AVR-Chat

The variable PS or the 4V bandgap provide the reference.  

With REFS0 set, the external Vref is selected which means it's serving as an input.  

Problem is, the adc provides readings that are scaled against a 5V reference regardless of the actual Vref value.

Behaves the same in simulation so it's not a case of crispy fried chips.  

73/jeff/ac0c


From: n1ist 
Sent: Friday, January 22, 2010 8:50 PM
To: AVR-Chat@yahoogroups.com 
Subject: [AVR-Chat] Re: Vref on ATTiny861a


  
If you set the ADC to use an external reference, then Vref is an INPUT and you need to feed in the reference there. If you set the ADC to use anything else (Avcc, internal references), then Vref is an OUTPUT and the only thing you should have connected to it is a 100n filter cap. Feeding a voltage into Vref when it is trying to source will cause all sorts of problems, including the processor failing at elevated temperatures...

--- In AVR-Chat@yahoogroups.com, "Jeff Blaine AC0C" <keepwalking188@...> wrote:
>
> Graham,
> 
> Can you expand a bit more?
> 
> Vref is tied to this ps so that I can set the Vref to various levels for testing. Because the device did not seem to recognize the 4.0v bandgap ref, I put this in to see if varying the Vref would be reflected in different ADC readings. It does not. 
> 
> Thanks.
> 
> 
> From: ecros_technology 
> Sent: Friday, January 22, 2010 8:10 PM
> To: AVR-Chat@yahoogroups.com 
> Subject: [AVR-Chat] Re: Vref on ATTiny861a
> 
> 
> 
> --- In AVR-Chat@yahoogroups.com, "Jeff Blaine AC0C" <keepwalking188@> wrote:
> 
> > Vref is tied to a variable ps ...
> 
> Well, that's your problem, then.
> 
> Graham.
> 
> 
> 
> 
> 
> [Non-text portions of this message have been removed]
>





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