In both windowed modes, the timer starts with the first window defined by a 32-bit field in the First_Wind register. The first window is considered the closed window. This is followed by a second window defined by another 32-bit field in the Second_Wind register. The second window is considered the open window. The protocol to keep the watchdog timer operating correctly depends on the window mode and the optional features that are enabled. In the simplest mode, the timer is restarted during the open window time.
Normally, if software tries to restart the timer before the open window time period, a bad event occurs and the timer can generate a system interrupt or reset.
The Q&A token window mode requires the system software to write three one-byte responses during the first window (closed window) and a one-byte write in the second window (open window). The value of the token responses are based on the seed, feedback, and an answer count value. The seed and feedback values are programmed by software. The answer count value increments by one for each token response.
The two window timer modes include: