A 1.024 kHz clock (CLK_WDT) is sourced from the internal Ultra Low-Power Oscillator, OSC32K. Due to the ultra low-power design, the oscillator is less accurate than other oscillators featured in the device, and hence the exact time-out period may vary from device to device. This variation must be taken into consideration when designing software that uses the WDT, to ensure that the time-out periods used are valid for all devices. Refer to the Electrical Characteristics section for more specific information.

The WDT clock (CLK_WDT) is asynchronous to the peripheral clock. Due to this asynchronicity, writing to the WDT Control A (WDT.CTRLA) register will require synchronization between the clock domains. Refer to Synchronization for further details.