SH7760 Datasheet, PDF (408/1345 Pages) Renesas Technology Corp – SuperHTM RISC engine

The right of access to the bus is transferred at the end of bus cycles.
When the bus release request signal (BREQ) is asserted, this LSI releases the bus as soon as the
current bus cycle being executed ends and outputs the bus request acknowledge signal (BACK).
However, the bus is not released during multiple bus cycles generated due to a smaller data bus
width than the access size (such as when performing longword access to 8-bit bus width memory)
or during a 32-byte transfer such as a cache fill or write-back. The bus is also not released between
read and write cycles during execution of a TAS instruction, or between read and write cycles
when DMAC dual address transfer is executed. When BREQ is negated, BACK is negated and use
of the bus is resumed.
When a refresh request is generated, this LSI performs a refresh operation as soon as the current
bus cycle being executed ends. However, refresh operations are deferred during multiple bus
cycles generated due to a smaller data bus width than the access size (such as when performing
longword access to 8-bit bus width memory) and during a 32-byte transfer such as a cache fill or
write-back. The refresh operation is also deferred between read and write cycles during execution
of a TAS instruction, and between read and write cycles when DMAC dual address transfer is
executed. Refresh operations are also deferred in the bus release state.
Since the CPU in this LSI is connected to cache memory by a dedicated peripheral bus, the CPU
can still read from cache memory when the bus is being used by another bus master inside or
outside this LSI. When writing from the CPU, an off-chip write cycle is generated when write-
through has been set for the cache in this LSI, or when an access is made to a cache-off area. This
results in a delay until the bus is returned.
When this LSI wants to take back the bus in response to an internal memory refresh request, it
negates BACK. A device that asserts the off-chip bus release request receives the BACK negation,
and then negates BREQ to release the bus. In this way, the bus is returned to this LSI, and then
processing is performed.
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