Output compaction for high X-densities via improved input rotation compactor design

Testing requires checking whether the output response of a circuit or system is correct or has an error. Combinational linear compactors can be used to compact the output response for a large number of scan chains into a smaller number of outputs. While some compactor designs can guarantee observation of all scan chains in the presence of a small number of X's (unknowns), this may not be sufficient for designs with higher X densities. This paper presents a completely new methodology for designing an output compactor based on using an input rotator that is able to maintain high observability even in the presence of high X-densities while still achieving high compaction ratios. The key idea is to place a combinational rotator in front of a carefully designed XOR network that maximizes separation of the input dependence in adjacent inputs within a particular shift distance of the input rotator. A systematic procedure is presented for constructing such a compactor given a targeted compaction ratio and maximum shift distance. Experimental results show significant improvement in observability for high X-densities in comparison to existing state-of-the-art compaction methods including X-compact and Steiner compactors for the same compaction ratio. This results in higher fault coverage, better diagnosis, and less overhead when output compaction is employed in designs with high X-densities.