
213
CHAPTER 10 WATCH TIMER
User
’
s Manual U14260EJ3V1UD
10.4 Watch Timer Operations
10.4.1 Watch timer operation
The watch timer generates an interrupt request (INTWT) at specific time intervals (2
14
/f
W
seconds or 2
5
/f
W
seconds)
by using the main system clock or subsystem clock. The interrupt request is generated at the following time intervals
(where WTM3 = 0).
If main system clock (8.38 MHz) is selected: 0.25 seconds
If subsystem clock (32.768 kHz) is selected: 0.5 seconds
When bit 0 (WTM0) and bit 1 (WTM1) of the watch timer operation mode register (WTM) are set to 1, the count
operation starts, and 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, a zero-second start can be achieved for the watch timer by
setting WTM1 to 1 after clearing it to 0. In this case, however, the 9-bit prescaler is not cleared. Therefore, an error
up to 2
9
/f
W
seconds occurs in the first overflow (INTWT) after the zero-second start.
Remark
f
W
: Watch timer clock frequency (f
X
/2
7
or f
XT
)
10.4.2 Interval timer operation
The watch timer operates as interval timer that 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 operation mode register
(WTM). When bit 0 (WTM0) of WTM is set to 1, the count operation starts. When this bit is cleared to 0, the count
operation stops.
Table 10-2. Interval Timer Interval Time
WTM6
WTM5
WTM4
Interval
Time
When Operated at
f
X
= 12 MHz
Note
When Operated at
f
X
= 8.38 MHz
When Operated at
f
X
= 4.19 MHz
When Operated at
f
XT
= 32.768 kHz
0
0
0
2
4
/f
W
170
μ
s
244
μ
s
488
μ
s
488
μ
s
0
0
1
2
5
/f
W
341
μ
s
488
μ
s
977
μ
s
976
μ
s
0
1
0
2
6
/f
W
682
μ
s
977
μ
s
1.95 ms
1.95 ms
0
1
1
2
7
/f
W
1.36 ms
1.95 ms
3.91 ms
3.90 ms
1
0
0
2
8
/f
W
2.73 ms
3.91 ms
7.82 ms
7.81 ms
1
0
1
2
9
/f
W
5.46 ms
7.82 ms
15.6 ms
15.6 ms
Other than above
Setting prohibited
Note
Expanded-specification products of
μ
PD780078 Subseries only.
Remark
f
W
: Watch timer clock frequency (f
X
/2
7
or f
XT
)
f
X
: Main system clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency