AREF bypass capacitance on ATMega2560?

[Joerg]

Lasse Langwadt Christensen wrote:

On Tuesday, August 20, 2013 7:49:01 PM UTC+2, Joerg wrote:
Nico Coesel wrote:

Joerg <invalid@invalid.invalid> wrote:
LM wrote:
On Monday, August 19, 2013 11:14:22 PM UTC+3, Joerg wrote:
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.
http://www.analogconsultants.com/
2560 cpus are used in Arduino cards. They use 100nF there. Allthough
I must say I truly hate schemas made with eagle.
I use Eagle all the time. You can make schematics look just like Orcad
or any other.
A application note seems to use 0.33u. So anything will work.
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
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


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.


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

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.

And who xeroxes anymore these days? I still have a machine in my office

but that's only for legal papers and stuff.


even worse they can change numbers:

http://www.dkriesel.com/en/blog/2013/0802_xerox-workcentres_are_switching_written_numbers_when_scanning?

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/

 

[Nico Coesel]

Joerg <invalid@invalid.invalid> wrote:

Nico Coesel wrote:
Joerg <invalid@invalid.invalid> wrote:

LM wrote:
On Monday, August 19, 2013 11:14:22 PM UTC+3, Joerg wrote:
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.
http://www.analogconsultants.com/
2560 cpus are used in Arduino cards. They use 100nF there. Allthough
I must say I truly hate schemas made with eagle.

I use Eagle all the time. You can make schematics look just like Orcad
or any other.


A application note seems to use 0.33u. So anything will work.

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

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


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.


I hope Atmel didn't lie to you. Atmel likes to be creative with their
specs... I had to put Atmel on my blacklist.


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.

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...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

 

[Joerg]

Nico Coesel wrote:

Joerg <invalid@invalid.invalid> wrote:

Nico Coesel wrote:

[...]

I hope Atmel didn't lie to you. Atmel likes to be creative with their
specs... I had to put Atmel on my blacklist.

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.

I learned the hard way that none of Atmel's parts specced to run at
1.8V will work reliably at 1.8V.

Yikes, that does not sound good. What did they say about it?

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

LM wrote:

On Tuesday, August 20, 2013 8:49:01 PM UTC+3, Joerg wrote:
Nico Coesel wrote:

[...]

I hope Atmel didn't lie to you. Atmel likes to be creative with their
specs... I had to put Atmel on my blacklist.


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/

Atmel does not explain the USB pins of their Mega CPUs very well.

It can be worse. Many connector datasheets have no (!) amperage rating
for their contacts. That is kind of an important parameter.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[John Devereux]

Joerg <invalid@invalid.invalid> writes:

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.

MCU manufacturers are so generally so clueless with respect to the
analog aspects... I would just experiment with a devkit if at all
worried.


--

John Devereux

 

[Spehro Pefhany]

On Tue, 20 Aug 2013 11:54:54 -0700, Joerg <invalid@invalid.invalid>
wrote:

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.

It's important that they send the correct information to the NSA over
their Ethernet port.

 

[John Devereux]

Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> writes:

On Tue, 20 Aug 2013 11:54:54 -0700, Joerg <invalid@invalid.invalid
wrote:



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.

It's important that they send the correct information to the NSA over
their Ethernet port.

You know, funny, that does not even sound all that paranoid now.

--

John Devereux

 

[Joerg]

Klaus Kragelund wrote:

On Tuesday, August 20, 2013 9:44:25 PM UTC+2, Spehro Pefhany wrote:
On Tue, 20 Aug 2013 11:54:54 -0700, Joerg <invalid@invalid.invalid


wrote:



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.


It's important that they send the correct information to the NSA
over

their Ethernet port.

I think they may have some back-door installed in the copying
machines.

On another note, some machines must by law have a money bill
detector. If a bill is detected, they insert some pixels on the print
to identify the machine

Then a text appears on the little LCD "Dude, you're screwed!" and the
flashing of blue lights can be seen down at the street.

