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MPC7400 RISC Microprocessor Users Manual
MPX Bus Protocol
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Figure 9-22. Overlapped ARTRY and TS (with a Delayed AACK) in MPX Bus Mode
Cycle 1: The master has requested the bus and receives a qualiTed bus grant.
Cycle 2: The master begins the address tenure by driving TS and the address.
Cycle 3: The system responds with AACK, ending the address tenure. The master
receives another (parked) address bus grant.
Cycle 4: The master begins a new address tenure by driving TS and a new address.
Some snooping device, however, asserts ARTRY for the Trst transaction. Bus grant
remains parked to processor 0.
Cycle 5: The system delays AACK for the second transaction for some reason. BG0 is
negated to allow the retrying processor to request the bus. Processor 1 takes
advantage of this window of opportunity and requests the bus to perform a push of the
data that caused the retry.
Cycle 6: The system asserts AACK to terminate the second address tenure. Since the
window of opportunity has passed, processor 0 requests the address bus again to
retry its transaction. But the arbiter may NOT rearbitrate and grant the address bus to
processor 0 before the push requested in the window of opportunity.
Cycle 7: Even though this cycle would be the address retry window for the second
address tenure, no processor may assert ARTRY, because that transaction was
implicitly canceled by the ARTRY. (If AACK had not been delayed, an assertion of
ARTRY here could cause contention with the snooper that would be driving ARTRY
negated from the address retry window of the Trst address tenure.) A bus grant is
given to processor 1 to perform its push.
Cycle 8: Processor 1 begins its snoop push.
Cycle 9: The snoop push address tenure is acknowledged and terminated.
Cycle 10: The arbiter now grants processor 0 the address bus to retry its transaction.
SYSCLK
BR0
BG0
BR1
9
10
BG1
TS
AACK
ARTRY1
ADDR
1
2
3
4
5
6
7
8
Cycle