SH7708 Datasheet, PDF (313/633 Pages) Renesas Technology Corp – SuperH™ RISC engine

10.3.9 Waits between Access Cycles
A problem associated with higher external memory bus operating frequencies is that data buffer
turn-off on completion of a read from a low-speed device may be too slow, causing a collision
with data in the next access. This results in lower reliability or incorrect operation. To avoid this
problem, a data collision prevention feature has been provided. This memorizes the preceding
access area and the kind of read/write. If there is a possibility of a bus collision when the next
access is started, a wait cycle is inserted before the access cycle thus preventing a data collision.
There are two cases in which a wait cycle is inserted: when an access is followed by an access to a
different area, and when a read access is followed by a write access from the SH7708 Series.
When the SH7708 Series performs consecutive write cycles, the data transfer direction is fixed
(from the SH7708 Series to other memory) and there is no problem. With read accesses to the
same area, in principle, data is output from the same data buffer, and wait cycle insertion is not
performed. Bits AnIW1 and AnIW0 (n = 0–6) in WCR1 specify the number of idle cycles to be
inserted between access cycles when a physical space area access is followed by an access to
another area, or when the SH7708 Series performs a write access after a read access to physical
space area n. If there is originally space between accesses, the number of idle cycles inserted is the
specified number of idle cycles minus the number of empty cycles.
Waits are not inserted between accesses when bus arbitration is performed, since empty cycles are
inserted for arbitration purposes.
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