HD6413008VF25 Datasheet, PDF (180/688 Pages) Renesas Technology Corp – Old Company Name in Catalogs and Other Documents

6. Bus Controller
6.5 Idle Cycle
6.5.1 Operation
When the H8/3008 chip accesses external space, it can insert a 1-state idle cycle (T_i) between bus
cycles in the following cases: when read accesses between different areas occur consecutively, and
when a write cycle occurs immediately after a read cycle. By inserting an idle cycle it is possible,
for example, to avoid data collisions between ROM, which has a long output floating time, and
high-speed memory, I/O interfaces, and so on.
The initial value of the ICIS1 and ICIS0 bits in BCR is 1, so that idle cycle insertion is performed
in the initial state. If there are no data collisions, the ICIS bits can be cleared.
Consecutive Reads between Different Areas: If consecutive reads between different areas occur
while the ICIS1 bit is set to 1 in BCR, an idle cycle is inserted at the start of the second read cycle.
Figure 6.18 shows an example of the operation in this case. In this example, bus cycle A is a read
cycle from ROM with a long output floating time, and bus cycle B is a read cycle from SRAM,
each being located in a different area. In (a), an idle cycle is not inserted, and a collision occurs in
bus cycle B between the read data from ROM and that from SRAM. In (b), an idle cycle is
inserted, and a data collision is prevented.
φ
Address bus
Bus cycle A Bus cycle B
T1 T2 T3 T1 T2
φ
Address bus
Bus cycle A Bus cycle B
T1 T2 T3 Ti T1 T2
RD
RD
Data bus
Data bus
Data collision
Long buffer-off time
(a) Idle cycle not inserted
(b) Idle cycle inserted
Figure 6.18 Example of Idle Cycle Operation (ICIS1 = 1)
Write after Read: If an external write occurs after an external read while the ICIS0 bit is set to 1
in BCR, an idle cycle is inserted at the start of the write cycle.
Figure 6.19 shows an example of the operation in this case. In this example, bus cycle A is a read
cycle from ROM with a long output floating time, and bus cycle B is a CPU write cycle.
Rev.4.00 Aug. 20, 2007 Page 134 of 638
REJ09B0395-0400