Synchronisation circuit for addition of two asynchronously changing digital values in digital motor control circuits

Phased-based digital motor control circuits often require the addition of two asynchronously changing digital values, corresponding typically to the rotor angle and electrical field angle of the stator. When one angle is increasing and the other is decreasing, their sum is not monotonic and can lead to errors in subsequent circuits that rely on the edge transitions of the least significant bit of this sum. A simple circuit is described which removes these transition errors and provides a ´synchronous´ monotonic output.