CHAPTER 9 WATCH TIMER
Preliminary User’s Manual U16315EJ1V0UD
206
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 X1 input 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
11
×
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
:
X1 input clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency
f
W
: Watch timer clock frequency