The Gound of MPU and Driver is connected together. is a very innocent looking statement but could mean a lot based on the PHYSICAL implementation starting with the point where the MCU ground connects up with the driver ground; the size of cable used ; the dynamic performance of your power supply and the way you are drawing power to the MCU / driver circuits and so on.

Just as an example : The power supply is a separate unit. The MCU board is a separate unit and so is the H-Bridge driver unit. If you now run the ground cable from the driver unit via the MCU board and then finally link this with a single wire to the power supply ground - you sure can have plenty of fun - and lot of uncoded events can happen.

Raghu