Cautions For Watch Timer - NEC 78K0 Series User Manual

Figure 10-3. Operation Timing of Watch Timer/Interval Timer
5-bit counter
Count clock
Watch timer
interrupt INTWT
Interval timer
interrupt INTWTI
Remark f : Watch timer clock frequency
W
Figures in parentheses are for operation with f

10.5 Cautions for Watch Timer

When operation of the watch timer and 5-bit counter is enabled by the watch timer mode control register (WTM) (by
setting bits 0 (WTM0) and 1 (WTM1) of WTM to 1), the interval until the first interrupt request signal (INTWT) is
generated after the register is set does not exactly match the specification made with bits 2 and 3 (WTM2, WTM3) of
WTM. Subsequently, however, the INTWT signal is generated at the specified intervals.
Figure 10-4. Example of Generation of Watch Timer Interrupt Request Signal (INTWT)
It takes 0.515625 seconds for the first INTWT to be generated (2
INTWT is then generated every 0.5 seconds.
WTM0, WTM1
INTWT
292
CHAPTER 10 WATCH TIMER
0H
Start
Interrupt time of watch timer (0.5 s)
Interval time
(T)
= 32.768 kHz (WTM7 = 1, WTM3, WTM2 = 0, 0)
W
(When Interrupt Period = 0.5 s)
0.515625 s
Preliminary User's Manual U17260EJ3V1UD
Overflow
Interrupt time of watch timer (0.5 s)
T
× 1/32768 = 0.015625 s longer).
9
0.5 s 0.5 s
Overflow