Motorola MPC8260 PowerQUICC II, User Manual

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MOTOROLA Chapter 8. The 60x Bus 8-23
Part III. The Hardware Interface
8.4.4 Address Transfer Termination
Address transfer termination occurs with the assertion of the address acknowledge (AACK)
signal, or retried with the assertion of ARTRY. ARTRY must remain asserted until one
clock after AACK; the bus clock cycle after AACK is called the ARTRY window. The
MPC8260 controls assertion of AACK unless the cycle is claimed by an external slave, such
as an external L2 cache controller. Following the assertion of L2_HIT, the L2 cache
controller is responsible for asserting AACK. When AACK is asserted by the external slave,
it should be asserted for one clock cycle and then negated for one clock cycle before
entering a high-impedance state. The MPC8260 holds AACK in a high-impedance state
until it is required to assert AACK to terminate the address cycle.
The MPC8260 uses AACK to enforce a pipeline depth of one to its internal slaves.
NOTE
If the MPC8260 processor clock is conÞgured for 1x or 1.5x
clock mode, the ARTRY snoop response cannot be determined
in the minimum allowed address tenure period. For this clock
mode, AACK must not be asserted to the chip until at least the
third clock of the address tenure (one address wait state) to give
the processor time to assert ARTRY on the bus. For the other
clock conÞguration modes, the ARTRY snoop response can be
determined in the minimum address tenure period, and AACK
may be asserted as early as the second bus clock of the address
tenure (zero address wait states).
8.4.4.1 Address Retried with ARTRY
The address transfer can be terminated with the requirement to retry if ARTRY is asserted
during the address tenure and through the cycle following AACK. The assertion causes the
entire transaction (address and data tenure) to be rerun. As a snooping device, the MPC8260
processor asserts ARTRY for a snooped transaction that hits modiÞed data in the data cache
that must be written back to memory, or if the snooped transaction could not be serviced.
As a bus master, the MPC8260 responds to an assertion of ARTRY by aborting the bus
transaction and requesting the bus again, as shown in Figure 8-7. Note that after
recognizing an assertion of ARTRY and aborting the current transaction, the MPC8260
may not run the same transaction the next time it is granted the bus.