HD6417706 Datasheet, PDF (285/709 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer Super RISC engine Family/SH7700 Series

8.5.7 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 this LSI. When this LSI
performs consecutive write cycles, the data transfer direction is fixed (from this LSI 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, 2 to 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 this LSI 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.
CKIO
A25 to A0
T1
T2
Twait
T1
T2
Twait
T1
T2
RD/
D31 to D0
Area m read
Area n space read
Area n space write
Area m inter-access wait specification
Area n inter-access wait specification
Figure 8.39 Waits between Access Cycles
Rev. 4.00, 03/04, page 239 of 660