
MOTOROLA
Chapter 9. External Bus Interface
9-7
Bus Interface Signal Descriptions
BI
Burst inhibit
1
Low
I
Driven by the slave device to which the current
transaction was addressed. Indicates that the current
slave does not support burst mode.
O
Driven by the MPC565/MPC566 when the slave
device is controlled by the on-chip memory controller.
the MPC565/MPC566 also asserts BI for any external
master burst access to internal MPC565/MPC566
memory space.
Arbitration
BR
Bus request
1
Low
I
When the internal arbiter is enabled, BR assertion
indicates that an external master is requesting the bus.
O
Driven by the MPC565/MPC566 when the internal
arbiter is disabled and the chip is not parked.
BG
Bus grant
1
Low
O
When the internal arbiter is enabled, the
MPC565/MPC566 asserts this signal to indicate that
an external master may assume ownership of the bus
and begin a bus transaction. The BG signal should be
qualified by the master requesting the bus in order to
ensure it is the bus owner:
Qualified bus grant = BG &~ BB
I
When the internal arbiter is disabled, BG is sampled
and properly qualified by the MPC565/MPC566 when
an external bus transaction is to be executed by the
chip.
BB
Bus busy
1
Low
O
When the internal arbiter is enabled, the
MPC565/MPC566 asserts this signal to indicate that it
is the current owner of the bus.
When the internal arbiter is disabled, the
MPC565/MPC566 asserts this signal after the external
arbiter has granted the ownership of the bus to the
chip and it is ready to start the transaction.
I
When the internal arbiter is enabled, the
MPC565/MPC566 samples this signal to get indication
of when the external master ended its bus tenure (BB
negated).
When the internal arbiter is disabled, the BB is
sampledtoproperly qualifythe BG line when an
external bus transaction is to be executed by the chip.
RETRY
1
Low
I
In the case of regular transaction, this signal is driven
by the slave device to indicate that the
MPC565/MPC566 must relinquish the ownership of
thebus andretry thecycle.
O
When an external master owns the bus and the
internal MPC565/MPC566 bus initiates access to the
external bus at the same time, this signal is used to
cause the external master to relinquish the bus for one
clock to solve the contention.
Table 9-1. MPC565/MPC566 SIU Signals (continued)
Signal Name
Pins
Active
I/O
Description