AREF bypass capacitance on ATMega2560?

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

Sometimes. Other times you just need to proceed without too much hassle

Cheers

Klaus

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.

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

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:

I believe all (color?) printers insert micro print so print out can be identified regard less of what you print

-Lasse

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

Photoshop (and Paint Shop Pro) have a banknote detection algorithm:

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.

0.1uF

Nothing special.

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

It doesn't really matter.

Vladimir Vassilevsky DSP and Mixed Signal Designs

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

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

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

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

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.

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?"

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

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. On power-up, the external capacitor will look like a large load on the LDO voltage regulator. To prevent erroneous operation, the device is held in Reset while a constant current source charges the external capacitor. After the cap is fully charged, the device is released from Reset. For more information, refer to...

Best regards, Spehro Pefhany

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