In the center of the watchdog timer block diagram is the 17-bit counter, with the input fixed to the internal low speed oscillator and no other selection possible. Using the option byte, one can select the window size and the time of the watchdog timer. Usually in case of a watchdog timer overflow or underflow, if the window size check occurs, a system reset will be generated. As a second option the watchdog timer can also be used as an interval timer interrupt. That means if 75% of the overflow time of the watchdog timer occurs an internal interval interrupt will be generated. This can be used, for example in Stop Mode, to get a pre-warning that the watchdog timer will overflow soon and wake up the system to re-trigger the watchdog timer.