As suggested, a 555 would be completely pointless - the PIC can use a simple resistor and capacitor as it's oscillator, and will be at least as stable as a 555. But if you need any sort of precision, then use a crystal.
As Nigel said it is pointless to use a 555 as an oscillator for the PIC. However it is possible to make a 555 timer extremely stable when using film capacitors
for timing, and you don't need the Vcc to be highly regulated.
I don't see the point in using a PIC if you're just going to use an external timing RC network unless you wany long delays. A TS555 costs less than a PIC and will be as accurate as a PIC with an external RC timing network.
Please don't get what I'm saying confused, of course a using PIC using the internally trimmed on-chip RC oscillator is more accurate than a 555.
I don't see the point in using a PIC if you're just going to use an external timing RC network unless you wany long delays. A TS555 costs less than a PIC and will be as accurate as a PIC with an external RC timing network.
I think you're missing the point (or I am?), the question is about using a 555 to replace the crystal on a PIC - not about using a PIC to replace a 555.