I have developed a microcontroller based project in I have used 89S52 to control synchrous motors. Here I am experincing a problem of hanging of microcontroller anytime when motors gets starts and stops.
GetDeviceInfo has rightly pinted an important issue. In such cases, decoupling capacitors 0.1uF (i.e. 100n) connected across the supply in the control circuitary (may be one for each chip) is expected to solve the problem. Also use of diodes in reverse bias across the supply feeding the windings (to suppress back emf) may also be of help.
Decoupling capacitors across the microcontroller & Inductive kick killer Reverse bias diode across the motor will definitely resolve ur problem,
Another way to resolve the problem is to use two different power supplies one for controlling circuit & the other one for Motors
Regards
Hameeds