Unconstrained-Activation Functional-Detection Scan-Based Tests

Functional tests are important for detecting delay faults under functional operation conditions. Functional operation conditions can be created with a scan-based test by ensuring that the test would take the circuit into a reachable state. In this case, the functional capture cycles that follow, where the circuit traverses reachable states, can be used for fault detection. This article defines a type of scan-based test that is designed to take advantage of the functional capture cycles before the circuit enters a reachable state for activating delay faults. The tests are referred to as unconstrained-activation functional-detection or UAFD tests. In a c-cycle UAFD test for a fault, the first c-2 functional capture cycles activate the fault and take the circuit into a reachable state. The last two functional capture cycles detect the fault while taking the circuit through reachable states. Experimental results for benchmark circuits demonstrate the increase in the transition fault coverage achievable when non-reachable states are used for fault activation.