Here is the data sheet for the LED.
http://www.alldatasheet.com/datashee...SGBB557TA.html

You do not need a negative voltage. 0 volts is what you need. Ground is not negative.
Reread the post from skimask.

RC6 = PORTC.6
Have you looked at the data sheet for the PIC you are planning to use? It explains the functions.

From the data sheet.
TX = Asynchronous serial transmit data output (EUSART module); takes
priority over port data. User must configure as output.

CX =Synchronous serial clock output (EUSART module); takes priority
over port data.
Synchronous serial clock input (EUSART module).