MOTOROLA
Chapter 14. MPC860 External Bus Interface
14-29
Part IV. Hardware Interface
Figure 14-24. Internal Bus Arbitration State Machine
14.4.6.4 External Bus Parking
During external arbitration, the MPC860 supports bus parking. If the MPC860 detects that
an external arbiter has asserted BG to it and BB is negated, the MPC860 starts a transfer
without asserting BR.
14.4.7 Address Transfer Phase-Related Signals
The following sections describe the address transfer phase-related signals.
14.4.7.1 Transfer Start
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 Þrst 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 Figure 14-22.
needs the bus
External owner
BG = 0
BB = TS
Idle
BG = 0
BB = TS
BB = 1, BR
= 1
BR = 1
BR
= 0
860 bus wait
BG = 1
BB = TS
860 owner
BG = 1
BB = 0
no longer
needs the bus
BB = 1
BB = 0
External master
release bus
Internal master
with higher priority than the
external device requires
the bus
still
needs the bus
BR = 0
External device which
has higher priority than the
current internal bus master
requests the bus
External master
requests bus
MPC860
MPC860
MPC860
MPC860
BB = 0
Summary of Contents for MPC860 PowerQUICC
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