Before I go ahead and design something like this, I would like to read some authorized opinions on this issue: is it a good idea to have 2 crystals/resonators (with different frequencies) on the board, selectable through a jumper? I don't see a reason this wouldn't work (and I did not try it yet), but I want to make sure that I don't break rules/principles (that may make me look ridiculous, like this question itself :) ). Thanks.
Hi, sure, this is possible, but maybe you also have to change the capacitators from each pin of the crystal to ground, if the frequencies are too different. Have a look to the avr data sheet. It is also possible to use two crystal frequency generator an a jumper to feed one of the outputs of a crystal oscillator to the input pin of the cpu (also for this have a look into the datashet).
My opinion is authorised only by myself, but if you do this, don't forget to make sure any libraries which depend on clock frequency (delays, serial bit rates) are made aware of any changes.
Most of the clock dependent code in the libraries and core handle the clock frequency through compile time options. This code needs to be recompiled to work correctly with different clock rates so there is a risk that changing the jumper without recompiling would cause timing errors.
What is the advantage of having two crystals?
Thanks again. The replies so far confirm that this solution is not laughable. This is the board I am working on: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1249788932 It will be compatible with Sanguino and uzebox. I am trying to cover both for cost reasons. Since I will get 10 PCBs, I would like to be able to spread their use as much as possible. The main difference between the 2 is, it seems, the crystal frequency. Any other suggestion is appreciated.