Testing digital circuits: studying the increment of the number of states and estimating the fault coverage

Testing of digital circuits is very important, especially for guaranteeing the correct and reliable functioning of electronic devices. One of the possibilities for deriving high quality test suites is using test generation methods for a corresponding Finite State Machine simulating the circuit behavior. In this paper, we estimate the number of implementation states whenever a circuit mutant is introduced. Experimental evaluation is performed for three types of mutants, namely Single Stuck-At Fault Mutants, Single Bridge Fault Mutants, and Hardly Detectable Fault Mutants. Experiments with the ITC'99 benchmarks (second release) show that in most cases the injection of a fault does not increase the number of states. Moreover, whenever the number of states is increased, the increment is on average 20%. Given this increment, we perform the experiments to showcase that for testing circuits with guaranteed fault coverage with respect to the listed faults, one can apply the W-method with the upper bound m = 1.2n states, for n states in the specification (circuit) FSM.