I have a signal fluctuate from 1.89V to 2.10V, i.e. the signal have 200mV fluctuation. I want to use it to control a relay. When the signal change from 2.1V to 1.89V, set the relay. When the signal change from 1.89V to 2.10V, reset the relay.

I use PIC16c73b's RA0 as A/D channel to sample the signal, and RA3 as the reference Vref(3V), and RB7 as the output to control the relay.

The question is that: When I debug the system step by step it works just as I expected, but when I run it all the way it does not work, i.e. when I change the signal, the relay does not work. The program is revised from the example program of INTAD.ASM.

The possible reason is that the change of the signal is lost. I don't know when and why it is lost.

(I'd say that I am new to PIC)

        LIST P=16C73b
;     ERRORLEVEL  -302
;
        include "P16c73b.inc"
;
flag    equ     20
TEMP    equ     21
adover  equ     0
oldad   equ     22
tmp     equ     23
first   equ     24
PORTB_TMP equ   25
status_tmp equ  26
#define differ 0x05
;adif    equ    1
;adgo    equ    2
;adie    equ    6
;gie     equ    7
;rp0     equ    5
;
        ORG     0x00
;
;
        goto    start
;
        org     0x04
        goto    service_ad      ;interrupt vector
;
;
        org     0x10
start
      movlw   0x00            ;init i/o ports
        clrf    PORTB
      BSF     STATUS, RP0     ;Bank1
      MOVWF   TRISB           ;PortB as outputs
      BCF     STATUS, RP0     ;Bank0
        clrf    oldad
        clrf    tmp
        movlw   0x01
        movwf   first
        movlw   0x00
        movwf   PORTB_TMP
;
        call    InitializeAD
;
update
        bcf     flag,adover     ;reset software a/d flag
        call    SetupDelay      ;setup delay >= 10uS.
        movf    adres, W
        movwf   oldad
        bcf     PIR1,adif       ;reset a/d int flag (ADIF)
        bsf     INTCON, peie    ;Enable peripheral interrupts
        bsf     INTCON,gie      ;enable global interrupt
        bsf     ADCON0,go       ;start new a/d conversion
loop
        btfsc   flag,adover     ;a/d over?
        goto    update          ;yes
        goto    loop            ;no then keep checking
;
service_ad
        btfss   PIR1,adif       ;ad interrupt?
        retfie                  ;no then ignore
        bsf     flag,adover     ;a/d done set
        movf    ADRES,W         ;get a/d value
;
        subwf   oldad, W        ;(oldad)-(adres)
        movwf   tmp
        movf    status, W
        movwf   status_tmp
        btfsc   first, 0                ;the first time?
        goto    TheFirst
      goto    FirstOut
TheFirst
        decf    first, F
        bcf     PORTB, 7
        bcf     PORTB_TMP, 0
        goto    out                     ;omit the first time
FirstOut
        btfss   status_tmp, 0
        goto    negative        ;adres>oldad
;
        movf    tmp, W          ;adres<=oldad
        sublw   differ          ;differ-((oldad)-(adres))
      btfsc   STATUS, 0
        goto    nochange                ;no change
        bsf     PORTB, 7                ;change
        bsf     PORTB_TMP, 0
        goto    out                     ;start new conv.
nochange
      btfsc   PORTB_TMP, 0
        goto    one
        bcf     PORTB, 7
        bcf     PORTB_TMP, 0
        goto    out
one
        bsf     PORTB, 7
        bsf     PORTB_TMP, 0
        goto    out
negative
        comf    tmp, W
        addlw   0x01            ;|(oldad)-(adres)|
        sublw   differ          ;differ-|(oldad)-(adres)|
      btfsc   STATUS, 0
        goto    nochange                ;no change
        bcf     PORTB, 7                ;change
        bcf     PORTB_TMP, 0
out
        return                  ;do not enable int
;
;
;InitializeAD, initializes and sets up the A/D hardware.
;select ch0 to ch3, RC OSC., a/d interrupt.
;
InitializeAD
        clrf    INTCON          ;clear all interrupts
        bsf     STATUS,rp0      ;select Bank1
        movlw   0x01            ;select ch0-ch3 as analog inputs...
        movwf   ADCON1          ;and RA3 as Vref
      bsf     PIE1, adie      ;Enable A/D interrupts
        bcf     STATUS,rp0      ;select Bank0
        movlw   0xc1            ;RC clock, turn on a/d, ...
        movwf   ADCON0          ;Channel 0 is selected
        return
;
;This routine is a software delay of 10uS for the a/d setup.
;At 4Mhz clock, the loop takes 3uS, so initialize TEMp with
;a value of 3 to give 9uS, plus the move etc should result in
;a total time of > 10uS.
SetupDelay
        movlw   0x03
        movwf   TEMP
SD
        decfsz  TEMP, F
        goto    SD
        return
;
;
        END
--

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