CHAPTER 9 WATCHDOG TIMER
Preliminary User’s Manual U16846EJ1V0UD
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(3) When the CPU clock is the Ring-OSC clock (f
R
) and the watchdog timer operation clock is the high-speed
system clock (f
XP
) when the STOP instruction is executed
When the STOP instruction is executed, operation of the watchdog timer is stopped. After STOP mode is
released, counting is stopped until the timing of <1> or <2>, whichever is earlier, and then counting is started
using the operation clock before the operation was stopped. At this time, the counter is not cleared to 0 but holds
its value.
<1> The oscillation stabilization time set by the oscillation stabilization time select register (OSTS) elapses.
<2> The CPU clock is switched to the high-speed system clock (f
XP
).
Figure 9-6. Operation in STOP Mode
(CPU Clock: Ring-OSC Clock, WDT Operation Clock: High-Speed System Clock)
<1> Timing when counting is started after the oscillation stabilization time set by the oscillation stabilization time
select register (OSTS) has elapsed
Watchdog timer
Operating
Operation stopped
Operating
f
R
f
XP
CPU operation
17 clocks
Normal operation
(Ring-OSC clock)
Clock supply stopped
Normal operation (Ring-OSC clock)
Oscillation
stopped
STOP
Oscillation stabilization time
(set by OSTS register)
<2> Timing when counting is started after the CPU clock is switched to the high-speed system clock (f
XP
)
Operating
Operation stopped
Operating
f
R
f
XP
f
R
→
f
XP
Note
CPU operation
17 clocks
Normal operation
(Ring-OSC clock)
Clock supply
stopped
Normal operation (Ring-OSC clock)
Normal operation
(high-speed system clock)
CPU clock
Oscillation
stopped
STOP
Oscillation stabilization time
(set by OSTS register)
Watchdog timer
Note Confirm the oscillation stabilization time of f
XP
using the oscillation stabilization time counter status register
(OSTC).
Waiting for the oscillation stabilization time is not required when the external RC oscillation clock is
selected as the high-speed system clock by the option byte. Therefore, the CPU clock can be switched
without reading the OSTC value.