18.6.2.5 Window Mode

In Window mode operation, the WDT uses two different time specifications: the WDT can only be cleared by writing 0xA5 to the CLEAR register after the closed window time-out period (TO_WDTW), during the subsequent Normal time-out period (TO_WDTW). If the WDT is cleared before the time window opens (before TO_WDTW is over), the WDT will issue a system reset. Both parameters TO_WDTW and TO_WDT are periods in a range from 8ms to 16s, so the total duration of the WDT time-out period is the sum of the two parameters. The closed window period is defined by the Window Period bits in the Configuration register (CONFIG.WINDOW), and the open window period is defined by the Period bits in the Configuration register (CONFIG.PER).

By default, the Early Warning interrupt is disabled. If it is desired, the Early Warning Interrupt Enable bit in the Interrupt Enable register (INTENSET.EW) must be written to '1'. The Early Warning Interrupt is disabled again by writing a '1' to the Early Warning Interrupt bit in the Interrupt Enable Clear (INTENCLR.EW) register. If the Early Warning interrupt is enabled in Window mode, the interrupt is generated at the start of the open window period, i.e. after TO_WDTW. The Window mode operation is illustrated in figure Window-Mode Operation.