I swear I've seen this somewhere before but now I tried looking around the website and also at the Atmel AVR168 docs and I don't really see clear enough an answer. I'd like to know what sort of tolerance the input voltage to the arduino has. I mean, if I bypass the regulator (well, it's likely I'm going to produce my own board...) and supply a voltage what is the tolerance? 10% (0.5V +-) The avr168 docs seem to indicate that the chip can run at at low as 3.3V and still maintain 16MHz but I did see somewhere that supplying less than 5V to the arduino could create shaky performance. So what's the real answer? I'd like to try to stay in the 4.7V to 5.2V range but if I end up with 4.5V should I expect trouble?
IIRC, the ATmega 8/48/88/168 have two variants: low voltage low speed (max 10MHz, down to 1.7V) and 5V high speed (20MHz). The specs for the 8 and the 48/88/168 are different.
I don't see the rating I'm looking for, but absolute max (i.e. smoke it if you go beyond this) is 6V. I see references to 4.5V-5.5V in several places, which leads me to agree with your 5V +/-10% assumption.
I have ran an ATmega168 on 3 AA batteries (nominal 4.5V, of course lower as the batteries age) and gotten very screwy behavior when programming (like wiping out the bootloader).
I think the ATmega328 has converged to a single part that can handle low voltage and high speed.
As long as you aren't writing to flash at lower voltages, you should be fine.
-j
Thanks, that's pretty much what I figured. I have a couple of other things connected to the arduino, the rest of which are 10% tolerant. But, it's still probably best that I get as close as possible to 5V to prevent that as an opportunity for screwiness.
Look for the section labeled "Speed Grades" in "Electrical Characteristics"
There is curve of operating frequency vs input voltage
Ahhh, skipped over that before. 16Mhz isn't explicitly listed but it appears as if the lower limit is about 4.3V or so at 16Mhz. It certainly appears that 4.5V is safe, even at 20Mhz.
I don't like the organization of the specs. It would be nice to have operating frequency in the table specified at a few common voltage ranges -- +5V +-10%, +3.3V +- 10%
with a reference + hyperlink to the graph.
(* jcl *)
I don't like the organization of the specs.
Hi jcl,
If you are refering to the Arduino site, I like the organization of the specs – or more specifically, I like the refreshing absence of technical detail, so the site is accessible to non-engineers. IMO most users on the Arduino.cc site buy boards configured to the appropriate frequency for the boards specified voltage.
For those producing their own boards (such as the OP), is it not reasonable to assume that the relevant datasheet will be consulted?
mem, I think he was referring to the ATmega datasheet.
-j
I was referring to the organization of the ATmega168 datasheet (which should have the refreshing technical details 
(* jcl *)