I am conceptualizing a design that involves ATMEGA2560. So the first few things I did was start re-reading the atmel doc and looking at some open-source designs.
I came across seeeduino MEGA:
http://www.seeedstudio.com/wiki/index.php?title=Seeeduino_Mega
They have a 5V/3.3V power supply switch (read the intro and specs). But according to atmel's doc, 2560 only works with 4.5V-5.5V:
I checked seeeduino MEGA's EAGLE file. They ARE indeed switching between 3.3V and 5V going to VCC. I guess with a 1280 that works as overclocking it since 16MHz needs about 4.5V but with a 2560 there is no way to have it work safely below 4.5V. Any thoughts?
FYI, I'm only interested in 5V operating voltage but since I came across this design from a reputable seller, I've got to make some sense out of it.
I think they are just overclocking it/undersupplying voltage. They are doing the same thing with their ATmega328P-based board, making it voltage selectable but clocking it at 16MHz. There is no choice of switching in an 8MHz crystal, so when you pick 3V3 you are overclocking. Adafruit does the same with their Metro 328.
Technically, for the ATmega2560, you are supposed to not only clock it with an 8MHz crystal, but also change the mega chip itself to the ATmega2560V version to run it at 3V3. Maybe it does have a ATmega2560V chip on there (still that would be overclocked if the crystal is 16MHz even at 5V).
I was not really able to understand the Seeed Eagle schematic, I could not definitively find what I thought was their switch, and the notes talk about a 5V/3V3 selector switch for I/O only on ports K and F. So I don't know what they really had in mind and how close the final product came to what they were thinking. Maybe the guy who wrote the note was thinking of sending a reference voltage to AREF (which is not too good of an idea in Arduinoland), but another guy actually put the switch in there and switched all of VCC (which is not in spec but perhaps OK for most of Arduinoland).
In my opinion there is a certain amount of slop in most Arduino boards and derivatives. And that seems to be OK for most applications. It is up to the hobbyist or developer to keep that stuff in mind and not be too lazy and just assume stuff was designed properly enough for the project being built.
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Thanks. I agree with everything you said. They are arduinolandish, aren't they? I'll make sure I don't do what they do.
Need Atmega2560V to run at 3.3V.
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