If you have used more than 1 AD input ( the multiplexing mode )
than is almost sure you haven't used the right delayed between various
channels ( 20uS ). I'm not sure if you don't have also some noise or bad
ground design.

regards,
Vasile
http://www.geocities.com/vsurducan


On Tue, 14 Jan 2003, Jai Dhar wrote:

> Hello,
>
> Have a question about the A/D on the 16f877. I gave my first shot on coding a
> simple converter that uses a divider from a pot as an input to one of the
> pins. Now I'm pretty sure I have my code set right, because when I read the
> results (I did about 5 test cases), they all seem pretty accurate, except with
> a few problems.
>
> Some of the pins had constantly a constantly changing voltage that was
> registering on my DMM. Most of the pins would be stable, either ~0 or 5.2V,
> but some of them, each on different tests, would be fluctuating, sometimes
> from 2.5-2.9V, and sometimes other values. Is this normal? Does the fact that
> my program keeps going in a loop affect this (even though I'm reading the Go/!
> Done flag to check when it's done)? And it seems to be different pins each
> time, so there doesn't appear to be a definite pattern. On some readings, all
> the pins are perfect. Any ideas?
>
> Thank you,
>
> Jai
>
> PS: Just in case, I have my Code below.
>
> clrf TRISB                 ;Set all of Port B and D to outputs
> clrf TRISD
>
> movlw 0xff
> movwf TRISA
>
> movlw 0x0E           ;Set Justification and set RA0 as the only analog input
> movwf ADCON1
>
>
> bcf STATUS, RP0         ;Switch back to Bank 0
>
> movlw 0xC1           ;Select A/D on, Channel 0 and RC Clock
> movwf ADCON0
>
> movlw 0x00
> movwf PORTB    ;initialize the ports to a blank value.
> movwf PORTD
>
> Start1
>
>
> bsf ADCON0, GO           ;Start the A/D.
> Loop1
> btfsc ADCON0, GO
> goto Loop1
>
> ;Finished The A/D conversion
>
> movf ADRESL, 0     ;Move the low byte into Port B (B<7>:B<6>)
> movwf PORTB
>
> movf ADRESH, 0     ;Move the high byte into Port D (D<7>:D<0>)
> movwf PORTD
>
>
> goto Start1   ;Start it all over again.
>
>
>
>
>
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