CHAPTER 9 WATCH TIMERUser’s Manual U16227EJ2V0UD2069.4 Watch Timer Operations9.4.1 Watch timer operationThe watch timer generates an interrupt request (INTWT) at a specific time interval by using the X1 input clock orsubsystem clock.When bit 0 (WTM0) and bit 1 (WTM1) of the watch timer operation mode register (WTM) are set to 1, the countoperation starts. When these bits are cleared 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 byclearing WTM1 to 0. In this case, however, the 11-bit prescaler is not cleared. Therefore, an error up to 2 11 × 1/f Wseconds 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 TimeWTM3 WTM2 Interrupt Time Selection When Operated at fXT = 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 s0 1 2 13/f W 0.25 s 0.105 s1 0 2 5/f W 977μs 410μs1 1 2 4/f W 488μs 205μsRemark f X : X1 input clock oscillation frequencyf XT: Subsystem clock oscillation frequencyf W: Watch timer clock frequency
