HFCLKSTART blocks changes to LFCLKSRC

[pabigot]

Using a nRF51822-QFAC-A1 on the nRF51-DK I noticed unexpected behavior when starting the HFCLK and LFCLK in rapid succession. This variant reproduces the problem at startup:

NRF_CLOCK->TASKS_HFCLKSTART = 1;
NRF_CLOCK->LFCLKSRC = CLOCK_LFCLKSRC_SRC_Xtal;
while (! NRF_CLOCK->EVENTS_HFCLKSTARTED) {
}
NRF_CLOCK->TASKS_LFCLKSTART = 1;

With this configuration both clocks start but NRF_CLOCK->LFCLKSTAT shows the clock source to be RC, not Xtal, and it never changes to Xtal. In fact, LFCLKSRC reads back 0 instead of 1 as it was assigned.

If I move the wait-for-started before the assignment to LFCLKSRC then the expected behavior is observed: LFCLKSTAT shows RC immediately, but switches to Xtal after the crystal has stabilized. Delays up to 500 us are not a substitute for waiting for EVENTS_HFCLKSTARTED.

Without trying more variations, it appears that writes to some clock registers are ignored between TASKS_HFCLKSTART and EVENTS_HFCLKSTARTED. I don't see a dependency of this sort documented in the nRF51 SRM v3.0 or in PAN v3.0; is this a known anomaly, or documented somewhere else?

[Susheel Nuguru]

I have tested this now. And I see that only selecting the the LFCLKSRC is ignored and trying to set some other register just works fine if done in between HFCLKSTART and EVENTS_HFCLKSTARTED.

It is not a big thing as you mentioned but I agree that this needs to be documented. I will create a internal ticket for the team to document this. It will be handled for sure (internal tickets get solved at some point), but cannot give you any time line.

Thanks for pointing this out.

[pabigot]

Didn't mean to imply I wasn't going to wait before counting on clock stability, just that I was looking for a way to run the start sequence in parallel without waiting between the two starts. In the framework I'm developing the wait-for-stability is delayed until the point stability is actually needed (e.g., TIMER or RTC startup); there's a lot of other non-timing-critical initialization that can occur while the clocks stabilize, and the RC sources are fine for that work.