Logic Redundancy - Adding Logic Redundancy

Adding Logic Redundancy

In some cases it may be desirable to add logic redundancy. One of those cases is to avoid race conditions whereby an output can fluctuate because different terms are "racing" to turn off and on. To explain this in more concrete terms the Karnaugh map to the right shows the minterms and maxterms for the following function:

The boxes represent the minimal AND/OR terms needed to implement this function:

The k-map visually shows where race conditions occur in the minimal expression by having gaps between minterms or gaps between maxterms. For example, the gap between the blue and green rectangles. If the input were to change to then a race will occur between turning off and turning off. If the blue term switches off before the green turns on then the output will fluctuate and may register as 0. Another race condition is between the blue and the red for transition of to .

The race condition is removed by adding in logic redundancy, which is contrary to the aims of using a k-map in the first place. Both minterm race conditions are covered by addition of the yellow term . (The maxterm race condition is covered by addition of the green-bordered grey term .)

In this case, the addition of logic redundancy has stabilized the output to avoid output fluctuations because terms are racing each other to change state.