1.3 Timer Clock

The WDT is clocked from the 1 kHz output from the internal 32 kHz Ultra Low Power (ULP) oscillator. This oscillator can also be used by the RTC timer and is used by the brown-out detection circuit, if used in Sampled mode. If any of these modules are configured to use the ULP oscillator the oscillator is running. The additional current consumption by enabling additional modules using the ULP RC oscillator is very low. Refer to the datasheet for more information about power consumption.

It is important to be aware that the clock for the WDT is not very accurate. This is due to the fact that the oscillator is designed to draw very little power to be able to use the WDT even in long-life battery powered applications. The downside of low power oscillators is low accuracy. The typical accuracy of the clock for the WDT is ±30% (refer to data sheet for exact information on accuracy of the clock). This means that the clock frequency can vary from one device to another. When designing software using the WDT, the device-to-device variation must be addressed to ensure that the timeout periods used are valid for all devices, and not only the ones used in the lab during development.

Further, the clock source may vary over temperature and supply voltage – though this variation is significantly less than the ±30% device-to-device variations. Refer to the data sheet for more information on this topic.