
Chapter 8. Signal Descriptions
8-27
MPX Bus Signal ConTguration
8.4.2.1 Bus Request (BR)Output
Following are the state meaning and timing comments for the BR output signal on the
MPC7400 in MPX bus mode.
State Meaning
Asserted
Same as 60x bus interface
Negated
Same as 60x bus interface
Timing Comments
Assertion
Same as 60x bus interface
Negation
Note that BR is negated during the cycle in which the
processor is asserting TS unless the processor has another pending
transaction to perform in MPX bus mode.
High ImpedanceSame as 60x bus interface
8.4.2.2 Bus Grant (BG)Input
Following are the state meaning and timing comments for the BG output signal on the
MPC7400 in MPX bus mode.
State Meaning
Asserted
Indicates that the MPC7400 may, with the proper
qualiTcation, begin a bus transaction. A qualiTed bus grant is
determined from the bus state as follows:
QBG = BG ¥
ARTRY ¥
TS ¥
(latched state variables)
Negated
Indicates that the MPC7400 is not granted next address
bus ownership.
Timing Comments
Assertion
May occur on any cycle.
NegationMay occur whenever the MPC7400 must be prevented
from starting a bus transaction. The MPC7400 may still assume
address bus ownership on the cycle BG is negated if BG was asserted
the previous cycle with other bus grant qualiTcations. Negation must
occur in every cycle the arbiter delays AACK. Since AACK is not in
the qualiTed bus grant equation and ABB is not generated by the
MPC7400 in MPX bus mode, the bus arbiter must negate BG in
every cycle the arbiter is delaying AACK in order to prevent a
qualiTed bus grant.
8.4.2.3 Address Bus Monitor (AMON)Output
The address bus monitor (AMON) signal is strictly optional in the MPX bus protocol.
Following are the state meaning and timing comments for AMON.
State Meaning
AssertedSame as 60x bus interface ABB signal
NegatedSame as 60x bus interface ABB signal