
Chapter 3. Memory Access Protocol
3-7
Figure 3-4 shows bus parking; a qualified bus grant exists on the clock edge following a
need_bus condition eliminating the two bus clock cycles required for arbitration. The
processor negates ABB for at least one bus clock cycle after AACK is asserted, even if it is
parked and another transaction is pending. Typically, the most recent bus master remains
parked; however, system designers can choose other schemes, such as providing
unrequested bus grants in situations where it is easy to correctly predict the next device
requesting bus mastership.
Figure 3-4. Address Bus Arbitration Showing Bus Parking
When the processor receives a qualified bus grant, it assumes address bus mastership by
asserting ABB and negating the BR output signal. Meanwhile, the processor drives the
requested address onto the address bus and asserts TS to indicate the start of a new
transaction. To avoid the bus hogging these processors, always assert ABB and TS
simultaneously and negate ABB the clock cycle following assertion of AACK; however, the
processors accommodate systems in which ABB is asserted early or removed late.
When designing external bus arbitration logic, note that the processor may assert BR
without using the bus after it receives the qualified bus grant. For example, if the 604 snoops
an
access
that
cancels
the
reservation
read-with-intent-to-modify-atomic (RWITMA) operation and for which it has asserted BR,
when the 604 is granted the bus, it no longer needs to perform the RWITMA operation;
therefore, the 604 does not assert ABB and does not use the bus for the read operation.
associated
with
a
queued
The 604 asserts BR for at least one clock cycle in these instances.
-1
0
1
need_bus
BR
bg
abb
artry
qualified BG
ABB