AVR-Chat

Kasper,
 
You are correct data sheet sheet does state that the cap at aref is for improved noise performance although in the register settings (ADMUX) it does say that when selecting the avcc or internal reference that a cap should be at aref 
(Internal 2.56V Voltage Reference with external capacitor at AREF pin).
 
Must admit i have always "assumed" that this cap should be there. You're right about the delay between switching references though.
 
The wave form at this pin is a little odd, it looks as though the waveform  is being integrated by something switching the reference, possibly through the on board reference select fet and some parasitic capcitance that is 'seen' at this pin, it seems to be very periodic (not noise) and consistant.
 
When using testboards i have sometimes left this cap off, and can't recall seeing something like this. Is the reference being switched on and off quickly, looks like around 2MHz ?
 
Regards
 


--- On Mon, 18/7/11, kkpdk <kkpyahoogroups@kasperkp.dk> wrote:


From: kkpdk <kkpyahoogroups@kasperkp.dk>
Subject: [AVR-Chat] Re: ATMega16A - AREF oscillating
To: AVR-Chat@yahoogroups.com
Date: Monday, 18 July, 2011, 11:55


  





--- In AVR-Chat@yahoogroups.com, STEVEN HOLDER <s.holder123@...> wrote:

> Does not look good does it, i assume you do not get this with the non 'a' version ?

Yes, we have been using them since they were 'new'.

> Is the AREF pin left unconnected? i assume you are using the recommended filter circuit on the AVCC line and that there are no "switching" siganals that could couple into the circuit while the conversion is taking place ?

It is filtered, yes. So is VCC, and they are very clean, <500uV. While ADC needs to be enabled for the 2.56V reference to be output, it does not need to be converting.

> What is the clock frequency of the device ? and what is the converer prescaler ?

11.0592MHz. And /64 for ~176kHz. 

> The data sheet does by impication suggest a capacitor should be placed at the aref pin.

It says noise performance. And Atmel says it works without, just as it does in parts without an AREF pin (such as Tiny13).

And while I do have room for the capacitor, it means a delay every time I switch the reference. And since both the capacitance at which this part stops oscillating, and the sink current and thus settling time is undocumented, I will be relying on a double-undocumented fix. Just lovely.

/Kasper Pedersen








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