May seem like an odd question, but I was wondering if anyone has an idea what a MCU such as the atmega328 would do if it was programmed for a external xtal such as the typical 16MHz arduino, except in place of the crystal there was just a light resistor (to prevent a direct short) between the pins of the caps. Would the MCU still attempt to start the crystal oscillating?
Assuming the xtal is just acting as a filter, and perhaps a means to maintain oscillations for at least a little while before acted upon again, it seems like the MCU would continue to attempt to oscillate the xtal. I know it wouldn't run but wonder if it's attempts would show up on a scope, or perhaps it may just timeout if no feedback response of the correct type is received.
Just curious.
Thanks,
John
The crystal also introduces a phase shift, necessary for oscillation.
Makes sense. 
Just found a link that seems to show an equivalent model, but which introduces an inductor to what I mentioned. I wonder what the MCU would do if presented with something like that. Where this is all going, is I'm just curious how the waveform would look without the crystal cleaning it up with it's defined resonant frequency. I wonder if it would also require an excessive amount of power from the MCU, or if it would, as I mentioned before, just timeout if it didn't eventually get a response it was expecting, or if it would even know.
Thanks,
John
The oscillator circuit is specifically designed for reliable operation with crystals, and with only a few allowed variations. See the processor data sheet for some of the details.
There are also technical documents on the manufacturer's web site that explain the limitations of the oscillator circuitry more fully.
There is an internal RC oscillator option. Start with section 13.2 of the [u]ATmega Datasheet[/u]. There's more information about accuracy & calibration in section 32.4.
I've never used it and I don't know much about it. But as a rule, you need an accurate clock for serial communications (including USB), so you might have trouble with the bootloader/serial monitor, etc. or with anything else time-related.
you might have trouble with the bootloader/serial monitor, etc. or with anything else time-related
The 8MHz internal RC oscillator is calibrated at the factory and in my experience, is almost always accurate enough for serial communications, at least up to 56 KBaud. I have had a few failures that might be attributed to timing errors, but I was never certain of the cause.