29.4.2 Switching from 32.768 kHz Crystal Oscillator to Embedded Always-on 64 kHz RC Oscillator

The sequence to switch from the 32.768 kHz crystal oscillator to the embedded always-on 64 kHz (typical) RC oscillator is the following:

1. Switch the main system bus clock to a source different from slow clock (PLL or Main Oscillator).
2. Switch from the 32.768 kHz crystal oscillator to the embedded RC oscillator by writing a 0 to the OSCSEL bit.
3. Wait 5 slow clock cycles for internal resynchronization.