AMD

PRELIMINARY

1–25

Am79C982

Note: When used in minimum mode.

19406B-13

X1

RST

SHCK

1

2

12

16

20

CRS

AUI

TP1

TP0

24

28

32

10

TP3

TP2

STR

AUI

TP1

TP0

TP3

TP2

SO

(Note)

AUI

AUI

Figure 6. bIMR4 Management Port Minimum Mode and Port Activity Monitor Signal Relationship

Port Activity Monitor

Two pins, CRS and STR, are used to serially output the

state of the internal Carrier Sense signals from the AUI

and the eight (four) TP ports. This function together with

external hardware and/or software can be used to moni-

tor repeater receive and/or collision activity.

The resolution of the CRS signal is 2 ms. The incoming

data is sampled repeatedly during each 2 ms period. If

any activity occurs (regardless of length) during any

2 ms period, this activity will be latched. At the start of

the next 2 ms period, the bIMR device will examine the

latches for each port. For any port in which activity

occurred, the corresponding bit in the CRS output

stream will remain set for the 2 ms period. This means

that during any 2 ms time interval the CRS output bit

stream represents carrier activity that occurred in the

preceding 2 ms period (see Figure 7). During the last

1000 ns of the 2 ms period, the CRS signal is reset to

LOW. Figure 7 illustrates this by showing the output of

the register in the recommended LED drive circuitry of

Figure 8.

The CRS pin is used to indicate carrier sense for all nine

ports of the device (five for the Am79C982-4). This pin

outputs a ten bit stream that repeats every microsecond.

During this period there are ten bit times (100 ns). Each

port has a “time slot” in this repeating bit stream (see

Figure 4). For example, activity on the AUI port is repre-

sented by the state of the CRS pin during the second

100 ns period of the one microsecond cycle.

Because the one microsecond sequence is repeated

unchanged for most of the longer 2 ms cycle, any LED

driven by the latch and shift register shown in Figure 3

and 5 will remain on for at least 2 ms. This minimizes the

need for external pulse stretching logic.

1000 ns

2 ms

TPX

or AUI

CRS

19406B-14

Figure 7. Carrier Sense Signal Output Corresponding to the

States of AUI or Twisted-Pair Port Activity