Even restrooms have such features:

http://www.funnycorner.net/funny-pictures/4347/183.jpg

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Spehro Pefhany]

On Tue, 20 Aug 2013 13:33:11 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

On Tuesday, August 20, 2013 9:44:25 PM UTC+2, Spehro Pefhany wrote:
On Tue, 20 Aug 2013 11:54:54 -0700, Joerg <invalid@invalid.invalid

wrote:







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.



It's important that they send the correct information to the NSA over

their Ethernet port.

I think they may have some back-door installed in the copying machines.

On another note, some machines must by law have a money bill detector. If a bill is detected, they insert some pixels on the print to identify the machine

Regards

Klaus

Many color printers put an ID dot code on the output page regardless
of the source.

http://w2.eff.org/Privacy/printers/docucolor/

http://www.washingtonpost.com/wp-dyn/content/article/2005/10/18/AR2005101801663.html

Photoshop (and Paint Shop Pro) have a banknote detection algorithm:
http://fstoppers.com/photoshop-wont-let-you-work-with-images-of-currency

 

[Nico Coesel]

Joerg <invalid@invalid.invalid> wrote:

Nico Coesel wrote:
Joerg <invalid@invalid.invalid> wrote:

Nico Coesel wrote:

[...]

I hope Atmel didn't lie to you. Atmel likes to be creative with their
specs... I had to put Atmel on my blacklist.

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.

I learned the hard way that none of Atmel's parts specced to run at
1.8V will work reliably at 1.8V.


Yikes, that does not sound good. What did they say about it?

Not much. Tech support didn't respond after I concluded the device
isn't suitable for running on 1.8V. I guess they knew they lost a
customer. I solved the problem by adding an extra 2V regulator in the
next production runs.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

 

[Vladimir Vassilevsky]

On 8/19/2013 3:14 PM, Joerg wrote:

Folks,
What's the usual capacitance?

0.1uF

> Any stability issues there?

Nothing special.

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.

Do not. Use Vcc as reference.
Internal reference is inaccurate.

I saw people
using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
as usual.

It doesn't really matter.

Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com

 

[Jim Stewart]

Vladimir Vassilevsky wrote:

On 8/19/2013 3:14 PM, Joerg wrote:

Folks,
What's the usual capacitance?

0.1uF

Any stability issues there?

Nothing special.

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.

Do not. Use Vcc as reference.
Internal reference is inaccurate.

I noticed that awhile ago. The specs on my
LDO regulator were about 5x better than the
specs on the internal "reference".

 

[Joerg]

Vladimir Vassilevsky wrote:

On 8/19/2013 3:14 PM, Joerg wrote:

Folks,
What's the usual capacitance?

0.1uF

Any stability issues there?

Nothing special.

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.

Do not. Use Vcc as reference.
Internal reference is inaccurate.

Yeah, sure looks ghastly. I provided my own reference from another board
and filtered it with 0.1uF.

I saw people
using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
as usual.

It doesn't really matter.

However, not everyone knows that because they don't say much about the
innards of the chip. It's my first ATMega case, or maybe the 2nd.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Tim Williams]

"Joerg" <invalid@invalid.invalid> wrote in message
news:b7l1blFlu93U2@mid.individual.net...

I saw people
using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
as usual.

It doesn't really matter.


However, not everyone knows that because they don't say much about the
innards of the chip. It's my first ATMega case, or maybe the 2nd.

Finally gave up and bit the uC bullet? ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

 

[Klaus Kragelund]

On Thursday, August 22, 2013 1:38:28 AM UTC+2, Joerg wrote:

Vladimir Vassilevsky wrote:

On 8/19/2013 3:14 PM, Joerg wrote:



Folks,

What's the usual capacitance?



0.1uF



Any stability issues there?



Nothing special.



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.



Do not. Use Vcc as reference.

Internal reference is inaccurate.





Yeah, sure looks ghastly. I provided my own reference from another board

and filtered it with 0.1uF.





I saw people

using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,

as usual.



It doesn't really matter.





However, not everyone knows that because they don't say much about the

innards of the chip. It's my first ATMega case, or maybe the 2nd.

