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15.3
Operation
15.3.1
Watchdog Timer Operation
To use the WDT as a watchdog timer, set the WT/IT bit in TCSR and TME bit to 1. Software
must prevent TCNT overflows by rewriting the TCNT value (normally be writing H'00) before
overflows occurs. This ensures that TCNT does not overflow while the system is operating
normally. If TCNT overflows without being rewritten because of a system crash or other error, in
the WDT0 the WDTOVF signal is output. This is shown in figure 15-4. This WDTOVF signal can
be used to reset the system. The WDTOVF signal is output for 132 states when RSTE = 1, and for
130 states when RSTE = 0.
If TCNT overflows when 1 is set in the RSTE bit in RSTCSR, a signal that resets the H8S/2633
Series internally is generated at the same time as the WDTOVF signal. This reset can be selected
as a power-on reset or a manual reset, depending on the setting of the RSTS bit in RSTCSR. 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 RES pin reset has priority and the WOVF bit in RSTCSR is cleared to 0.
In the case of WDT1, the chip is reset, or an NMI interrupt request is generated, for 516 system
clock periods (516ø) (515 or 516 states when the clock source is øSUB (PSS = 1)). This is
illustrated in figure 15-4 (b).
An NMI request from the watchdog timer and an interrupt request from the NMI pin are both
treated as having the same vector. So, avoid handling an NMI request from the watchdog timer
and an interrupt request from the NMI pin at the same time.
576
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