Does the "0 - 20MHz @ 4.5 - 5.5V" in the datasheet for the atmega328p also apply if the chip is driven by a 20MHz self powered oscillator instead of a regular crystal and it's circuitry? Can it be powered by 3.3v if using the oscillator? I'm guessing it can't be, but just wanted to double check.
Yes, it can be driven by an external oscillator.
If the AVR running at 3.3V, you oscillator should have a 3.3V output (not 5V)
Using an external oscillator does NOT absolve you from the voltage vs clockrate curve; I wouldn't expect a 3.3V AVR to run with a 20MHz oscillator.
Yea, was kinda stupid of me even asking for an opinion - more of a "wishful thinking" sort of thing. But thanks for at least responding. 
Figure 32-1 in the '328 datasheet shows what it can do graphically. 9.8 mhz at 3.3 volts is the max, 8 MHz is the accepted practical clock rate. Anything else is overclocking and not guaranteed to work.
There are reports (on this board) that they can be clocked to at least 32 MHz at 5 volts with external oscillators. But, you cannot use eeprom at those rates and you must drop back into spec in order to reflash. There was no mention of operating temperature range so as is always the case, YMMV.
Not that you asked, but if you need small and fast, consider a Teensy.
avr_fred:
Figure 32-1 in the '328 datasheet shows what it can do graphically. 9.8 mhz at 3.3 volts is the max, 8 MHz is the accepted practical clock rate. Anything else is overclocking and not guaranteed to work.There are reports (on this board) that they can be clocked to at least 32 MHz at 5 volts with external oscillators. But, you cannot use eeprom at those rates and you must drop back into spec in order to reflash. There was no mention of operating temperature range so as is always the case, YMMV.
Not that you asked, but if you need small and fast, consider a Teensy.
- got it. Thanks Fred.
May want to look at the datasheet yourself, this is what I see.
for ref Linear interpolation - Wikipedia
Duoh. That's what you get when you use the google calculator on an iPad instead of picking up the rpn machine you've used for forty years... 12.8 @3.3 is what I get with my trusty hp. Should've seen/used the 10mhz at 2.7v as a sanity check. Mea culpa.
ha, ha. . . gotta love this forum - glad I found it. Has enlightened me, and kept me from making some mistakes lately, I may have made otherwise.
Thanks guys.
You can generally get 16mhz out of a '328p @ 3.3v, even though it's out of spec.
20mhz is pretty aggressive, though, that I wouldn't expect to work.
There's some other dude here who routinely overclocks atmegas, and he indicated that the weakest link is usually the crystal drive, and that external clock sources can get you further above spec'ed speeds, though. It might well work like that. It will likely be particularly sensitive to adverse conditions though (ie, it probably won't work over the whole temperature range, and make sure it's got a nice stable power supply).