Its just a bypassing capacitor for the reference of the ADC (and what other analog uses the REF)

I have never seen a microcontroller datasheet that spelled out that it should be a certain value. It corresponds to bypassingm your average bandgap IC...

Cheers

Klaus

 

[Joerg]

Klaus Kragelund wrote:

On Thursday, August 22, 2013 1:38:28 AM UTC+2, Joerg wrote:
Vladimir Vassilevsky wrote:

On 8/19/2013 3:14 PM, Joerg wrote:
Folks, What's the usual capacitance?
0.1uF
Any stability issues there?
Nothing special.
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.
Do not. Use Vcc as reference. Internal reference is inaccurate.


Yeah, sure looks ghastly. I provided my own reference from another
board

and filtered it with 0.1uF.





I saw people using 0.1uF and 0.47uF. The datasheet is silent
about stuff like that, as usual.
It doesn't really matter.


However, not everyone knows that because they don't say much about
the

innards of the chip. It's my first ATMega case, or maybe the 2nd.


Its just a bypassing capacitor for the reference of the ADC (and what
other analog uses the REF)

I have never seen a microcontroller datasheet that spelled out that
it should be a certain value. It corresponds to bypassingm your
average bandgap IC...

Yeah, but if you bypass, for example, the simple TL431 with the wrong
value of capacitance it can really sing the blues. Becomes unstable.
It's best to know if there is any risk of that. Other companies such as
Analog Devices do spell out the ref bypass values in their datasheets.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Tim Williams wrote:

"Joerg" <invalid@invalid.invalid> wrote in message
news:b7l1blFlu93U2@mid.individual.net...
I saw people
using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
as usual.
It doesn't really matter.

However, not everyone knows that because they don't say much about the
innards of the chip. It's my first ATMega case, or maybe the 2nd.

Finally gave up and bit the uC bullet? ;-)

Sometimes you need it. A while ago I even had a switcher design that
would have been totally impossible to do without a uC. But it does raise
eyebrows if I request timers and port pins and MIPS, probably because of
my analog background. "YOU want some of the uC resources? What for?"

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Tim Williams]

"Klaus Kragelund" <klauskvik@hotmail.com> wrote in message
news:ad74b41c-8232-48d5-8016-070e5c7a1107@googlegroups.com...

Its just a bypassing capacitor for the reference of the ADC (and what
other analog uses the REF)

I have never seen a microcontroller datasheet that spelled out that it
should be a certain value. It corresponds to bypassingm your average
bandgap IC...

No, I don't think it's that. If it's configured for internal reference,
yes, but otherwise, it's just a bypass to keep sampling and whatever
clean. As a result, one would assume it should be pretty simple (just
bypassing for external noise and low impedance), unless using the internal
reference, in which case, some stability concern might be warranted (give
or take how crappy the reference is... which, as has been mentioned, is
rather awful to begin with!).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

 

[Tim Williams]

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:imgd191h4qe9urhrpnsq4tg1k6mir76hb1@4ax.com...

Here's a similar requirement on a Microchip part:

The LDO voltage regulator requires an external bypass
capacitor for stability. The VUSB pin is required to have
an external bypass capacitor. It is recommended that
the capacitor be a ceramic cap between 0.22 to 0.47 uF.
snip

Considering the simplest absolute on-chip tolerances are factors of 2 to 5
(e.g., current limit on an average LDO), I wouldn't feel comfortable at
any point in that ridiculously narrow range!

The TL431 is, of course, an upside-down LDO, and must be compensated
accordingly. At least it's dominant-pole compensated, so it goes as
easily as any op-amp (keeping in mind the open collector output).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

 

[Spehro Pefhany]

On Thu, 22 Aug 2013 10:46:38 -0700, the renowned Joerg
<invalid@invalid.invalid> wrote:

Yeah, but if you bypass, for example, the simple TL431 with the wrong
value of capacitance it can really sing the blues. Becomes unstable.
It's best to know if there is any risk of that. Other companies such as
Analog Devices do spell out the ref bypass values in their datasheets.

I've seen errata too.. I forget the wimpy term they used..
"fluctuations" or something like that, when they really meant
"oscillate like a banshee".


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com