On Undetectable Faults and Fault Diagnosis

The presence of an undetectable fault u(i) may modify the response of a detectable fault d(j) to a test set used for fault diagnosis. This may impact the accuracy of fault diagnosis based on the responses of single faults. Many state-of-the-art diagnosis processes are based on the responses of single stuck-at faults even though their goal is to diagnose defects (including multiple defects) that are different from stuck-at faults. Therefore, we study the effects of undetectable single stuck-at faults on the accuracy of fault diagnosis based on the responses of single stuck-at faults. For this purpose, we consider the cases where the response of a double stuck-at fault u(i)&d(j), which consists of an undetectable fault u(i) and a detectable fault d(j) , is different from the response of the single fault d(j) . We show that there are significant, yet manageable, numbers of such faults in benchmark circuits under test sets used for fault diagnosis. In all these cases, a fault diagnosis process based on single stuck-at faults may not identify the locations of d(j) and u(i) as candidate defect sites if a defect affects the sites of d(j) and u(i). We conclude that it is important to consider u(i)&d(j) during fault diagnosis in order not to preclude the sites of d(j) and u(i) as candidate defect sites.