PIC16F883 over 12V input with resistors?
The board you are using has a three-terminal regulator that drops the input voltage to 5 volts.  It is rated only to 16 volts and requires 7 volts to operate.  The MTS2916A motor driver requires a minimum 4.5 volts on its logic input, which is supplied from this regulator.  Since the regulator has a voltage drop associated with it, the board will not work with either 4.5 or 18V on the input.

You can remove jumper JP2 on the board and apply your 4.5 - 18 volts directly to the jumper pin connected to VLoad on the MTS2916A, and use a separate battery or supply (sharing the same ground) of 7-16 volts on the J1 input.  A standard 9V battery would be fine here.

To use the RX pin as an input you could use a resistor divider to bring your 18 volts down to a logic level:







simulate this circuit &ndash; Schematic created using CircuitLab