retrolefty:
[gripe]
(why the Arduino boards don't use the same crystal for both chips anyway is beyond me - and why use a crappy RESONATOR on the microcontroller when some applications may need some semblance of precision?)
[/gripe]It seems to me the very first version of the Uno (and maybe mega2560) had crystal resonators and the story was they couldn't pass FCC standards for emissions tests without changing to the ceramic resonator. Not sure that was fact or conjecture, but I'm pretty sure that story was put out there by someone.
I've heard that story too... and I find it difficult to believe. Maybe the relatively long run required to bring the clock signal across from one chip to the other (i.e. using one crystal for both) could be an RFI problem, but using a separate crystal for each chip shouldn't be.
I think the real reason was to save costs. The 16U2 needs a stable and accurate clock given that it's the USB interface, but the microcontroller doesn't normally need an accurate clock (unless one needs to do accurate timing) so it gets the resonator.
I've actually had to replace the resonator with a crystal on a 2560R3 board for a few projects that required accurate timing.
I would gladly pay an extra dollar for a board to get a crystal instead of a resonator (or at least give me two through-holes to install my own).