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That is assuming that the clock frequency is fixed. I don't know the level of this course but for beginners changing the clock speed, and given the numbers devide too beautifully to be a coincidence I think just a prescaler will fulfill the requirement, with a verry crude sine, but good enough I think.
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You're probably right since one freq is 2.5 times the other. If he's lucky, the instructor was showing a clumsy way to do it so they would appreciate the next lesson on how to do it more elegantly .:)