CHAPTER 9 WATCH TIMER
User’s Manual U16899EJ2V0UD
228
9.4 Watch Timer Operations
9.4.1 Watch timer operation
The watch timer generates an interrupt request (INTWT) at a specific time interval by using the high-speed system
clock or subsystem clock.
When bit 0 (WTM0) and bit 1 (WTM1) of the watch timer operation mode register (WTM) are set to 1, the count
operation starts. When these bits are set to 0, the 5-bit counter is cleared and the count operation stops.
When the interval timer is simultaneously operated, zero-second start can be achieved only for the watch timer by
setting WTM1 to 0. In this case, however, the 11-bit prescaler is not cleared. Therefore, an error up to 2
9
×
1/f
W
seconds occurs in the first overflow (INTWT) after zero-second start.
The interrupt request is generated at the following time intervals.
Table 9-4. Watch Timer Interrupt Time
WTM3
WTM2
Interrupt Time Selection
When Operated at f
XT
= 32.768 kHz
(WTM7 = 1)
When Operated at f
X
= 10 MHz
(WTM7 = 0)
0 0
2
14
/f
W
0.5 s
0.210 s
0 1
2
13
/f
W
0.25 s
0.105 s
1 0
2
5
/f
W
977
µ
s 410
µ
s
1 1
2
4
/f
W
488
µ
s 205
µ
s
Remark f
X
: High-speed system clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency
f
W
: Watch timer clock frequency
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