All PICs have protection diodes on the pins which prevent the pins going outside the range Vdd+0.3V and Vss-0.3V.
Because your feed is via a high impedance resistor you wont damage the PIC but when you are expecting the output to be -15V it will only be -0.3V
I am guessing that because you have -15V in your circuit you have dual supply rails so could you not use an Opamp to invert the pic output to be in the range 0V to -5V. You would then have -ve voltage to drive your PIC.
Another possibility is to use a hardware PWM in the PIC to generate a negative voltage. Search for "negative voltage" and "LCD".
Hope that helps
Keith
www.diyha.co.uk
www.kat5.tv
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