Exploiting X-correlation in output compression via superset X-canceling

An alternative to masking unknown (X) values before the compactor (i.e., X-masking) is to capture X´s in the MISR and cancel them out after compaction (i.e., X-canceling). Existing X-canceling methodologies require a number of control bits to perform the X-canceling that is linear in the number of X´s to be canceled. This paper describes a new methodology for X-canceling which can exploit the fact that the scan cells in which X´s are captured tend to be highly correlated in order to significantly reduce the number of control bits required for X-canceling. X´s tend to be generated in certain portions of the design, and hence certain scan cells capture X´s with much higher frequency than other scan cells. Instead of custom generating the control bits to cancel out only the X´s in one MISR signature, the proposed approach finds a general superset solution which can cancel out the X´s for many MISR signatures. This allows the same control bits to be reused many times thereby significantly improving the amount of compression that can be obtained. Architectures for implementing superset X-canceling are described along with experimental results.