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Am79C976
8/01/00
P R E L I M I N A R Y
-
The FIFO thresholds should be greater than or equal to
the cache line size to maximize the use of the MRL and
MRM commands. 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 access, AD[1:0]
will both be 0 indicating a linear burst order. Note that
during a burst read operation, all byte lanes will always
be active. The Am79C976 controller will internally dis-
card unneeded bytes.
The Am79C976 controller will always perform only a
single burst read transaction per bus mastership pe-
riod, where
transaction
is defined as one address
phase and one or multiple data phases. The
Am79C976 controller supports zero wait state read cy-
cles. It 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.
The device may insert IRDY wait states in the middle of
a burst read transaction.
Figure 12 shows a typical burst read access. The
Am79C976 controller arbitrates for the bus, is granted
tem memory, and then releases the bus. In the exam-
ple, the memory system extends the data phase of
each access by one wait state.
The Am79C976 controller uses non-burst cycles to
write descriptors when SWSTYLE (BCR20, bits 7-0) is
0 or 2. All Am79C976 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 Am79C976 controller
may perform 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 asserted
for the last transaction. The Am79C976 supports zero
wait state write cycles. (See the section
Descriptor
DMA Transfers
for the only exception.) It asserts IRDY
immediately after the address phase.
Figure 13 shows two non-burst write transactions. The
first transaction has two wait states. The Am79C976
device supports zero wait state non-burst write cycles.
FRAME
CLK
AD
IRDY
TRDY
C/BE
DEVSEL
REQ
GNT
PAR
DEVSEL is sampled
ADDR
0110
PAR
1
2
3
4
5
6
7
8
10
9
11
DATA
ADDR
DATA
PAR
PAR
PAR
0000
0110
0000
22929B13