Krupski:
Yes, that's the worst part of a resonator. They drift all over the road like a drunk driver as the temperature changes.
A couple of weeks ago I was looking for a frequency vs temperature graph for a typical ceramic resonator. The only thing I could find was this guy's tests with an Arduino. Here's what he measured:
That more or less matched what I had measured with my Uno -- a linear-looking decline with increasing internal AVR temperature. (I think his internal thermometer was more poorly calibrated than mine, which wasn't very well calibrated either):
So not really too drunken... but still not what you'd be looking for if interested in a stable clock. A crystal will also drift with temperature. Some of the curves are parabolic. Even the DS3234 will drift noticeably if the temperature changes quickly relative to it's default ~1/min conversion rate.
Krupski:
Plus, I wish the Arduino used a clock source that was an integer multiple of standard serial baud rates. The 16.000 clock results in an actual baud rate of 9615 for 9600 (+0.16% error) and 117647 for 115200 (+2.12% error).
It seems like everybody and their dog wants to make a clock out of an Arduino. But how many people run into problems with serial communication because their crystal or resonator is off? I've been transmitting without errors at 57600 on an ATtiny with an uncalibrated internal 8MHz RC oscillator.
It appears that the Zero has a built-in TCXO @ 32.768kHz and accompanying RTC (not too unlike the Teensy). A lot of future Arduino clock builders will no doubt be made happy by this.