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ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
Figure 10-7. Watchdog timer.
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake the
device from sleep-modes, and also as a general system timer. One example is to limit the maximum time allowed
for certain operations, giving an interrupt when the operation has run longer than expected. In System Reset mode,
the WDT gives a reset when the timer expires. This is typically used to prevent system hang-up in case of runaway
code. The third mode, Interrupt and System Reset mode, combines the other two modes by first giving an interrupt
and then switch to System Reset mode. This mode will for instance allow a safe shutdown by saving critical param-
eters before a system reset.
The “Watchdog Timer Always On” (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset
mode. With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are locked to
1 and 0 respectively. To further ensure program security, alterations to the Watchdog set-up must follow timed
sequences. The sequence for clearing WDE and changing time-out configuration is as follows:
1.
In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and WDE. A logic one
must be written to WDE regardless of the previous value of the WDE bit.
2.
Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP) as desired, but with
the WDCE bit cleared. This must be done in one operation.
The following code example shows one assembly and one C function for turning off the Watchdog Timer. The
example assumes that interrupts are controlled (for example by disabling interrupts globally) so that no interrupts
will occur during the execution of these functions.
128kHz
OSCILLATOR
MCU RESET
INTERRUPT
WDIE
WDIF
OSC/2K
OSC/4K
OSC/8K
WDP3