HD6475348R Datasheet, PDF (265/487 Pages) Hitachi Semiconductor – Single-Chip Microcomputer

13.3.3 Operation in Software Standby Mode
The watchdog timer has a special function in recovery from software standby mode. Specific
watchdog timer settings are required when the software standby mode is used.
Before Transition to the Software Standby Mode: The TME bit must be cleared to 0 to stop
the watchdog timer counter before a transition to the software standby mode. The chip cannot
enter the software standby mode while the TME bit is set to 1. Before entering the software
standby mode, software should also set the clock select bits (CKS2 to CKS0) to a value that
makes the timer overflow interval equal to or greater than the stabilization time of the clock
oscillator.
Recovery from the Software Standby Mode: Recovery from the software standby mode can be
triggered by an NMI request. In this case the recovery proceeds as follows:
When an NMI request signal is received, the clock oscillator starts running and the watchdog
timer starts counting at the rate selected by the clock select bits before the software standby mode
was entered. When the count overflows from H'FF to H'00, the ø clock is presumed to be stable
and usable, clock signals are supplied to all modules on the chip, the standby mode ends, and the
NMI interrupt-handling routine starts executing.
13.3.4 Setting of Overflow Flag
The OVF bit is set to 1 when the timer count overflows in the interval timer mode.
Simultaneously, the WDT module requests an interval timer interrupt. The timing is shown in
figure 13-6.
ø
TCNT
Internal overflow
signal
H'FF H'00
OVF
Figure 13-6 Setting of OVF Bit
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