Q:

Must I disable my high frequency oscillators before going into EM2, EM3 or EM4? Which oscillator is running when I wake up again?

A:

When entering EM2, EM3 or EM4, all high frequency oscillators are disabled by hardware. When waking up from EM2/EM3, the HFRCO will be selected and running at the frequency band it was set to when entering EM2/EM3. If you want to use HFXO, this must be enabled and selected manually after a wake-up.

To exit EM4 the device must be reset and after the reset the EFM32 will run off the HFRCO at 14 MHz.

No oscillators are disabled when entering EM1.