
Chapter 9. System Interface Operation
9-39
MPX Bus Protocol
9.6.1.1.1 QualiTed Bus Grant in MPX Bus Mode
In MPX bus mode, the MPC7400 has a different equation from 60x bus mode for the
qualiTcation of BG. The internally-generated address bus busy (
abb
) is not used to qualify
BG. This removes the timing dependence of a qualiTed bus grant on the AACK signal,
which is used to terminate the internally-generated
abb
in 60x bus mode. However, this
means that in MPX bus mode the system arbiter must not assert BG to a second master until
the cycle after AACK is asserted to end an address tenure from the Trst master. Another
consequence of removing the dependence on the internally-generated
abb
is that BG must
be negated in every cycle that AACK is delayed from the clock cycle after the assertion of
TS. The TS signal is still used to qualify bus grant in order to optimize speculative bus
parking.
To gain access to the address bus, a bus master asserts BR and holds it asserted until it
detects a qualiTed bus grant (QBG). The equation for a qualiTed bus grant for the MPC7400
in MPX bus mode is:
QBG = BG & ARTRY & TS & (latched state variables)
where òl(fā)atched state variablesó include latched ARTRY. Thus, a qualiTed bus grant occurs
when BG is asserted, ARTRY is not asserted in the current or the preceding cycle, and TS
is not asserted by this or any other processor.
A typical arbitration sequence is illustrated in Figure 9-19. When the MPC7400 needs to
perform a bus access, it asserts BR to the arbiter (non-parked case). Xs in the Tgure mark
the values of the signals that allow a qualiTed bus grant.
Figure 9-19. MPX Bus Address Bus ArbitrationNon-Parked Case
Arbiter designs must ensure that no more than one address bus master can be granted the
bus at one time (that is, bus grants must be mutually exclusive). In single-master
applications, BG can effectively be tied asserted, always granting the bus (called bus
parking). However, as explained above, BG must be negated in every cycle that the arbiter
delays AACK.
SYSCLK
1
2
3
0
need_bus
BR
BG
ARTRY
TS
Arbitrate
Assume control of bus