Steve Smith <PICLIST@MITVMA.MIT.EDU> wrote:

> The data book on this subject has led me to confuse myself (again).
> The point is as follows. configuration of the A-D is as follows
> ADCON1 equ b'00000100' this implies that RA0 and RA1 are analouge
> inputs and Vref is Vdd problem comes here RA3 is also analouge
> input.
>
> RA3 is required as digital output and its Tris flag is set to 0
> therefore a digital output can be sent refering to THE BOOK fig 5-1
> the hardware implies that a digital input is not possable during a
> A-D conversion no problem, but a digital output seems possable and
> after trying this it appears to work. However the port info changes
> on the basis of read modify write surely at this point the data on
> the digital input is unstable due to being analouge will this cause
> a problem with code or can the RA3 be leagaly used as digital output
> when ADCON1 is set as above ? Can sombody clarify this dilema

Steve:

If a port pin is configured as an analog input by ADCON1 and as a
digital output by TRISA, read-modify-write instructions will
read it as a "0".

This may cause a problem with your code, but it won't be due to any
"unstable" data on the digital input; instead, any problems it causes
will be of the usual read-modify-write variety.  For instance:

    BSF     PORTA,1 ;After this instruction executes, RA1 goes high.

    BSF     PORTA,2 ;After this instruction executes, RA2 goes
                    ;high, but RA1 GOES LOW, since the PIC reads
                    ;"0" for all pins configured as analog inputs.

One way to overcome this problem is to use a "shadow" register:

    SHADOW  EQU     [any register]

    ....

    CLRF    PORTA   ;Make sure that SHADOW and PORTA are in sync.
    CLRF    SHADOW  ;

    ....

    BSF     SHADOW,1    ;After these instructions are executed,
    MOVF    SHADOW,W    ;RA1 will go high.
    MOVWF   PORTA       ;

    BSF     SHADOW,2    ;After these instructions are executed,
    MOVF    SHADOW,W    ;RA2 will go high without affecting RA1.
    MOVWF   PORTA       ;

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
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