Datasheet

The Watchdog 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 Watchdog
System Reset Enable (WDE) in Watchdog Timer Control Register (WDTCSR.WDCE and
WDTCSR.WDE). A logic one must be written to WDTCSR.WDE regardless of the previous value of
the WDTCSR.WDE.
2. Within the next four clock cycles, write the WDTCSR.WDE and Watchdog prescaler bits group
(WDTCSR.WDP) as desired, but with the WDTCSR.WDCE cleared. This must be done in one
operation.
The following examples show a function for turning off the Watchdog Timer. The
examples assume that interrupts are controlled (e.g. by disabling interrupts globally) so
that no interrupts will occur during the execution of these functions.
Assembly Code Example
WDT_off:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Clear WDRF in MCUSR
in r16, MCUSR
andi r16, (0xff & (0<<WDRF))
out MCUSR, r16
; Write '1' to WDCE and WDE
; Keep old prescaler setting to prevent unintentional time-out
lds r16, WDTCSR
ori r16, (1<<WDCE) | (1<<WDE)
sts WDTCSR, r16
; Turn off WDT
ldi r16, (0<<WDE)
sts WDTCSR, r16
; Turn on global interrupt
sei
ret
C Code Example
void WDT_off(void)
{
__disable_interrupt();
__watchdog_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
/* Write logical one to WDCE and WDE */
/* Keep old prescaler setting to prevent unintentional time-out */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCSR = 0x00;
__enable_interrupt();
}
Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or
brown-out condition, the device will be reset and the Watchdog Timer will stay enabled. If
the code is not set up to handle the Watchdog, this might lead to an eternal loop of time-
out resets. To avoid this situation, the application software should always clear the
ATmega48PA/88PA/168PA
System Control and Reset
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 83