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10.4 Watch Timer Operations
10.4.1 Watch timer operation
When the 8.38-MHz main system clock is used, the timer operates as a watch timer with a 0.25-second interval.
The watch timer generates interrupt requests at a constant time interval.
When bit 0 (WTM0) and bit 1 (WTM1) of the watch timer mode control register are set to 1, the count operation
starts. When set to 0, the 5-bit counter is cleared and the count operation stops.
For simultaneous operation of the interval timer, zero-second start can be set only for the watch timer by setting
WTM1 to 0. However, since the 9-bit prescaler is not cleared the first overflow of the watch timer (INTWT) after zero-
second start may include an error of up to 2
10.4.2 Interval timer operation
The watch timer operates as interval timer which generates interrupt request repeatedly at an interval of the preset
count value.
The interval time can be selected with bits 4 to 6 (WTM4 to WTM6) of the watch timer mode control register (WTM).
WTM6
WTM5
0
0
0
0
0
1
0
1
1
0
1
0
Other than above
Remark f
: Watch timer clock frequency
W
136
CHAPTER 10 WATCH TIMER
× 1/f
9
.
W
Table 10-3. Interval Timer Interval Time
WTM4
Interval Time
× 1/f
4
0
2
W
× 1/f
5
1
2
W
× 1/f
6
0
2
W
× 1/f
7
1
2
W
× 1/f
8
0
2
W
× 1/f
9
1
2
W
Setting prohibited
When Operated at
When Operated at
f
= 65.4 kHz
f
W
W
244 µ s
3.91 ms
489 µ s
7.81 ms
978 µ s
15.6 ms
1.96 ms
31.3 ms
3.91 ms
62.5 ms
7.82 ms
125 ms
= 4.09 kHz
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