; sensor.asm

; Program to get interrupts from an external sensor connected to RB0.  In particular,
; this program is designed to detect and count the number sensor events from a
; QRB1134 emitter/sensor pair.

        LIST P=16F877, R=DEC

        INCLUDE "P16F877.inc"


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

; Define start of vars

RAM        EQU   0x20

; Variables

Temp       EQU   RAM+0
Count      EQU   RAM+1        ; Counter for events
UpdateFlag EQU   RAM+2


; Temps for interrupt handling.  Put in 0x70 to 0x7F as they are available across all 
; RAM banks.

W_Temp     EQU   0x70
Stat_Temp  EQU   0x71

; Start of the Program

        ORG       3
        goto      Start

; ======================================================================= ISR Routine

        ORG       4
isr
; Save the current state

        movwf     W_Temp
        swapf     STATUS,W
        CLRF      STATUS
        movwf     Stat_Temp


; Handle the interrupt on RB0

        btfss     INTCON,INTF      ; do we have one
        goto      restore          ; if not, we are done

; reneable interrupt

        bcf       INTCON,INTF

; Do what we came to do, set a flag.

        bsf       UpdateFlag,0

restore

; Restore the prior system state

        swapf     Stat_Temp,W
        movwf     STATUS
        swapf     W_Temp,F
        swapf     W_Temp,W
       
        retfie


; ========================================================================= Main program 

        ORG       0x50
Start

; Set  A4-A0 to be output and RD0-RD7 to be output for the LED bank and set
; RB0 for input

        bsf       STATUS, RP0

        clrf      TRISA
        clrf      TRISD
        clrf      TRISB
        bsf       TRISB,0

        bcf       STATUS, RP0

; Initialize the Count and display

        clrf      Count
        movf      Count,W
        call      DispLedBar


; Initialize the flag

        clrf      UpdateFlag

; Initialize the interrupts

        call      InitInts

; In the loop, watch for the flag indicating that an event has occurred 
; and update the Counter and display.

mainloop

        btfss     UpdateFlag,0
        goto      mainloop

        incf      Count,F
        movf      Count,W
        call      DispLedBar
        bcf       UpdateFlag,0

        goto      mainloop


; Initialize interrupts

InitInts

; Set up RB0 to interrupt on the falling edge, so that interrupts as the voltage
; drops.

        bsf       STATUS, RP0      ; OPTION_REG is in bank 1

        bsf       OPTION_REG,0     ; pullups
        bcf       OPTION_REG,0     ; interrupt on falling edge
        
        bcf       STATUS, RP0

        
; Enable the interrupts

        bsf      INTCON,GIE
        bsf      INTCON,INTE
        bcf      INTCON,INTF

        return

; -----------------------------------------------
; Output a value to the LED bar

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