Fujitsu F2MC-8FX Hardware Manual page 213

CHAPTER 13 WATCH PRESCALER
Note:
If the interval time set for the watch prescaler is shorter than the oscillation stabilization wait time of
the subclock, an interrupt request of the watch prescaler is generated during the oscillation
stabilization wait time of the subclock required for recovery by an external interrupt upon the
transition from the subclock mode or the sub PLL clock mode to the stop mode. To prevent this, set
the interrupt request enable bit (WPCR:WTIE) in the watch prescaler control register to "0" to disable
interrupts of the watch prescaler when entering the stop mode during the subclock mode or the sub
PLL clock mode.
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