AM79C970 datasheet(60/168 Pages) AMD

60 / 168 page

P R E L I M I N A R Y

AMD

1-927

Am79C970

Collision Detect Function

Activity on both twisted pair signals RXD

± and TXD±

constitutes a collision, thereby causing the COL signal

to be activated. (COL is used by the LED control circuits)

COL will remain active until one of the two colliding sig-

nals changes from active to idle. However, transmission

attempt in Link Fail state results in LCAR and CERR in-

dication. COL stays active for 2 bit times at the end of

a collision.

Signal Quality Error (SQE) Test

(Heartbeat) Function

The SQE function is disabled when the 10BASE-T port

is selected.

Jabber Function

The Jabber function inhibits the twisted pair transmit

function of the T-MAU TXD

± is active for an excessive

period (20 ms – 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) the COL signal is asserted. Once the

transmit data stream to the T-MAU is removed, an “un-

jab” time of 250 ms – 750 ms will elapse before the

T-MAU COL and re-enables the transmit circuitry.

Power Down

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

ings mode. This feature is useful in battery powered or

low duty cycle systems. The T-MAU will go into power

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

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

Savings Modes section 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.

Once H_RESET ends, coma mode is disabled, and the

T-MAU is selected. The T-MAU will remain in the reset

state for up to 10

µs. Immediately after the reset condi-

tion 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.

The T-MAU circuitry will always go into power down

mode if H_RESET is asserted, coma mode is enabled,

or the T-MAU is not selected.

10BASE-T Interface Connection

Figure 26 shows the proper 10BASE-T network inter-

face design. Refer to the

PCnet Family Technical Man-

ual (PID #18216A) for more design details, and refer to

Appendix A for a list of compatible 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 (Am79C980) and the IMR+ (Am79C981).

18220C-28

XMT

Filter

RCV

Filter

RJ45

Connector

Filter &

Transformer

Module

TXP+

TXD-

TXP-

TXD+

RXD+

RXD-

PCnet-PCI

TD+

TD-

RD+

RD-

61.9

Ω

422

Ω

61.9

Ω

422

Ω

100

Ω

1.21 K

Ω

1:1

Figure 26. 10BASE-T Interface Connection

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