Test enrichment for path delay faults using multiple sets of target faults

Test sets for path delay faults in circuits with large numbers of paths are typically generated for path delay faults associated with the longest circuit paths. We show that such test sets may not detect faults associated with the next-to-longest paths. This may lead to undetected failures since shorter paths may fail without any of the longest paths failing. In addition, paths that appear to be shorter may actually be longer than the longest paths if the procedure used for estimating path length is inaccurate. We propose a test enrichment procedure that increases significantly the number of faults associated with the next-to-longest paths that are detected by a (compact) test set. This is achieved by allowing the underlying test generation procedure the flexibility of detecting or not detecting the faults associated with the next-to-longest paths. Faults associated with next.-to-longest paths are detected without increasing the number of tests beyond that required to detect the faults associated with the longest paths. The proposed procedure thus improves the quality of the test set without increasing its size.