Response compaction with any number of unknowns using a new LFSR architecture

This paper presents a new test response compaction technique with any number of unknown logic values (X´s) in the test response bits. The technique leverages an X-tolerant response compactor (X-compact), and forces X´s that are not tolerated by X-Compact to known values. The data required to designate the X´s not tolerated by the X-compactor, also called mask data, is stored in a compressed format on the tester and decompressed on-chip. The authors applied this technique to four industrial designs and obtained 26-fold to 60-fold reduction in test response data volume with no impact on test quality.