[68] CLOCK: EVENTS_HFCLKSTARTED can be generated before HFCLK is stable

This anomaly applies to IC Rev. Rev 1, build codes QFAA-B00, QFAB-B00, CIAA-B00.

It was inherited from the previous IC revision Engineering C.

Symptoms

EVENTS_HFCLKSTARTED may come before HFXO is started.

Conditions

When using a 32 MHz crystal with start-up longer than 400 µs.

Consequences

Performance of radio and peripheral requiring HFXO will be degraded until the crystal is stable.

Workaround

32 MHz crystal oscillator startup time must be verified by the user. If the worst-case startup time is shorter than 400 µs, no workaround is required. If the startup time can be longer than 400 µs, the software must ensure, using a timer, that the crystal has had enough time to start up before using peripherals that require the HFXO. The Radio requires the HFXO to be stable before use. The ADC, TIMERs, and TEMP sensor for example can use the HFXO as a reference for improved accuracy.