HFCLKSTART blocks changes to LFCLKSRC

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:


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?