Actually,

My code does not connect directly to a keyboard. It receives data from the PICs serial port which is connected to a PC. The PC is running HyperTerm or TerraTerm. When you press the "UP" and "DOWN" arrow on a PC keyboard, HyperTerm actually sends out 3 ASCII characters (the first one being an <ESC>).

So, on a HyperTerm session, when the program starts, the PIC asks what frequency you want the PWM to be -
312Khz,156Khz,78Khz or 39Khz. It uses a table to set PR2 to the correct value, and then writes the desired PWM Value into CCPR1L and CCP1CON. You increment or decrement the
PWM value by pressing the UP and DOWN arrow keys.