Rev. 1.0, 09/02, page 287 of 568
13.3 Operation
13.3.1
Watchdog Timer Mode Operation
To use the WDT as a watchdog timer, set the WT/
IT
bit in TCSR and the TME bit to 1. Software
must prevent TCNT overflows by rewriting the TCNT value (normally by writing H
′
00) before
overflow occurs. This ensures that TCNT does not overflow while the system is operating
normally. If TCNT overflows without being rewritten because of a system malfunction or other
error, the WOVF bit in RSTCSR is set to 1. If the RSTE bit in RSTCSR is set to 1, an internal
reset is issued. This is shown in figure 13.2. At this time, select the power-on reset by clearing the
RSTS bit in RSTCSR to 0. The internal reset signal is output for 518 states.
If a reset caused by a signal input to the
RES
pin occurs at the same time as a reset caused by a
WDT overflow, the reset by the
RES
pin has priority and the WOVF bit in RSTCSR is cleared to
0.
TCNT value
H'00
Time
H'FF
WT/ =1
TME=1
Write H'00
to TCNT
WT/ =1
TME=1
Write H'00
to TCNT
518 states
Internal reset signal
*
WT/
TME
Note:
*
The internal reset signal is generated only if the RSTE bit is set to 1.
Overflow
internal reset is
generated
WOVF=1
: Timer mode select bit
: Timer enable bit
Legend
′
Figure 13.2 Example of WDT0 Watchdog Timer Operation
13.3.2 Interval
Timer
Mode
When the WDT is used as an interval timer, an interval timer interrupt (WOVI) is generated each
time the TCNT overflows. Therefore, an interrupt can be generated at intervals.
When the TCNT overflows in interval timer mode, an interval timer interrupt (WOVI) is requested
at the time the OVF bit of the TCSR is set to 1.
Summary of Contents for H8S/2627
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