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Chapter 11
Watchdog Timer
Preliminary User’s Manual U14913EE1V0UM00
11.3 Operation
11.3.1 Operating as watchdog timer
Set bit 6 (WDTM) of the watchdog timer mode register (WDTM) to 1 to operate as a watchdog timer to
detect program runaway and generate an RESET signal.
Setting bit 7 (WDTEN) of WDTM to 1 starts the count. After counting starts, if WDTEN is set to 1 again
within the set time interval for runaway detection, the watchdog timer is cleared and counting starts
again.
If WDTEN is not set to 1 and the runaway detection time has elapsed, a non-maskable interrupt (NMI-
WDT) or a RESET is generated.
The watchdog timer stops running in the STOP mode. Consequently, set WDTEN to 1 and clear the
watchdog timer before entering the STOP mode.
Caution: Sometimes, the actual runaway detection time is a maximum of 0.5% less than the set
time.
Table 11-2: Runaway Detection Time by Watchdog Timer
Clock
Runaway detection time
f
SUB
= 4 MHz
f
SUB
= 5 MHz
2
15
/f
CKSEL1
1.04 s
0.839 s
2
14
/f
CKSEL1
0.52 s
0.419 s
2
6
/f
CKSEL1
0.002 s
0.0016 s
2
5
/f
CKSEL1
0.001 s
0.0008 s
2
15
/f
CKSEL2
4.19 s
3.355 s
2
14
/f
CKSEL2
2.097 s
1.677 s
2
6
/f
CKSEL2
0.008 s
0.006 s
2
5
/f
CKSEL2
0.004 s
0.003 s
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