Sequentially untestable faults identified without search ("simple implications beat exhaustive search!")

This paper presents a novel fault-independent algorithm for identifying untestable faults in sequential circuits. The algorithm is based on a simple concept that a fault which requires an illegal combination of values as a necessary condition for its detection is untestable. It uses implications to find a subset of such faults whose detection requires conflicts on certain lines in the circuit. No global reset state is assumed and no state transition information is needed. Our fault-independent algorithm identifies untestable faults without any search as opposed to exhaustive search done by fault-oriented test generation algorithms. Results on benchmark and real circuits indicate that we find a large number of untestable faults, much faster (up to 3 orders of magnitude) than a test-generation-based algorithm that targeted the faults identified by our algorithm. Moreover, many faults identified as untestable by our approach were aborted when targeted by a sequential test generator.