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15.3
Operation
15.3.1
Watchdog Timer Mode
To use the WDT as a watchdog timer, set the WT/IT bit and the TME bit in TCSR to 1. While the
WDT is used as a watchdog timer, if TCNT overflows without being rewritten because of a
system malfunction or another error, an internal reset or NMI interrupt request is generated. TCNT
does not overflow while the system is operating normally. Software must prevent TCNT
overflows by rewriting the TCNT value (normally writing H'00) before overflows occurs.
If the RST/NMI bit in TCSR is set to 1, when the TCNT overflows, an internal reset signal for this
LSI is issued for 518 system clocks as shown in figure 15.2. If the RST/NMI bit is cleared to 0,
when the TCNT overflows, an NMI interrupt request is generated.
An internal reset request from the watchdog timer and a reset input from the RES pin are
processed in the same vector. A reset source can be identified by the state of the XRST bit in
SYSCR. 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 XRST bit in SYSCR is set to 1.
An NMI interrupt request from the watchdog timer and an interrupt request from the NMI pin are
processed in the same vector. Do not handle an NMI interrupt request from the watchdog timer
and an interrupt request from the NMI pin at the same time.
H'FF
H'00
Internal reset signal
[Legend]
WT/ :
Timer mode select bit
TME:
Timer enable bit
OVF:
Overflow flag
Note: * After the OVF bit becomes 1, it is cleared to 0 by an internal reset.
The XRST bit is also cleared to 0.
Figure 15.2 Watchdog Timer Mode (RST/NMI
Rev. 1.00, 09/03, page 420 of 704
TCNT value
WT/
= 1
Write H'00 to
TME = 1
TCNT
Overflow
OVF = 1*
WT/
= 1
TME = 1
518 system clocks
NMI = 1) Operation
NMI
NMI
Time
Write H'00 to
TCNT
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