AM79C973 datasheet(49/304 Pages) AMD

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Am79C973/Am79C975

P R E L I M INARY

Basic Non-Burst Write Transfer

By default, the Am79C973/Am79C975 controller uses

non-burst cycles in all bus master write operations. All

Am79C973/Am79C975 controller non-burst write ac-

cesses are of the PCI command type Memory Write

(type 7). The byte enable signals indicate the byte

lanes that have valid data. The Am79C973/Am79C975

controller typically performs more than one non-burst

write transaction within a single bus mastership period.

FRAME is dropped between consecutive non-burst

write cycles. REQ, however, stays asserted until

FRAME is asser ted for the last transaction. The

Am79C973/Am79C975 supports zero wait state write

cycles except with descriptor write transfers. (See the

section Descriptor DMA Transfers for the only excep-

tion.) It asserts IRDY immediately after the address

phase.

Figure 14 shows two non-burst write transactions. The

first transaction has two wait states. The target inserts

one wait state by asserting DEVSEL one clock late and

another wait state by also asserting TRDY one clock

late. The second transaction shows a zero wait state

write cycle. The target asserts DEVSEL and TRDY in

the same cycle as the Am79C973/Am79C975 control-

ler asserts IRDY.

Basic Burst Write Transfer

The Am79C973/Am79C975 controller supports burst

mode for all bus master write operations. The burst

mode must be enabled by setting BWRITE (BCR18, bit

5). To allow burst transfers in descriptor write opera-

tions, the Am79C973/Am79C975 controller must also

be programmed to use SWSTYLE 3 (BCR20, bits 7-0).

All Am79C973/Am79C975 controller burst write trans-

fers are of the PCI command type Memory Write (type

7). AD[1:0] will both be 0 during the address phase in-

dicating a linear burst order. The byte enable signals in-

dicate the byte lanes that have valid data.

The Am79C973/Am79C975 controller will always per-

form a single burst write transaction per bus mastership

period, where transaction is defined as one address

ph as e an d o n e or mul t i p l e da ta p h a s es. T h e

Am79C973/Am79C975 controller supports zero wait

state write cycles except with the case of descriptor

write transfers. (See the section Descriptor DMA Trans-

fers for the only exception.) The device asserts IRDY

immediately after the address phase and at the same

time starts sampling DEVSEL. FRAME is deasserted

when the next to last data phase is completed.

Figure 14. Non-Burst Write Transfer

FRAME

CLK

IRDY

TRDY

C/BE

DEVSEL

REQ

GNT

PAR

DEVSEL

is sampled

ADDR

0111

PAR

2345678

DATA

ADDR

DATA

PAR

0111

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