A new methodology involving effect-cause analysis has been demonstrated for the diagnosis of path delay faults. The paper illustrates a structural representation, called the suspect circuit, of all the possible path delay faults in a faulty circuit. This representation has been used to design efficient algorithms that enable us to manipulate the suspect faults without having to enumerate them explicitly. Procedures for removing fault-free paths from the list of suspect faults have been implemented to improve the diagnostic resolution. Moreover, efficient data structures are used to complement the procedures and reduce the memory footprint of the algorithms. Results indicate that the diagnostic resolution obtained is very high and includes all possible causes of the observed delay faults.

At-speed robust path delay testing is more desirable than the commonly employed slow-fast delay testing due to lower test application time, lower area overhead, and increased possibility of detecting unmodeled faults that cause errors only during at-speed circuit operation. However, it has been observed in practice that at-speed application of tests generated by existing test generators can lead to their invalidation. In this paper, we first present techniques to modify tests generated by existing test generators to avoid invalidation during at-speed testing. We then present a new procedure to generate tests suitable for at-speed delay testing of combinational circuits. Experimental results show that (a) at-speed application of test sets generated by existing generators leads to significant test invalidation, where the degree of invalidation is approximately proportional to the degree of compactness of the test set, and (b) the at-speed robust path delay tests generated by the proposed test generator are significantly shorter than those obtained by modifying the tests generated by existing generators.

A new methodology to diagnose delay failures is described. Key characteristics of the methodology are: (a) path-oriented diagnosis, (b) effect-cause reasoning, and (c) utilization of information obtained from the passing vectors. Two new representations are developed to make manageable the complexity of a path-oriented methodology. The results of diagnosis are: (a) proven to include all possible causes of observed delay errors, and (b) empirically found to have very high resolution.

Conditions are derived for robust testing of a path delay fault via a sequence of vectors applied at-speed. A simulator has been developed that uses the above conditions, along with the knowledge of paths that are robustly tested by the previous vectors, to determine the fault coverage obtained by such testing. The results demonstrate that the existing fault simulators can overestimate robust path delay fault coverage by 5-15%.