Window watchdog is a slightly more complex implementation of the timeout watchdog. In window watchdog there is a timeout period, like in the timeout watchdog; however, a watchdog trigger cannot be issued at any time during the window period unlike timeout. In window watchdog the window period is split evenly into a closed window period and an open window period, where the watchdog trigger must be sent during the open window period. This can be the second half of the window length – so for 128ms window the second 64ms is where the watchdog must trigger – however this is not that simple. The window, closed window, and open window periods all include tolerances so the tolerances are not exact. The safe trigger area can actually take up less than half of the total window period. The safe trigger area size can be calculated by taking the minimum total window time and subtracting the maximum closed window time where the start of the safe trigger area is the maximum closed window time and the end of the safe trigger area is the minimum window length. The TCAN28XX line of devices uses an internal oscillator with a +/-10% accuracy range to set up the window periods. If the window is set to 128ms, that means realistically, the closed and open windows are 64ms each – however with the ±10% tolerance this is unwise to use the nominal timing. First, finding the maximum closed window time, which is 1.1 × 64 = 70.4ms. Next, the minimum window period, which is 0.9 × 128ms = 115.2ms. Finally, with these two values the safe trigger area in this case is 70.4ms to 115.2ms instead of the 65ms to 128ms that can exist without tolerance.

Figure 5-11 Window Watchdog Timing