Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 403

13.4.6.4 External Bus Parking
During external arbitration, the MPC885 supports bus parking. If the MPC885 detects that an external
arbiter has asserted BG to it and BB is negated, the MPC885 starts a transfer without asserting BR.
13.4.7 Address Transfer Phase-Related Signals
The following sections describe the address transfer phase-related signals.
13.4.7.1 Transfer Start (TS)
The transfer start signal (TS) indicates the beginning of a transaction on the bus addressing a slave device.
A device should assert TS only after the arbitration protocol has granted mastership. TS is asserted only
for the first cycle of the transaction and is negated in the successive clock cycles until the end of the
transaction. To avoid contention, the master should three-state this signal when it relinquishes the bus. This
situation indicates that an external pull-up resistor should be connected to TS to avoid having a slave
recognize this signal as asserted when no master drives it; see
Freescale Semiconductor
BB = 0
External master
requests bus
BR = 1
External master
release bus
Idle
BB = three-state
BG = 1
MPC885
needs the bus
MPC885
no longer
needs the bus
BR = 0
External device which
has higher priority than the
current internal bus master
requests the bus
Figure 13-24. Internal Bus Arbitration State Machine
MPC885 PowerQUICC Family Reference Manual, Rev. 2
External owner
BB = three-state MPC885
Internal master
BG = 0
with higher priority than the
external device requires
the bus
BB = three-state
BB = 1
885 owner
BG = 1
BB = 0
MPC885
still
needs the bus
Figure 13-22.
External Bus Interface
BB = 0
885 owner
BG = 1
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