Hybrid selector for high-X scan compression

Scan testing and scan compression are widely used, but ever more complex designs require higher compression, while the increased density of unknown (X) values reduces effective compression. In this paper, we present a new selector design which blocks all Xs while allowing more observability of non-X scan cells and which requires fewer input control values. Supported by novel test generation algorithms, the selector enables very high compression even if the density of unknown values is very high and varies every shift. Results on industrial designs with various X densities demonstrate consistently high compression and test coverage.