CHAPTER 8 WATCH TIMER
8.4 Watch Timer Operation
8.4.1 Operation as watch timer
The main system clock (4.19 MHz) or subsystem clock (32.768 kHz) is used to enable the watch timer to operate
at 0.5-second intervals.
The watch timer is used to generate an interrupt request at specified intervals.
By setting bits 0 and 1 (WTM0 and WTM1) of the watch timer mode control register (WTM) to 1, the watch timer
starts counting. By setting them to 0, the 5-bit counter is cleared and the watch timer stops counting.
It is possible to start the watch timer only from zero seconds by clearing WTM1 to 0 when the interval timer and
watch timer operate at the same time. In this case, however, an error of up to 29 × 1/fW seconds may occur in the
overflow (INTWT) after the zero-second start of the watch timer because the 9-bit prescaler is not cleared to 0.
8.4.2 Operation as interval timer
The interval timer is used to repeatedly generate an interrupt request at the interval specified by a preset count
value.
The interval can be selected by bits 4 to 6 (WTM4 to WTM6) of the watch timer mode control register (WTM).
Table 8-3. Interval Time of Interval Timer
WTM6
WTM5
WTM4
Interval
24 × 1/fW
At fX = 5.0 MHz
409.6 µs
At fX = 4.19 MHz
489 µs
At fXT = 32.768 kHz
488 µs
0
0
0
0
1
1
0
0
1
1
0
0
0
1
0
1
0
1
25 × 1/fW
819.2 µs
1.64 ms
3.28 ms
6.55 ms
13.1 ms
978 µs
977 µs
26 × 1/fW
1.96 ms
3.91 ms
7.82 ms
15.6 ms
1.95 ms
27 × 1/fW
3.91 ms
28 × 1/fW
7.81 ms
29 × 1/fW
15.6 ms
Other than above
Setting prohibited
Remarks 1. fX: Main system clock oscillation frequency
2. fXT: Subsystem clock oscillation frequency
3. fW: Watch timer clock frequency
User’s Manual U15075EJ1V0UM00
174
NEC [ NEC ]
