Selecting state variables for improved on-line testability through output response comparison of identical circuits

The existence of multiple copies of the same functional units in a design allows on-line testing to be performed by comparing the output responses of identical circuits when identical input sequences are applied to them. We extend the output response comparison scheme for identical sequential circuits in order to increase the fault coverage and reduce the fault latency of an unknown input sequence. The extension is based on using state variables in addition to primary outputs as part of the output response comparison scheme. The proposed procedure orders the state variables of the circuits such that each additional state variable in the ordered list has the highest possible impact on the on-line testability of the circuits. Depending on other constraints, the first state variables in the list can be selected for inclusion in the output response comparison scheme.