AREF bypass capacitance on ATMega2560?

Folks,
What's the usual capacitance? Any stability issues there? I was planning
on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
order to be able to use its internal bandgap reference. I saw people
using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
as usual.
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
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Are you sure the datasheet does not explain it. I have read that data sheet and it explained filtering for minimum noise. I don't remember any details but filtering the ADC supply was there. What about some example schematics . I am sure I got values somehow (pulled from sleeve?).
Leif
Reply to
LM
Atmel mentions "a capacitor" but no values. I've parsed it several times. I could write to them but IME such things take their time and I wanted to submit to layout preferably tomorrow.
The ones I found had it tied to AVCC.
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
first app note on the site:
formatting link

33n
-Lasse
Reply to
Lasse Langwadt Christensen
That is a much smaller device, the ATMega88, different silicon. For the ATMega2560 I have never seen anything that small in schematics on the web. All between 0.1uF and 0.47uF but they were not from Atmel.
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
Well, I used 100nF there. How I got it I don't remember. I may have applied some exact scientific method like making a guess (or picked a piece muesli ) and it worked well enough. I don't remember any large problems just there but Atmel's Datasheets are not perfect. If I now go and look at the datash eet and find the answer then... Are you sure you read it through.
Reply to
LM
2560 cpus are used in Arduino cards. They use 100nF there. Allthough I must say I truly hate schemas made with eagle.
A application note seems to use 0.33u. So anything will work.
Reply to
LM
I use Eagle all the time. You can make schematics look just like Orcad or any other.
Well, I wanted to be sure so I asked Atmel. Filled out their club membership application. Surprisingly I received an answer this morning: They say 100nF. In American that would be 0.1uF :-)
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
Forgot to mention, unless I really missed it the Arduino Mega 2560 has no cap at all there:
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--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
Now Xerox that diagram 10 times (copy from a copy from a copy...). That dot will dissapear and make you look stupid because you seem to have written 01uF. Thats why we use 100nf, 1k5, etc in Europe :-)
I hope Atmel didn't lie to you. Atmel likes to be creative with their specs... I had to put Atmel on my blacklist.
--
Failure does not prove something is impossible, failure simply 
indicates you are not using the right tools... 
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Reply to
Nico Coesel
There is one, 100nF (C3). But any value will do (almost)....
Cheers
Klaus
Reply to
Klaus Kragelund
Indeed! Didn't see it. I really hate such netlist style schematics, better to draw it right at the pin.
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
We over here in the lands of the Wild West will see the gap and immediately know there's a dot. That can be a whole 'nother story if it's casually written as .1uF which I try to avoid.
And who xeroxes anymore these days? I still have a machine in my office but that's only for legal papers and stuff.
So far, in over 25 years, I had only one screw-up with Atmel. One of their 8051 series uC would not reliably run at full spec'd clock speed and I had to back off to 75%. They did fess up and apologize though, something only very few and good companies (such as LTC) do when caught with a bug.
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
Now you know why I hate eagle.
Reply to
LM
guess it is just a matter of being brought up with SI units and standard use
0.1uF and 100nF is the same number of letters
even worse they can change numbers:
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-Lasse
Reply to
Lasse Langwadt Christensen
Atmel does not explain the USB pins of their Mega CPUs very well.
Reply to
LM
That's not at all Eagle's fault. Any CAD can be used to draw such chopped up schematics, most that I've see were done in Orcad and Protel but only because those are very popular programs. After all, you won't likely hate a particular brand of car just because one crashed into yours after its driver screwed up :-)
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
Yeah, it's just a custom I guess. Right after my degree I started working at an American company. It was in Germany but English was spoken inside the building, units were imperial, and schematics were US-style with wiggly-line resistors and all that.
Copying machines should never ever second-guess what is on a document. That is IMO a very bad design and I would never buy a machine like that.
[...]
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
I learned the hard way that none of Atmel's parts specced to run at 1.8V will work reliably at 1.8V.
--
Failure does not prove something is impossible, failure simply 
indicates you are not using the right tools... 
 Click to see the full signature
Reply to
Nico Coesel
[...]
Yikes, that does not sound good. What did they say about it?
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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