showing a pic output up to 15V via high resistance

[Simon]

I'm building a project where it would be advantageous for me to be able to let 15V be seen on the PWM output of the PIC via a 100+ Kohm resistor. Will I do any damage ?

[jimmc]

Simple answer is... no damage.
Of course the PIC pin will never rise to more than approximately Vdd + 0.7v (even if Tri-stated or configured as an input).

Looking at the data-sheet for the 16F628 (as an example) Section 17.0 - Electrical specifications - Absolute Maximum ratings
The output clamp current for Vo < 0 or Vo > Vdd is + 20mA .
Similarly the input clamp current for Vi < 0 or Vi > Vdd is + 20mA so your 100uA max. is safe with a 2000% margin. ((15v-5v)/100k)

(There is an overall limit of 300mA for Vss and 250mA for Vdd, so not all I/O pins can take the maximum current simultaneously.)

Jim

[Simon]

Thanks, I thought there was some sort of protection circuitry but not sure if it was just there as extreme last resort

[Zero999]

Yes, thre's no chance of it doing any harm. The PIC has built-in protection diodes and the current will be a mere 100µA. There's even an application note by Microchip which involves connecting the mains to an I/O pin via a 1M resistor.