nrf51822: Is it possible to wake up chip from systems off mode using the built in WDT and an extern crystal

[tyler]

I am using an Nrf51822 for a project. I looked in the datasheet and it said that there is a built in WDT for the chip. It also says that clocks can't be used while in systems off mode. Because of this I am wondering if it is possible to use an extern crystal and the built in WDT to wake up the device from systems off mode?

[endnode]

No, watchdog is using LF CLK and as all clock sources are OFF during SYSTEM OFF then it cannot run. The fact that you are willing to supply the clock externally doesn't change the fact;) Also note (see Reference Manual) that WDT is reset when chip enters SYSTEM OFF and it configuration is cleared during reset (which is what happens when nRF5x wakes from SYSTEM OFF) so it wouldn't be really helpful...

You can still simulate this by some external component (e.g. by low power RTC chip which will send signal over GPIO PIN once timer expires and this can wake nRF5x if it is in SYSTEM OFF and this wake-up method is configured.