
MOTOROLA
Chapter 9. External Bus Interface
9-7
Bus Interface Signal Descriptions
BI / STS
Burst inhibit/
Special Transfer Start
1Low
I
Burst Inhibit: Driven by the slave device to which the
current transaction was addressed. Indicates that the
current slave does not support burst mode.
O
Burst Inhibit: Driven by the MPC561/MPC563 when
the slave device is controlled by the on-chip Memory
Controller. The MPC561/MPC563 also asserts BI for
any external master burst access to internal
MPC561/MPC563 memory space.
Special Transfer Start: Driven by the
MPC561/MPC563 when it owns the external bus.
Indicates the start of a transaction on the external bus
or signals the beginning of an internal transaction in
show cycle mode.
Arbitration
BR
Bus request
1Low
I
When the internal arbiter is enabled, BR assertion
indicates that an external master is requesting the bus.
O
Driven by the MPC561/MPC563 when the internal
arbiter is disabled and the chip is not parked.
BG
Bus grant
1Low
O
When the internal arbiter is enabled, the
MPC561/MPC563 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 MPC561/MPC563 when
an external bus transaction is to be executed by the
chip.
BB
Bus busy
1Low
O
When the internal arbiter is enabled, the
MPC561/MPC563 asserts this signal to indicate that it
is the current owner of the bus.
When the internal arbiter is disabled, the
MPC561/MPC563 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
MPC561/MPC563 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
sampled to properly qualify the BG line when an
external bus transaction is to be executed by the chip.
Table 9-1. MPC561/MPC563 BIU Signals (continued)
Signal Name
Pins
Active
I/O
Description