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8.10 Bus Arbitration
The SuperH microcomputer can release the bus to external devices when they request the bus. It
has two internal bus masters, the CPU and the DMAC. Priorities for releasing the bus for these
two are as follows.
Bus request from external device > refresh > DMAC > CPU
Thus, an external device has priority when it generates a bus request, even when the DMAC is
carrying out a burst transfer.
Note that when a refresh request is generated while the bus is released to an external device,
BACK goes high and the bus can be acquired to perform refreshing upon receipt of a BREQ =
high response from the external device. Input all bus requests from external devices to the BREQ
pin. The signal indicating that the bus has been released is output from the BACK pin. Figure 8.35
illustrates the bus release procedure.
Bus released
BREQ received
Bus acquisition
BREQ = low
BACK = low
acknowledgeBACK
Bus request
External device
SuperH
Bus release response
Address, data, strobe pin:
High impedance
Strobe pin:
High-level output
Figure 8.35 Bus Release Procedure
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