ADC Information
Resistors R1 and R2 (see schematic) can provide a high and low reference voltage to the PIC's ADC. This is useful if you are sampling a signal that doesn't span the entire 0-3.3V range. You want to fit your sampling across the entire 10-bits of resolution. In order to utilize these resistors you must change the ADC mode in the PIC's firmware. You can find ADC modes listed in the 16f873.h header file in the CCSC compiler. Please see the ADC section of the 16F873 datasheet.