Choicing crystal or external oscillator for time critical project on PIC18F

Not sure if you are still around or gave up, but in order to create a stable VCXO clock locked to the PAL chroma burst, you need a few timing signals like a "sandcastle" window detector to sample and hold the phase error during this period for the PLL to work.

So use this chip to get your clock and you are good to go.

Since a crystal has an extremely narrow bandwidth (BW) of 1/3000 of the centre frequency, that BW (~1KHz) will determine the best case low pass response delay and lock up time of the Xtal, however once many colour bursts are received or around 10mS of accumulated bursts) , it will stay in sync until the next horizontal line, so that only minor phase corrections can be done during the burst.

It is extremely difficult (but not impossible) to make a PLL lock up in one cycle and then be stable until the next burst. It would require precise measurement of frequency and phase with a 100x clock to get within 1%. using the horizontal sync with 2nS stability. The Horz Sync acts as a freq reference for another PLL loop from the same VCXO with less than 1ppm error and very low phase noise, which sounds easy, but is not cheap.

Apart from the crystal sounding like it's actually a resonator (part number?), you also need to unlock the OSCCON register to switch clock sources. The compiler provides built in functions to do this, as described in the datasheet.

Here is an example from the datasheet of the code to switch over to Primary Oscillator with PLL (you need to check/set the divider/multiplier bits according to your frequency - this is for 20Mhz crystal and 80Mhz operating freq):

int main()
{
// Configure PLL prescaler, PLL postscaler, PLL divisor
PLLFBD=30; // M = 32
CLKDIVbits.PLLPOST=0; // N1 = 2
CLKDIVbits.PLLPRE=2; // N2 = 4
// Initiate Clock Switch to Primary Oscillator with PLL (NOSC = 0b011)
__builtin_write_OSCCONH(0x03);
__builtin_write_OSCCONL(0x01);
// Wait for Clock switch to occur
while (OSCCONbits.COSC != 0b011);
// Wait for PLL to lock
while(OSCCONbits.LOCK!=1) {};
}