Toggle-Based Masking Scheme for Clustered Unknown Response Bits

Masking schemes typically suffer from over-masking of bits that may possess fault effect information, degrading test quality levels. Unknown response bits (x´s) exhibit a clustered distribution in responses due to structural proximity of x sources. In this work, we propose a toggle-based masking scheme that is capable of delivering very high observability levels in the case of clustered x distributions. The proposed scheme assigns a single-bit state to each chain, dictating whether the chain will be masked or observed. Clustered distribution of x´s enables an infrequent switching of state information, minimizing the amount of mask data that selectively toggles the state of chains. Thus, only a few mask channels are needed to control the proposed masking hardware, enabling the blocking of all x´s while over-masking a small number of non-x bits. Capability to mask all x´s enables the use of a MISR in conjunction, and thus eliminates the need for any scan-out channels, translating into enhanced parallelism in multi-site testing. Results on industrial test cases show that the proposed masking scheme is capable of minimizing or even eliminating over-masking, delivering near-optimal test quality levels.