PIC16F877A blinking leds example. Only blinks PORTA.

blink.asm

;*******************************************************************************
; Exercise 01
; Description: This program adds a number to the Working register.
; Author: Bangon Kali 4/6/2014
;*******************************************************************************

;*******************************************************************************
; Register file map
ADCON1 equ 9fh
STATUS equ 03h
TRISA equ 85h
PORTA equ 05h

COUNT1 equ 20h ;First counter for our delay loops (General Purpose Register)
COUNT2 equ 21h ;Second counter for our delay loops (General Purpose Register)
;*******************************************************************************

RP0 equ 05h
RP1 equ 06h

; Use the org directive to force program start at reset vector
        org 00h

; Program starts here
        clrw                    ; Clear the working register

        BCF STATUS, RP0         ;
        BCF STATUS, RP1         ; Bank0
        CLRF PORTA              ; Initialize PORTA by
                                ; clearing output
                                ; data latches
        BSF STATUS, RP0         ; Select Bank 1
        MOVLW 0x06              ; Configure all pins
        MOVWF ADCON1            ; as digital inputs


        MOVLW 0x00              ; Value used to
                                ; initialize data
                                ; direction

        MOVWF TRISA             ; Set RA<3:0> as inputs
                                ; RA<5:4> as outputs
                                ; TRISA<7:6>are always
                                ; read as '0'.
        BCF STATUS, RP0         ; Clear
        BCF STATUS, RP1         ; Bank0

START   MOVLW 0xff              ; Turn all leds on move to Wreg
        MOVWF PORTA             ; Turn all leds on PORTA

        MOVLW 0xff
        MOVWF COUNT1            ; Set Count1 to 0xFF
        MOVWF COUNT2            ; Set Count1 to 0xFF

LOOP1   DECFSZ COUNT1, 1        ; Subtract 1 from 255
        GOTO LOOP1              ; If COUNT is Zero then carry on.
        DECFSZ COUNT2, 1        ; Subtract 1 from 255
        GOTO LOOP1              ; Go back to the start of loop.
                                ; This delay counts down from 255 to 0
                                ; 255 times.

        MOVLW 0x00              ; Turn all leds on move to Wreg
        MOVWF PORTA             ; Turn all leds on PORTA

        MOVLW 0xff
        MOVWF COUNT1            ; Set Count1 to 0xFF
        MOVWF COUNT2            ; Set Count1 to 0xFF

LOOP2   DECFSZ COUNT1, 1        ; Subtract 1 from 255
        GOTO LOOP2              ; If COUNT is Zero then carry on.
        DECFSZ COUNT2, 1        ; Subtract 1 from 255
        GOTO LOOP2              ; Go back to the start of loop.
                                ; This delay counts down from 255 to 0
                                ; 255 times.

        GOTO START

        end