Chapter 9. System Interface Operation
9-45
MPX Bus Protocol
Note that the snoop response in MPX bus mode is composed of the ARTRY signal, the
shared (SHD[0:1]) signals, and the HIT signal. These signals must be driven in the address
retry window (as deTned for 60x bus interface) and remain asserted until the end of the
window. See Section 9.3.3.1, òAddress Retry Window and QualiTed ARTRY,ó for more
information.
9.6.1.4.1 Address Retry (ARTRY) in MPX Bus Mode
The following list identiTes the cases in which ARTRY is asserted in the address retry
window (when using MPX bus mode):
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No device on the bus has the required buffer space to handle the transaction.
A device has a transient pipeline collision.
A snooping device must push modiTed data to maintain coherency and data
intervention is not enabled (L1_INTVEN and L2_INTVEN bits cleared in
MSSCR0).
A snooping device must push modiTed data to maintain coherency and data
intervention is enabled but intervention is not possible for this transaction because
the transaction is not for a full cache-line (detected by WT or CI asserted).
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Note that in some instances, a single master may assert both ARTRY and HIT in response
to a snooped transaction. In this case, the ARTRY has precedence and the HIT signal is
ignored.
ARTRY assertions in MPX bus mode can have additional implications because the
MPC7400 allows new address tenures to begin without a dead cycle in between. The
MPC7400 allows a new address tenure from the same master to begin the cycle after
AACK, which overlaps the address retry window of the previous address tenure. If this
happens, the system and all bus devices must recognize that the second TS is implicitly
retried as well. As with the 60x interface, however, ARTRY does not affect the termination
of an address tenureaddress tenures are only terminated by AACK. Therefore, even if an
address tenure is to be retried by ARTRY for the previous address tenure, AACK is asserted
to terminate the second address tenure (shown in Figure 9-22).
Note that in Figure 9-22, the AACK for the second address tenure is delayed by a clock
cycle. This is to demonstrate how a system must handle a delayed AACK overlapping the
window of opportunity after a retry. In this case the address bus grant for the snoop push
requested in the window of opportunity must be delayed until at least the cycle after AACK.
This may be required to avoid critical timing conicts between ARTRY and AACK. Note
that the grant could be delayed further to avoid any critical timing conicts between AACK
and the bus grant, if necessary. In any case, if the address tenure of the second transaction
extends past the window of opportunity after the assertion of ARTRY, the arbiter must not
rearbitrate and grant the address bus to any device that may have requested the bus after the
window of opportunity but before the address tenure for the snoop push.