29.3.3.2 Window Mode

In Window mode operation, the WDT uses two different time-out intervals, a closed window time-out interval (TOWDTW) and an open window time-out interval (TOWDT):
  • TOWDTW defines an interval ranging from 244 μs to 250 ms where WDT reset is not allowed. If the WDR instruction is executed during this interval, the WDT will issue a system reset.
  • TOWDT, which is also configurable from 244 μs to 250 ms, defines the duration of the open interval during which the WDT can (and needs to) be reset. The open interval will always follow the closed interval, so the total duration of the time-out interval is the sum of the closed window and the open window time-out intervals.

When enabling the Window mode or leaving Debug mode, the window is activated after the first WDR instruction.

The figure below shows a typical timing scheme for the WDT operating in Window mode.

Figure 29-3. Window Mode Operation

The Window mode is enabled by writing a non-zero value to the WINDOW bit field in the Control A (CTRLA) register and disabled by writing it to 0x0.