Renesas H8S/2158, User Manual

Page: 195 / 929

Section 6 Bus Controller
Rev. 3.00 Jan 25, 2006 page 141 of 872
REJ09B0286-0300
6.8 Idle Cycle
When this LSI accesses the external address space, it can insert a 1-state idle cycle (T
I
) between
bus cycles 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 with a long output floating time, and
high-speed memory and I/O interfaces.
If an external write occurs after an external read while the ICIS bit is set to 1 in BCR, an idle cycle
is inserted at the start of the write cycle.
Figure 6.20 shows examples of idle cycle operation. In these examples, bus cycle A is a read cycle
for ROM with a long output floating time, and bus cycle B is a CPU write cycle. In figure 6.20 (a),
with no idle cycle inserted, a collision occurs in bus cycle B between the read data from ROM and
the CPU write data. In figure 6.20 (b), an idle cycle is inserted, thus preventing data collision.
T
1
Address bus
φ
Bus cycle A
Data bus
T
2
T
3
T
1
T
2
Bus cycle B
Long output floating time
Data collision
(a) No idle cycle insertion
T
1
Address bus
φ
Bus cycle A
Data bus
T
2
T
3
T
I
T
1
Bus cycle B
(b) Idle cycle insertion
T
2
WR WR
RD RD
Figure 6.20 Examples of Idle Cycle Operation