then you have to consider how it uses the clock. In "the bible" there's a description, for example, of how the MCU interacts with the serial port and the timers, etc. If you want everything tightly coupled, which would be the only reason for using the same oscillator, then you'd want to study that out very carefully.

If you're using loosely coupled communications, then there's no real need for precisely synchronized oscillators. If you don't need the syncrhonization, then your next concern should be the effects of having that 12 MHz or faster clock running around the board. It's much easier on the signal-quality guy's nerves if you distribute a slow clock and sunchronize with that, if it's at all necessary.

Think required rather than desired. Never put in any feature you don't absolutely require. If you leave a feature out, it's for sure that it won't cause trouble.

RE