AM79C90PC datasheet(15/62 Pages) AMD

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P R E L I M I N A R Y

AMD

Am79C90

DAL0–DAL15

DAS

READ

READY

(Output from

C-LANCE)

HOLD

ADR

17881B-10

Write Data

O.D.

Figure 4. Bus Slave Write Timing

Read Sequence (Master Mode)

A read cycle is begun by placing a valid address on

DAL00 – DAL15 and A16 – A23. The BYTE MASK sig-

nals are asserted to indicate a word, upper byte or lower

byte memory reference. READ indicates the type of cy-

cle. ALE or

AS is pulsed, and the trailing edge of either

can be used to latch addresses. DAL00 – DAL15 go into

a 3-state mode, and

DAS falls LOW to signal the begin-

ning of the memory access. The memory responds by

placing

READY LOW to indicate that the DAL lines have

valid data. The C-LANCE then latches memory data on

the rising edge of

DAS, which in turn ends the memory

cycle and

READY returns HIGH. Refer to Figure 5-1.

The bus transceiver controls,

DALI and DALO, are used

to control the bus transceivers.

DALI directs data toward

the C-LANCE, and

DALO directs data or addresses

away from the C-LANCE. During a read cycle,

DALO

goes inactive before

DALI becomes active to avoid

“spiking” of the bus transceivers.

Write Sequence (Master Mode)

The write cycle is similar to the read cycle except that the

DAL00 – DAL15 lines change from containing ad-

dresses to data after either ALE or

AS goes inactive.

After data is valid on the bus,

DAS goes active. Data to

memory is held valid after

DAS goes inactive. Refer to

Figure 5-2.

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