Your subroutine "Output_A2D_High_Byte" has only one instruction:

        movwf   PORTB

Move WHAT to PortB?  The stuff in W is gabarge from the delay routine your
have!  Better read the ADC value from ADRESH to W first


thomas

>From: Graham North <gs_piclist@HOTMAIL.COM>
>Reply-To: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [PIC]: 16F877 ADC Probs
>Date: Mon, 23 Apr 2001 20:13:28 -0000
>
>Hi I'm having some problems with the ADC on the 16F877. Am I correct in
>thinking that I can connect the sampling pin straight to a voltage (is this
>limited to 0 -> 5 Volts). Is some sort of protection recomended.
>
>I can not find reference to either of these questions in the data sheet.
>
>My code at the moment is reading pin A 0 and then displaying the highest 8
>bits to port B, but the value on port B will not change!
>
>I have included the code, I don't usually like including large amounts of
>code, but this has got me stumped!
>
>Thanks for any help,
>
>Graham North
>
>
>; ************************** Main  Code **************************
>
>Main
>
>        call    Startup_Init
>        call    InitPorts
>        call    InitPWM
>        call    Init_ADC
>
>Loop
>
>        call    ADC_Read
>
>        bsf     PORTE,0                 ; Used for testing
>
>        call    Delay
>
>        call    Output_A2D_High_Byte
>
>        bcf     PORTE,0                 ; Used for testing
>
>        call    Delay
>
>        goto Loop
>
>; ************************* Subroutines **************************
>
>
>Output_A2D_High_Byte
>
>; Outputs the 8bit value returned from the A2D
>
>        movwf   PORTB
>
>        return
>
>
>InitPWM
>
>; Sets up PWM output
>
>        BANK1                   ; Select bank 1.
>
>        bcf     TRISC,2         ; Set pin as output by clearing
>                                ; TRISC<2> bit.
>
>        movlw   B'00111111'     ; Set PWM period.
>        movwf   PR2             ; Period = [(PR2)+1] * 4 *
>                                ; TOSC * (TMR2 prescale value)
>                                ; Page 61 data sheet.
>
>        BANK0                   ; Select bank 0.
>
>        movlw   B'00100000'     ; Set PWM duty cycle.
>        movwf   CCPR1L          ; Duty Cycle = (CCPR1L:CCP1CON
>        bsf     CCP1CON,5       ; <5:4>) * TOSC * (TMR2
>        bsf     CCP1CON,4       ; prescale value)
>                                ; Page 61 data sheet.
>
>        movlw   B'00000110'     ; Start TIMER2 and set TMR2
>        movwf   T2CON           ; prescale value to 1:16
>                                ; (bit 1 = 1, bit 0 = x).
>                                ; Page 55 data sheet.
>
>        movlw   B'00001100'     ; Start PWM (bits 3-0 = 11xx)
>        movwf   CCP1CON         ; Page 58 data sheet.
>
>        return
>
>
>InitPorts
>
>; INITIALISE PORTS
>; binary used to see individual pin level
>
>        movlw b'00000000'       ; all port pins = low
>        movwf PORTA
>        movlw b'00000000'
>        movwf PORTB
>        movlw b'00000000'
>        movwf PORTC
>        movlw b'00000000'
>        movwf PORTD
>        movlw b'00000000'
>        movwf PORTE
>
>        BANK1                   ; Select Bank 1 for TRIS registers
>
>; binary used to see individual pin IO status  MSB -> LSB
>
>        movlw b'00000001'       ; Port A Bit 0 = input
>        movwf TRISA
>        movlw b'00000000'       ; Port B = outputs
>        movwf TRISB
>        movlw b'01000000'       ; Port C Bit 6 = input
>        movwf TRISC
>        movlw b'11111111'       ; Port D = inputs
>        movwf TRISD
>        movlw b'00000000'       ; Port E = outputs
>        movwf TRISE
>
>        movlw b'00000110'       ; all analog pins = digital
>        movwf ADCON1
>
>        BANK0                   ; Select Bank 0
>
>        return
>
>
>Startup_Init
>
>; Setting up the Special Function Registers
>
>        movlw   b'10010000'
>        movwf   INTCON
>
>        movwf   b'11111111'
>        movwf   OPTION_REG
>
>        return
>
>
>Init_ADC
>
>; Initialize ADC to use Fosc/2 and input line 000
>; input line can be selected on a per conversion basis
>; using GetADC
>
>        bsf     STATUS,RP0
>
>        clrf    TRISA           ; setup PORT A for input
>        comf    TRISA
>
>        bcf     PIE1,ADIE       ; disable ADC interrupt
>
>        bcf     STATUS,RP0
>
>        movlw   B'00000001'     ; initialize ADC
>        movwf   ADCON0
>
>        bsf     STATUS,RP0
>        clrf    ADCON1
>        bcf     STATUS,RP0
>
>        bcf     PIR1,ADIF       ; clear conversion complete flag
>
>        return
>
>
>ADC_Read
>
>; Read a value from the internal 10 bit A2D and return it in the working
>register.
>
>        btfsc   ADCON0,GO       ; wait for conversion to finish
>        goto    ADC_Read
>
>        bcf     PIR1,ADIF       ; clear conversion complete flag
>
>        movf    ADRESH,W        ; return upper 8 bits of result
>
>        return
>
>Delay
>
>; 1/2 SEC DELAY SUBROUTINE WITH 4MHz CLOCK
>
>        movlw 01h
>        movwf NbHi
>        movlw 03h
>        movwf NbLo
>        movlw 8Ah
>        movwf NaHi
>        movlw 5Bh
>        movwf NaLo
>
>DeLoop0 decfsz NaLo,F
>        goto DeLoop0
>        decfsz NaHi,F
>        goto DeLoop0
>        decfsz NbLo,F
>        goto DeLoop0
>        decfsz NbHi,F
>        goto DeLoop0
>        return
>
>
>
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