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I Return from Watch Mode
The watch mode is cancelled by a reset factor or when an interrupt is generated.
G
Return by reset factor
When the watch mode is cancelled by a reset factor, the mode transits to the main clock mode after the
watch mode is cancelled, transiting to the reset sequence.
Notes:
• To return from watch mode to main clock mode using the external reset pin (RST pin),
input the Low level for at least oscillator's oscillation time* + 100 µs + 16 machine
cycles (main clock).*:The oscillation time for the oscillator is the period of time taken
until its amplitude reaches 90%.It takes several to dozens of ms for crystal oscillators,
hundreds of µs to several ms for FAR/ceramic oscillators, and 0 ms for external clocks.
• There is no sub-clock in MB90F897S.
G
Return by interrupt
When an interrupt request higher than the interrupt level (IL) of 7 is generated from the watch timer and
external interrupt in the watch mode, the watch mode is cancelled. After the watch mode is cancelled, as
with normal interrupt processing, the generated interrupt request is identified according to the settings of
the I flag in the condition code register (CCR), the interrupt level mask register (ILM), and the interrupt
control register (ICR). In the sub-timer mode, no oscillation stabilization wait time is generated and the
interrupt request is identified immediately after return from the watch mode.
• When the CPU is not ready to accept any interrupt request, the next instruction to the currently
executing instruction is executed.
• When the CPU is ready to accept any interrupt request, it branches immediately to the interrupt
processing routine.
Notes:
• When handling an interrupt, the CPU usually services the interrupt after executing the
instruction that follows the one specifying the watch mode.
• There is no sub-clock in MB90F897S.
CHAPTER 3 CPU
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