Chapter 9. System Interface Operation
9-23
60x Address Bus Tenure
devices negate address bus requests, and they do not qualify address bus grants. This cycle
is the window of opportunity for the snooping master that asserted ARTRY and needs to
perform a snoop copyback operation. Thus, the snooping master that asserted ARTRY is
the only device allowed to assert BR. Note that a nonclocked bus arbiter may detect the
assertion of address bus request by the bus master that asserted ARTRY, and return a
qualiTed bus grant one cycle earlier than shown in Figure 9-7.
When the MPC7400 asserts ARTRY due to a snoop operation and is ready to perform the
snoop push, it always asserts BR in the window of opportunity to obtain bus mastership for
the copyback cycle. (A copyback operation due to a snoop hit to a modiTed block is
sometimes referred to as a snoop push.) Note that the copyback is a non-global (GBL
negated) transaction. External devices on the 60x bus must not assert ARTRY for
non-global transactions.
Note that even if the MPC7400 asserts BR in the window of opportunity for a snoop push,
it may be several bus cycles later before the MPC7400 is able to perform the necessary
transaction. The timing of TS may be dependent on resource constraints and may require
forward progress on the data bus. The bus arbiter should keep BG asserted until it detects
that BR is negated or TS is asserted from the MPC7400 indicating that the snoop copyback
has begun. The system should ensure that no other address tenures occur until the current
snoop push from the MPC7400 is completed.
It may occur in some systems that the MPC7400 was unable to perform a pending snoop
copyback when a new snoop operation is performed. In this case, the MPC7400 requests
the bus in the window of opportunity if it hits on the new snooped address, and it performs
the snoop copyback operation for the earlier snooped address rather than for the current
snooped address in order to clear its internal snoop queue.
Figure 9-7. Snooped Address Cycle with ARTRY
5
6
7
8
SYSCLK
ABB
ADDR
ARTRY
AACK
TS
BR
qual BG
ABB
4
3
2
1