27.4.1 Switching from Embedded 32 kHz RC Oscillator to 32.768 kHz Crystal Oscillator

The sequence to switch from the embedded 32 kHz (typical) slow RC oscillator to the 32.768 kHz crystal oscillator is the following:

1. Switch the main system bus clock to a source different from slow clock (PLL or Main Oscillator) through the Power Management Controller.
2. Switch from the embedded 32 kHz RC oscillator to the 32.768 kHz crystal oscillator by writing a 1 to the TD_OSCSEL bit.
3. Wait 39000 slow clock cycles for internal resynchronization.