15.4 Watchdog Timer

The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1MHz. This is the typical value at V_CC = 5V. See characterization data for typical values at other V_CC levels. By controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as shown in the figure below. The WDR – Watchdog Reset – instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs. Eight different clock cycle periods can be selected to determine the reset period. If the reset period expires without another Watchdog Reset, the ATmega8A resets and executes from the Reset Vector. For timing details on the Watchdog Reset, refer to the Watchdog Reset.

To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be followed when the Watchdog is disabled. Refer to the description of the Watchdog Timer Control Register for details.