Using partial masking in X-chains to increase output compaction for an X-canceling MISR

An X-Canceling MISR [Touba 07] provides the ability to tolerate unknowns (X's) in the output response with very little loss of observability of non-X values. When the density of X's is low, an X-Canceling MISR is extremely efficient as the number of control bits depends only on the total number of X's in the output response. However, for higher X-densities, an X-Canceling MISR becomes less efficient. This paper describes a very effective approach for using an X-Canceling MISR for designs with high X-density. It utilizes the idea of stitching together scan cells that capture the largest number of X's into "X-chains" as was proposed in [Wohl 08]: In the proposed approach, a partial X-masking approach is used for the X-chains to eliminate the vast majority of the X's at very little cost in terms of control bits. Only the X's coming from the scan cells not in the X-chains plus X's that are left unmasked in the X-chains need to be handled by the X-canceling MISR thereby significantly reducing the total number of control bits required. Experimental results show an order of magnitude improvement in the output compaction can be achieved.