Hi,
It depends on:
* which package you are using: Only valid for packages: TQFP-100, TFBGA-120, TQFP-128,
* what external device you are using: a bare XTAL (using XIN32 and XOUT32) or a XTAL_OSCILLATOR (using only XIN32)?
oscillator: no problem
bare XTAL: Don´t use PC00 as switching (changing states frequently) output or input.
I guess the signal of PC00 will couple into the oscillator and may cause problems (jitter ..)
So the signal on PC00 may be a slowly changing analog signal.
I guess it´s O.K. to use it to switch a LED ON and OFF (every couple of seconds), but it´s not O.K. to use it as PWM to dim a LED.
* Do a PDF search for "PC00", "PA00", "PA01", "XIN32", "XOUT32"
* read section 60.6.3
Klaus