A selective X-masking for test responses in the presence of unknown values

As the unknown values (X´s) appear in the output response, a large amount of scan cells may not be observed during test. In this paper we present a flexible X-masking logic called selective X-masking to handle the majority of X´s, while the X-tolerant compactor is applied to handle the remained X´s. The objective of this approach is to improve the observability of the test responses. The results show that this scheme can achieve a significant improvement of observability for output responses and reduce significantly the data overhead of the X-masking.