ADC Reference voltage

dongags5 · Posted: Tue Sep 16, 2014 9:12 am Post subject: ADC Reference voltage

I am unsure about the Reference voltage on the ATMEGA8535.
I use an inductor and capacitor from the VCC to supply the AVCC ADC pin and just use a bypass cap on the AREF pin and of course the ADC ground pin is grounded.
I use the following command
Config Adc = Single , Prescaler = Auto , Reference = Avcc

When I measure the voltage at the AREF pin I get the same voltage as I measure at the AVCC pin which is 5V which surprises me as I was expecting a reference voltage of 2.56V. Maybe what is measured at that pin is not the actual reference voltage?

I have an LCD display and look at the output from Getadc(2) i.e. the channel I use to apply an external voltage to the ADC.
Why is it that 255 (the maximum byte) is displayed when the DC input voltage is about 1.25V? I thought it would require an input voltage of twice that i.e. the same as the reference voltage (2.56V) or if the reference is 5V would be nearly twice that.

I hope that someone can throw some clarity on this - thanks.

Don

(BASCOM-AVR version : 2.0.7.5. , Latest : 2.0.7.7 )

Paulvk · Joined: 28 Jul 2006 Posts: 1197 Location: SYDNEY

Hello Don

Config Adc = Single , Prescaler = Auto , Reference = Avcc <<<<<<< its working as expected you told it to use AVCC

Try
Config Adc = Single , Prescaler = Auto , Reference =INTERNAL_2.56

I do not have a 8535 to try

Regards Paul

Visovian · Joined: 31 Oct 2007 Posts: 584 Location: Czech

dongags5 · Posted: Wed Sep 17, 2014 1:07 pm Post subject:

Thanks Paul,
I don't understand why there needs to be a s/w directive to have the AVcc reference when simply connecting the hardware AVcc to the reference pin would do the same thing if the chip only has the AVcc reference option. I tried the internal_2.56 directive and the compiler doesn't come up with an error. I got the impression that the ATMEGA8535 only accepts the Reference= AVcc command. I didn't measure the voltage at the reference pin when the _2.56 was specified but I suppose it would not be brought out and may not even be 2.56V - will check when I have time.

Thanks Visovian, you are quite right but I don't need high accuracy so a byte variable is fine.

cheers

Don

Evert :-) · Joined: 18 Feb 2005 Posts: 2114

Arera · Joined: 23 Sep 2007 Posts: 375 Location: Wuppertal, Germany

Hi Don
You must use a word for the adc. A byte would not reduce accuracy, but lead to completely false results.

dongags5 · Posted: Fri Sep 19, 2014 8:25 am Post subject:

Thanks Evert, I am using the correct regfile M8535.dat

In the Bascom help file, for the 8535 chip there seems to be only 2 modes: OFF and AVCC but as Paul suggested earlier I can use the internal reference of 2.56V. I measured 2.56V at the AREF pin confirming it to be correct.

Thanks Arera, I restrict my maximum input level so that I can use the byte variable rather than word as the high byte is zero. (ADC Result Registers ADCL is only used). Results are as expected and with sufficient accuracy for the requirement.

cheers

Don
Smile

Arera · Joined: 23 Sep 2007 Posts: 375 Location: Wuppertal, Germany

OK, you know what you are doing!
I thought you expectet to decrease the ADC's resolution from 10 to 8 bit by using a Byte....

Deanus · Joined: 26 May 2006 Posts: 188 Location: Adelaide

Hi dongags5,

With regards to the reference voltage, according to the data sheet,
You can choose the internal reference (2.56V) , OR the External reference VCC (Chip supply voltage) ,
But you CANNOT or should I say SHOULD NOT apply an external reference voltage and choose the internal reference via the switches.

This will have the effect of applying VCC chip supply voltage to the output of an opamp and will possibly destroy the internal reference opamp.
It explains it all in the Analogue reference section of the data sheet. Page 213 on my datasheet.

IF you supply an external Aref, then you MUST choose