;************************************************************************
;                                                                       *
;   Filename:       BA_L4-PB_LED-shadow.asm                             *
;   Date:           15/9/07                                             *
;   File Version:   1.0                                                 *
;                                                                       *
;   Author:         David Meiklejohn                                    *
;   Company:        Gooligum Electronics                                *
;                                                                       *
;************************************************************************
;                                                                       *
;   Architecture:   Baseline PIC                                        *
;   Processor:      12F508/509                                          *
;                                                                       *
;************************************************************************
;                                                                       *
;   Files required: none                                                *
;                                                                       *
;************************************************************************
;                                                                       *
;   Description:    Lesson 4, example 1b                                *
;                                                                       *
;   Demonstrates use of shadow register when reading a switch           *
;                                                                       *
;   Turns on LED when pushbutton on GP3 is pressed (active low)         *
;                                                                       *
;************************************************************************
;                                                                       *
;   Pin assignments:                                                    *
;       GP1 - LED                                                       *
;       GP3 - pushbutton switch                                         *
;                                                                       *
;************************************************************************

    list        p=12F509       
    #include    <p12F509.inc>  

                ; int reset, no code protect, no watchdog, 4Mhz int clock
    __CONFIG    _MCLRE_OFF & _CP_OFF & _WDT_OFF & _IntRC_OSC


;***** VARIABLE DEFINITIONS
        UDATA_SHR
sGPIO   res 1                   ; shadow copy of GPIO


;************************************************************************
RESET   CODE    0x000           ; effective reset vector
        movwf   OSCCAL          ; update OSCCAL with factory cal value 


;***** MAIN PROGRAM

;***** Initialisation
start				
        movlw   b'111101'       ; configure GP1 (only) as an output
        tris    GPIO            ; (GP3 is an input)

;***** Main loop
loop
        clrf    sGPIO           ; assume button up -> LED off
        btfss   GPIO,3          ; if button pressed (GP3 low)
        bsf     sGPIO,1         ;   turn on LED

        movf    sGPIO,w         ; copy shadow to GPIO
        movwf   GPIO

        goto    loop            ; repeat forever


        END