12.2.4 PLL Input Switch
Switching between the PLL and any non-PLL source is handled in the same manner as any other clock source change.
When the NOSC selects a source with a PLL, the system
continues to operate using the current oscillator until the new oscillator is ready.
When the new source is ready, the associated Status bit in the Oscillator Status (OSCSTAT) register is set, and once the PLL is locked and ready for use, the
PLL is Ready (PLLR) bit is set. Once both the source and PLL are ready, the
switch will complete.