Using Alias for A2D input names

[Edward LaBudde]

Hi all, newbie here.  Have encountered an “interesting” problem.  I have built a power monitor that measures the amps and volts of a battery charger.   I used an alias for the two A2D channels as “Amps” and “Volts” thusly:

#define Amps PORTA.0
#define Volts PORTA.1

Using this code
Amp_sample=readAD(Amps)    '(Amps)
Volt_sample=readAD(Volts)        '(Volts)

It does not work correctly.  It works only when the two channel inputs are tided together.
When the two inputs have separate sources the readings are wrong, and there is a significant interaction between the two channels.

Here is the assembler code it produces:
.ASM File
SAMPLE
    clrf    ADREADPORT
    btfsc    PORTA,0
    incf    ADREADPORT,F
    call    FN_READAD
    movf    READAD,W
    movwf    AMP_SAMPLE
    clrf    ADREADPORT
    btfsc    PORTA,1
    incf    ADREADPORT,F
    call    FN_READAD
    movf    READAD,W
    movwf    VOLT_SAMPLE
   
If I use the ports directly it works correctly.
As in the following:

Amp_sample=readAD(AN0)        '(Amps)
Volt_sample=readAD(AN1)        '(Volts)

.ASM  File
SAMPLE
    clrf    ADREADPORT
    call    FN_READAD
    movf    READAD,W
    movwf    AMP_SAMPLE
    movlw    1
    movwf    ADREADPORT
    call    FN_READAD
    movf    READAD,W
    movwf    VOLT_SAMPLE

Does anyone have any idea why there is a problem?

Regards, Ed.
   

[kent_twt4]

Had to look at the old RV voltmeter project.  Just change the define to:

#define volt AN0

I think you have to view this quirk, as an effort to standardize across device lines regardless of what port AN0 is actually on.

[Edward LaBudde]

Kent, thanks for the response.  Another "undocumented feature" no doubt. 
And just when I was getting to think I was getting the hang of things.  GCB has to keep me in my place!

Regards, Ed.