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G2 PowerPC Core Reference Manual
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MOTOROLA
Address Bus Tenure
latency for a transaction. The core always negates core_abb_out for at least one bus clock
cycle after core_aack is asserted, even if it is parked and has another transaction pending.
Typically, bus parking is provided to the device that was the most recent bus master.
However, system designers may choose other schemes, such as providing unrequested bus
grants in situations where it is easy to correctly predict the next device requesting bus
mastership.
When the core receives a qualified bus grant, it assumes address bus mastership by
asserting core_abb_out and negating core_br. Meanwhile, the core drives the address for
the requested access onto the address bus and asserts core_ts_out to indicate the start of a
new transaction.
Note that the core may assert core_br without using the bus after it receives the qualified
bus grant when external bus arbitration logic is designed. For example, in a system using
bus snooping, if the core asserts core_br to perform a replacement copy-back operation,
another device can invalidate that line before the core is granted mastership of the bus. In
that case, once the core is granted the bus, it no longer needs to perform the copy-back
operation; therefore, the core does not assert core_abb_out and does not use the bus for the
copy-back operation. Note that the core asserts core_br for at least 1 clock cycle in these
instances.
Figure 9-3. Address Bus Arbitration
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