Stanley,

The oversimplified answer is that you need to know where your current is flowing, and what effect it has on devices that share the same path.

The microcontroller must have clean power, as you already know. One way to get that clean power is to make sure that, after the voltage regulator, the power and ground don't share any path with the motor. (Your motor is on unregulated power.) One key place to attend to is the "ground" for the power driver device should be connected on the unregulated side of the voltage regulator. If the driver has a bypass cap, it too cannot be connected to the uC "ground".

Note that uC "ground" and power driver "ground" are the same net at one point (near the regulator), but they never share a common path once they have been split apart.

To help make your uC power quieter, be sure that the uC power and the uC "ground" have adequate bypass capacitors. Remember do not "ground" these bypass caps to anything that carries high current. 0.1 uF capacitors are more important than high value electrolytics.

-Mike