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Let's say I have a uC with an internal clock running at 8MHz. I'm aware that external crystals can have higher frequencies than internal clocks, so that's one obvious reason to use them. But I've heard strange things about not being able to use PWM with external oscillators because you can't ensure that the frequency is exactly as specified, or something? This could be entirely wrong, so I guess I'm just looking for clarification: is the only difference between using an external oscillator and the internal clock the frequency?
Is it possible, then, to use a single oscillator for multiple uCs? I'm going to be setting up connected network of them to control a robot (mainly because my sensors require 16 pins each and I have 16 sensors...). I have no worries about using more than one oscillator, I'm just thinking of simplifying things a bit.
Also, out of curiosity, if someone were to implement a real-time controller on a uC, I'm going to guess they'd need a very accurate clock as well, right? Or is it correct to say that you're only as accurate as your clock, but you can be guaranteed that the code will execute everything within that accuracy?