On the generation of scan-based test sets with reachable states for testing under functional operation conditions

Design-for-testability (DFT) for synchronous sequential circuits allows the generation and application of tests that rely on non-functional operation of the circuit. This can result in unnecessary yield loss due to the detection of faults that do not affect normal circuit operation. Considering single stuck-at faults in full-scan circuits, a test vector consists of a primary input vector U and a state S .We say that the test vector consisting of U and S relies on non-functional operation if S is an unreachable state, i.e., a state that cannot be reached from all the circuit states. Our goal is to obtain test sets with states S that are reachable states. Given a test set C, the solution we explore is based on a simulation-based procedure to identify reachable states that can replace unreachable states in C. No modifications are required to the test generation procedure and no sequential test generation is needed. Our results demonstrate that the proposed procedure is able to produce test sets that detect many of the circuit faults, which are detectable using scan, and practically all the sequentially irredundant faults, by using test vectors with reachable states. The procedure is applicable to any type of scan-based test set, including test sets for delay faults.