External Bus Interface
MOTOROLA
MPC823e REFERENCE MANUAL
13-29
EXTERNAL BUS
13
INTERFACE
13.4.6.2 BUS GRANT SIGNAL.
The BG
signal is asserted by the arbiter to indicate that the
bus is granted to the requesting device. The BG
signal can be negated after BR is negated.
The current bus master may choose to keep BG
asserted to park the bus and maintain
ownership without rearbitrating until another master makes a request. This reduces
arbitration time, which then improves performance. When configured for external central
arbitration, the BG
becomes an input signal to the MPC823e from the external arbiter. When
the internal on-chip arbiter is used, this signal is an output from the internal arbiter to the
external bus master.
13.4.6.3 BUS BUSY SIGNAL.
The BB
signal indicates that the current bus master is using
the bus. New masters must not begin transferring until this signal is negated. The bus owner
must not relinquish or negate this signal until its transfer is complete. To avoid contention on
the BB
signal, masters must three-state this signal when it gets a logical 1 value. This
situation implies that the connection of an external pull-up resistor is needed to ensure that
a master acquiring the bus recognizes that the BB
signal is negated, regardless of how
many cycles have passed since the previous master relinquished the bus. Refer to
Figure 13-21 for more information.
Figure 13-21. Basic Bus Busy Connection
EXTERNAL BUS
MPC823e
SLAVE 2
MASTER
TS
BB
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