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User’s Manual U19780EJ2V0UD
656
CHAPTER 30 CAUTIONS FOR WAIT
30.1 Cautions for Wait
This product has two internal system buses.
One is a CPU bus and the other is a peripheral bus that interfaces with the low-speed peripheral hardware.
Because the clock of the CPU bus and the clock of the peripheral bus are asynchronous, unexpected illegal data
may be passed if an access to the CPU conflicts with an access to the peripheral hardware.
When accessing the peripheral hardware that may cause a conflict, therefore, the CPU repeatedly executes
processing, until the correct data is passed.
As a result, the CPU does not start the next instruction processing but waits. If this happens, the number of
execution clocks of an instruction increases by the number of wait clocks (for the number of wait clocks, see
Table 30-
1
). This must be noted when real-time processing is performed.
30.2 Peripheral Hardware That Generates Wait
Table 30-1 lists the registers that issue a wait request when accessed by the CPU, and the number of CPU wait
clocks.
Table 30-1. Registers That Generate Wait and Number of CPU Wait Clocks
Peripheral Hardware
Register
Access
Number of Wait Clocks
Serial interface UART6
ASIS6
Read
1 clock (fixed)
Serial interface IICA
IICAS0
Read
1 clock (fixed)
Caution When the peripheral hardware clock (f
PRS
) is stopped, do not access the registers listed above
using an access method in which a wait request is issued.
Remark
The clock is the CPU clock (f
CPU
).