H8SSLASH2225 datasheet(281/1071 Pages) RENESAS

281 / 1071 page

Section 7 Bus Controller

Rev. 5.00 Aug 08, 2006 page 195 of 982

REJ09B0054-0500

(2) Write after Read

If an external write occurs after an external read while the ICIS0 bit in BCRH is set to 1, an

idle cycle is inserted at the start of the write cycle.

Figure 7.22 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.

In (a), an idle cycle is not inserted, and a collision occurs in cycle B between the read data

from ROM and the CPU write data. In (b), an idle cycle is inserted, and a data collision is

prevented.

Address bus

φφ

Bus cycle A

Data bus

Bus cycle B

Long output floating time

Data collision

(a) Idle cycle not inserted

(ICIS0 = 0)

Address bus

Bus cycle A

Data bus

Bus cycle B

(b) Idle cycle inserted

(Initial value ICIS0 = 1)

CS (area A)

CS (area B)

HWR

CS (area A)

CS (area B)

Figure 7.22 Example of Idle Cycle Operation (2)

(3) Relationship between Chip Select (

CS) Signal and Read (RD

RD) Signal

Depending on the system’s load conditions, the

RD signal may lag behind the CS signal. An

example is shown in figure 7.23.

In this case, with the setting for no idle cycle insertion (a), there may be a period of overlap

between the bus cycle A

RD signal and the bus cycle B CS signal.

Setting idle cycle insertion, as in (b), however, will prevent any overlap between the

RD and

CS signals.

In the initial state after reset release, idle cycle insertion (b) is set.

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