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CHAPTER 8 WATCH TIMER
Preliminary User’s Manual U13420EJ2V0UM00
8.4 Watch Timer Operations
8.4.1 Watch timer operation
When the 32.768-kHz subsystem clock or 8.38-MHz main system clock is used, the timer operates as a watch
timer with a 0.5-second or 0.25-second interval.
The watch timer generates an interrupt request (INTWT) at the constant time interval.
When bit 0 (WTM0) and bit 1 (WTM1) of the watch timer mode control register (WTM) is set to 1, the 5-bit counter
is cleared and the count operation stops.
While the interval timer is simultaneously operating, zero-second start only for the watch timer can be achieved
by setting WTM1 to 0. In that case, however, 9-bit prescaler is not cleared, so that an error of 2
9
×
1/f
W
at maximum
may occur in the first overflow (INTWT) after the zero-second start of watch timer.
8.4.2 Interval timer operation
The watch timer operates as interval timer which generates interrupt requests (INTWTI) 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).
Table 8-3. Interval Timer Interval Time
WTM6
WTM5
WTM4
Interval Time
When Operated at
When Operated at
When Operated at
f
X
= 8.38 MHz
f
X
= 4.19 MHz
f
XT
= 32.768 kHz
0
0
0
2
4
×
1/f
W
244
µ
s
489
µ
s
488
µ
s
0
0
1
2
5
×
1/f
W
489
µ
s
978
µ
s
977
µ
s
0
1
0
2
6
×
1/f
W
978
µ
s
1.96 ms
1.95 ms
0
1
1
2
7
×
1/f
W
1.96 ms
3.91 ms
3.91 ms
1
0
0
2
8
×
1/f
W
3.91 ms
7.82 ms
7.81 ms
1
0
1
2
9
×
1/f
W
7.82 ms
15.6 ms
15.6 ms
Other than above
Setting prohibited
Remark
f
X
: Main system clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency
f
W
: Watch timer clock frequency
Содержание mPD780065 Series
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