;pbtest.asm

; Test the push buttons by putting the value on the led bar
; The push buttons are interfaced through RB7-RB0.  The diagram in the Raven manual is not
; quite correct.  PB1 is the black button, and is connected to RB7, not RB4.  Similarly,
; RB6 is connected to yellow, RB5 to red and RB4 to green.
;
; Be careful! One of the settings that is necessary in order for the buttons to work
; is to clear bit 7 of the option register (also known as the NOT_RBPU bit).  When cleared,
; a weak pullup resistor is applied to the B pins, resulting in a default value of 1 on
; all of the pins.  Then, when the button is pressed, the value on the pin is driven
; low (zero).
;

        LIST P=16F877, R=DEC

        INCLUDE "P16F877.inc"

; Definitions

;  __CONFIG _CP_OFF & _DEBUG_OFF & _WRT_ENABLE_OFF & _CPD_OFF & _LVP_OFF & _BODEN_OFF & _PWRTE_ON & _WDT_OFF & _HS_OSC 


; Variables
ScratchPad  EQU     0x20
Temp        EQU     ScratchPad+0



; Start of the Program

        ORG       3
        goto      Start

        ORG       0x50 
Start
        bsf       STATUS,RP0
        clrf      TRISA
        clrf      TRISD
        movlw     b'11110000'
        movwf     TRISB
        bcf       OPTION_REG, 7       ; This controls the pullup registers
        bcf       STATUS,RP0

pbloop

; Get the state of the push button outputs

        movlw     0x1C
        movwf     PORTA

        fill (nop), 10           ; Give the buffer outputs time to settle

        movf      PORTB,W
        andlw     0xF0

; and output them for all to see

        call DispLedBar

        goto      pbloop


DispLedBar

; Enable the led bar and put the value in W there.

        movwf     Temp             ; save W
        movlw     0x12
        movwf     PORTA

        comf      Temp,W
        movwf     PORTD

        return


        END