I've used the '877 A/D with no problems at all. I always use a small filter
(series R, C to ground) on the inputs, and make sure that the 5V supply is
well filtered and bypassed, and that the layout is such that noise is
avoided. In my circuits, if I short the I/P pin to ground, the result of
the A/D conversion is zero, as it should be. If I put a known, stable
voltage on the I/P, the A/D conversion value is stable.

If you are reading a varying voltage on the I/P pin with a DVM, you
definitely have something wrong.

Larry

At 11:08 PM 1/14/2003 -0500, you 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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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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