Crystal oscillator tied to supply voltage?

[ErnieM]

Actually, oscillators like a little noise: one old guy trick from the days men were men and oscillators were built on a single transistor and gains were very finite one trick to make sure an oscillator oscillated was to put a cheap carbon resistor in the signal path. The resistor did nothing but generate some nice thermal noise pulses that were enough to kick start the oscillations.

If your power supply has enough noise riding on it to disturbs the oscillator then it's gonna break something else before the frequency goes off.

As the resonator operates at a small multiple (ummm, FOUR?) of the processor frequency (the major power line noise source) you dang well better optimize your capacitors for that frequency. But that's pretty simple anyway, just bypass anything using Vcc at the part. Ceramic caps are good and cheap.

[Gusse]

If your power supply has enough noise riding on it to disturbs the oscillator then it's gonna break something else before the frequency goes off.

PIC's can operate in quite large voltage range (2.0 - 5.5V, some smaller 4.2 -5.5V). So (small) voltage drops in supply are not problematic for PIC itself, but might be high enough to upset XTAL, if caps are connected to Vdd. Therefore you have to pay attention to PDN impedances at frequencies you are operating. If you have resonaces in PDN at peak current frecuencies, that will generate lots of noise to power supply. And that noise can couple to XTAL, if caps are also connected to Vdd.

Therefore it is more save to use GND for XTAL caps.

BR,
-Gusse-