To answer your question the following information would be required:

* What is the nominal speed of the motor
* Do you want to control the speed over a wide range or do you just want to keep a constant speed (in the latter case you might consider using some other design than a uC circuit)
* What is the required regulation accuracy
* What type of motor do you drive (AC or DC, sync or async, etc)
* What type of encoder do you use