Design and analysis of multiple weight linear compactors of responses containing unknown values

Occurrence of unknown values in scan chains in response to test vectors is a common phenomenon. This paper presents a method for designing matrices for linear test output compactors by using rows of multiple weights. Compared to previously proposed compactors, the method reduces the masking caused by unknowns by an order of magnitude provided that the unknowns are non-uniformally distributed among the scan chains. Also, using multiple rather than single weight compactors increases the compaction ratio and reduces the hardware overhead. The effectiveness of multiple weight compactors is demonstrated through analysis, simulations and experiments with test response from an industrial design.