CHAPTER 4 BUS CONTROL FUNCTION
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User’s Manual U15195EJ5V0UD
4.6.2 External wait function
When an extremely slow device, an I/O, or an asynchronous system is connected, an arbitrary number of wait
states can be inserted in the bus cycle by the external wait pin (WAIT) for synchronization with the external device.
Just as with programmable waits, accessing internal ROM, internal RAM, and on-chip peripheral I/O areas cannot
be controlled by external waits.
The external WAIT signal can be input asynchronously to CLKOUT and is sampled at the falling edge of the
CLKOUT signal in the T2 and TW states of the bus cycle. If the setup/hold time is not satisfied within the sampling
timing, a wait state may or may not be inserted in the next state.
4.6.3 Relationship
between
programmable wait and external wait
A wait cycle is inserted as the result of an OR operation between the wait cycles specified by the set value of the
programmable wait and the wait cycles controlled by the WAIT pin.
Wait control
Programmable wait
Wait by WAIT pin
For example, if the timings of the programmable wait and the WAIT pin signal are as illustrated below, three wait
states will be inserted in the bus cycle.
Figure 4-2. Example of Wait Insertion
CLKOUT
WAIT pin
Wait from WAIT pin
Programmable wait
Wait control
T2
TW
TW
TW
T3
Remark
The circles indicate the sampling timing.
Содержание PD703114
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