I don't see a settling time delay between the time that you
select the A/D channel and when you start the A/D conversion
(setting the GO bit). The data book mentions that the steps
for conversion are:

1. Configure the A/D module
2. Configure the A/D interrupt, if necessary
3. Wait the required sampling time
4. Start the conversion

Here is a snippet of the code I use:

get_start
        bcf     ADCON0, 3       ;switch to CH2
        bsf     ADCON0, 4       ;switch to CH2
        bcf     ADCON0, 5       ;switch to CH2
        call    sample_delay    ;provide sampling time
        bsf     ADCON0, 2       ;start conversion

settle2
        btfsc   ADCON0, 2       ;A/D over?
        goto    settle2 ;if no, then loop

        movf    ADRES, W        ;get A/D value
        movwf   START           ;store start value
.
.
.

sample_delay
        movlw           30
        movwf           TEMP3           ;initialize counter
SD
        decfsz  TEMP3, F        ;decrement counter and check if zero
        goto            SD              ;if no, loop again
        return                  ;if yes, return to main loop and begin A/D

This isn't even remotely optimized but it has always worked
fine.

Ryan

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of
> Thomas C. Sefranek
> Sent: Monday, November 08, 1999 7:35 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: PIC16C73A A-D
>
>
> I'm still looking for clues as to why the PIC
> will not do the entire voltage
> range with it's A-D.
>

<snip>

> Clear_IRQs CLRF INTCON ; Clear all Interrupts.
>   BCF PIR1, ADIF ; Clear any random A-D IRQ.
> Start_A2D BSF ADCON0, GO ; Start the A to D processes.
> ;
> ;******************************* Scheduler
> ************************************
> ; The Scheduler checks all task STATEs for
> "Active", (Non zero)
> ; activates the associated task for each state.
> ;*************************************************
> *****************************
> ;
> Scheduler BTFSC ADCON0,2 ; Is the A-D Ready?
>   GOTO Scheduler ; No, Return to scheduler loop.
> ;******************************* NEW SWR