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Am79C976
8/01/00
P R E L I M I N A R Y
-
The Am79C976 controller supports burst mode for all
bus master write operations. To allow burst transfers in
descriptor write operations, the Am79C976 controller
must be programmed to use SWSTYLE 3, 4, or 5
(BCR20, bits 7-0).
The controller uses the following rules to determine
whether to use the PCI Memory Write (MW) command
or the Memory Write and Invalidate (MWI) command
for burst write transfers.
—
When a transfer starts on a cache line boundary,
and there is at least a cache line of data to trans-
fer, use MWI.
—
When a transfer does not start on a cache line
boundary, use MW. (The external PCI bridge
should stop the transfer at the cache line bound-
ary if it can make good use of the MWI com-
mand.)
—
Stop the MWI transfer at a cache line boundary
if there is less than 1 cache line of data left to
transfer.
The Receive FIFO threshold should be greater than or
equal to the cache line size to maximize the use of the
MWI command. If the PCI bridge stops a transfer, the
Am79C976 device waits until the FIFO threshold con-
ditions are met before resuming the transfer.
During the address phase of a burst write transfer
AD[1:0] will both be 0 indicating a linear burst order.
The byte enable signals indicate which byte lanes have
valid data.
The Am79C976 controller will always perform a single
burst write transaction per bus mastership period,
where transaction is defined as one address phase and
one or multiple data phases. The Am79C976 controller
supports zero wait state write cycles when using the
Memory Write command. When using Memory Write
and Invalidate commands, the device may insert IRDY
wait states anywhere in the transaction.
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 shows a typical burst write access. The
Am79C976 controller arbitrates for the bus, is granted
access, and writes four 32-bit words (DWords) to the
system memory and then releases the bus. In this ex-
FRAME
CLK
AD
IRDY
TRDY
C/BE
DEVSEL
REQ
GNT
PAR
DEVSEL is sampled
ADDR
0111
PAR
1
2
3
4
5
6
7
8
10
9
DATA
ADDR
DATA
PAR
PAR
PAR
BE
0111
BE
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