Toggle-masking scheme for x-filtering

High quality screening of chips may require aggressive solutions such as faster-than-at-speed testing, which may generate responses with high density of unknown x's. Recently, we proposed a toggle-masking approach capable of masking all the unknown x's and minimizing the over-masked known bits for clustered distribution of unknown bits. In this work, we utilize our toggle-masking framework as a foundation, and transform this solution into an x-filter that allows a certain number/distribution of x's to pass, in order to further improve the observability levels. Naturally, the modified toggle-masking scheme is to be paired with another technique, such as an x-canceling MISR, which is capable of canceling the x's in the signature via post-processing operations. We propose different flavors of the proposed x-filter to be utilized with different versions of x-canceling MISR, which may suffer from test time increase and/or observability loss with high x-density responses. By proposing an x-filter that can adjust the number/distribution of x in-flow into the MISR, a perfect control over test time and observability is delivered, offering a wide spectrum of tradeoff solutions for the designers.