mmm....
has leido el datasheet del 18f2455?
EXAMPLE 10-1:INITIALIZING PORTA
CLRF PORTA; Initialize PORTA by clearing output data latches
CLRF LATA; Alternate method to clear output data latches
MOVLW 0Fh; Configure A/D
MOVWF ADCON1; for digital inputs
MOVLW 07h; Configure comparators
MOVWF CMCON; for digital input
MOVLW 0CFh; Value used to initialize data direction
MOVWF TRISA; Set RA<3:0> as inputs RA<5:4> as outputs
fijate no solo tiene conversores AD sino tambien comparadores... entonces te falta setear los comparadores.
TAREA: leer TODO el datasheet