

status signals (

LNKST, RCVPOL, XMT, RCV and COL)

signals are inactive.

After coming out of the power down mode, the T-MAU

will remain in the reset state for an additional 10

µs. Im-

mediately after the reset condition is removed, the

T-MAU will be forced into the Link Fail state. The T-MAU

will move to the Link Pass state only after 5–6 link beat

pulses and/or a single received message is detected on

the RD

± pair.

In snooze mode, the T-MAU receive circuitry will remain

enabled even while the

SLEEP pin is driven LOW.

10BASE-T Interface Connection

The figure below shows the proper 10BASE-T network

interface design. Refer to Appendix A for a list of com-

patible 10BASE-T filter/transformer modules.

Note that the recommended resistor values and filter

and transformer modules are the same as those used by

the IMR+ (Am79C981).

XMT

Filter

RCV

Filter

RJ45

Connector

Filter &

Transformer

Module

TXP+

TXD-

TXP-

TXD+

RXD+

RXD-

PCnet–PCI II

TD+

TD-

RD+

RD-

61.9

Ω

422

Ω

61.9

Ω

422

Ω

100

Ω

1.21 K

Ω

1:1

19436A-41

Figure 38. 10BASE-T Interface Connection

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