wdt accuracy

[Jo-Jo]

I am trying to set up the WDT on a nRF51822, but the timing doesn't seem quite right. If I load the CRV register with a count of 32768 I would expect it to be a one second time out (according to the data sheet), but it actually looks like it is around 900 ms. I toggle a GPIO when I reset, and when I feed the watchdog, so I can measure the timing on a scope. I have tried with settings that I think should be 4 and 6 seconds, and the results are the same. For instance, putting 32768*4 in the CRV register causes a reset to happen about every 3.6 seconds.

Am I missing something, have something setup funny, or does 32768 really equate to a 900 ms timeout?

[Stefan Birnir Sverrisson]

Hi

Yes I am seeing this too. However, you can overcome this problem by selecting the 32kHz crystal explicitly, then it should be accurate. Enable the 32kHz crystal with the following code before starting the watchdog timer:

NRF_CLOCK->LFCLKSRC = CLOCK_LFCLKSRC_SRC_Xtal;
NRF_CLOCK->TASKS_LFCLKSTART = 1;
while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0);
NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;