configuring AD converter in a 12F615

[tekart]

That would be awesome Darrell!

I have something working now using a 1N914 diode from the pin to ground and a 10K to plus from the pin. Sorta using the diode as a reference. But it's depending on the diode drop which may not be accurate in production.

A no-pin solution would be a better design. Have you posted this idea to the forum in the past? If not I'm sure that many of us that design battery powered products would benefit from your genius!

[Darrel Taylor]

Yes, I had posted it before. But it was for a different chip, and I can't find it now.
It's not too difficult, so I re-wrote it for the 12F615 and tested it on a LAB-X4.

The idea is to use the internal 0.6V reference (ADC channel 5).
The reference stays the same as VDD changes, so with VDD as the ADC's Vref, the value read changes too. You can use that change to calculate actual VDD without needing an external Analog Pin.

DEFINE ADC_BITS 10

LCD       VAR GPIO.1              ; serial pin for LCD
LCDbaud   CON 396                 ; serial baud rate

ADval     VAR WORD
VDD       VAR WORD

ANSEL  = 0                        ; All Pins are Digital
ADCON0.7 = 1                      ; Right Justify A/D result
HIGH LCD : PAUSE 2                ; Initialize serial data level

SEROUT2 LCD,LCDbaud,[$FE,1] : PAUSE 3 ; Clear LCD

GOSUB GetVDD

SEROUT2 LCD,LCDbaud,["A/D=",DEC ADval,"  ", _
                     "VDD= ",DEC VDD/10,".",DEC1 VDD]
STOP
;------------------------------------------------------------------
Vref_AD   CON 117           ; A/D reading for 0.6V Vref @ 5V VDD

GetVDD:
    VRCON.4 = 1                   ; Enable 0.6V Fixed reference
    PAUSE 10                      ; allow reference to stabilize
    ADCIN 5, ADval                ; read the 0.6V reference
    VDD = (Vref_AD * 50) / ADval  ; convert to volts
    VRCON.4 = 0                   ; turn off fixed reference
RETURN

The VDD result has 1 decimal place, so 5.0V will be 50.

HTH,

[tekart]

Awesome Darrell!

I plugged in your brilliant code and it works flawlessly!

Is it possible to get 2 decimals of resolution using the 10bit ADC? On a 3 Volt system that drops out a 2.0 Volts it could make a difference.

[Darrel Taylor]

Well as far as the math goes, you can get 2 decimals by multiplying *500 instead of 50. Then change the SEROUT2 to divide by 100 and use DEC2.

But those references aren't really accurate enough to get that kind of resolution reliably.
And they drift some with temperature, so you'll probably want to stick with 1 decimal place.

You could also do the same thing with an external "Precision" reference and get better resolution. But of course that uses a PIN, which are precious on an 8-pin chip.

[tekart]

Yeah, I though that would work. I tried X500 and got readings that varied by about .05 Volts even when I hit the PIC with a heat gun to see if there would be significant temperature drift. Not bad actually.

This is a totally awesome way to read the battery voltage Darrell. THANKS AGAIN. You are a friggin genius!

[klantz12]

Can anyone help me with configuring the AD converter in 16F877A on the labX1 board? I am using MicroCode Studio-pbp and i have tried the following code from the help button and from another post i read.


ADCON1=132 ' 128 + 4 = %10000100 'Left just 3 analog inputs rest digital
TrisA = 255 'all inputs for the a to d
Define ADC_BITS 10 ' Set number of bits in result
Define ADC_CLOCK 3 ' Set clock source (rc = 3)
Define ADC_SAMPLEUS 50 ' Set sampling time in microseconds

AdcIn 1, W0
AdcIn 3, W1


The error says that ADCIN needs a variable.

My assignment is just to read the AD input from the three potentiometers at A0 A1 and A3.

Pretty lost. Thanks! also, i didnt know how to start a new thread so sorry for posting in ya'lls.

Kay

[mackrackit]

didnt know how to start a new thread so sorry for posting in ya'lls.

Big button near the top of forum pages, "+ Post New Thread".

[tekart]

AdcIn 1, W0
AdcIn 3, W1

The error says that ADCIN needs a variable.

Kay, I'm assuming that you have migrated to the PIC world from Basic Stamps and you may not have defined your variables - something you do not need to do in Stamps. So you need:

W0 var byte

to define W0 as a byte variable - these definitions usually go near the top of the page.