CHAPTER 4 BUS CONTROL FUNCTION
User’s Manual U12688EJ4V0UM00
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4.6.2 External wait function
When an extremely slow device, I/O, or asynchronous system is connected, any number of wait states can be
inserted in a bus cycle by the external wait pin (WAIT) to synchronize with the external device.
Just as with programmable waits, access to internal ROM, internal RAM and internal peripheral I/O areas cannot
be controlled by external waits.
Input of the external WAIT signal can be done asynchronously to CLKOUT and is sampled at the falling edge of the
clock in the T1 and TW states of a bus cycle. If the setup/hold time in the sampling timing is not satisfied, a wait 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 a result of an OR operation between the wait cycle specified by the set value of
programmable wait and the wait cycle controlled by the WAIT pin. In other words, the number of wait cycles is
determined by whichever has the most cycles.
Wait control
Programmable wait
Wait by WAIT pin
For example, if the programmable wait is two waits, and the timing of the WAIT pin input signal is as illustrated
below, three wait states will be inserted in the bus cycle.
Figure 4-1. Example of Inserting Wait States
T1
TW
TW
TW
T2
CLKOUT
WAIT pin
Wait by WAIT pin
Programmable wait
Wait control
Remark
{
: Sampling timing
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