A hybrid fault simulator for synchronous sequential circuits

Fault simulation for synchronous sequential circuits is a very time-consuming task. The complexity of the task increases if there is no information available about the initial state of the circuit. In this case, an unknown initial state is assumed which is usually handled by introducing a three-valued logic. It is known that fault simulation based upon this logic only determines a lower bound for the fault coverage achieved by a test sequence. Therefore, we developed a hybrid fault simulator H-FS combining the advantages of a fault simulator using the three-valued logic and of an exact symbolic fault simulator based upon binary decision diagrams. H-FS is able to handle even the largest benchmark circuits and thereby determines fault coverages much more accurately than previous algorithms using the three-valued logic.