11.2.4 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.