The Watchdog Timer
The Watchdog Timer is clocked from a separate on-chip oscillator which runs at approximately 1MHz. This is the typical value at VCC = 5V.
By controlling the Watchdog Timer pre-scaler, the Watchdog reset interval can be adjusted from 16K to 2,048K cycles (nominally 16 - 2048 ms). The BASCOM RESET WATCHDOG - instruction resets the Watchdog Timer.
Eight different clock cycle periods can be selected to determine the reset period.
If the reset period expires without another Watchdog reset, the AT90Sxxxx resets and program execution starts at the reset vector address.