LAUSR

Observation points in the combinational logic of a circuit can aid in the diagnosis of logic defects by reducing the number of indistinguished fault pairs. This paper considers the use of observation points in the combinational logic for the diagnosis of scan chain defects under an approach to test application called transparent-scan. Tests that are used for the diagnosis of scan chain defects include functional capture cycles that propagate the effects of scan chain faults through the combinational logic. Scan chain faults can thus be distinguished based on the values that are propagated in their presence to an observation point in the combinational logic. The procedure for observation point insertion considers the following parameters with respect to scan chain faults: 1) the number of indistinguished fault pairs and 2) the distances, measured in numbers of scan cells, between the faults that cannot be distinguished. With lower distances, a set of candidates for a faulty scan chain is likely to contain faults in the same vicinity of the scan chain. This facilitates failure analysis. Experimental results demonstrate the effectiveness of observation points in the combinational logic for scan chain diagnosis under transparent-scan.