DIST: Deterministic In-System Test with X-masking

In-field and in-system deterministic tests begin to play a pivotal role in safety-critical applications (compliant with regulations such as ISO 26262), in large data centers, or in monitoring silicon aging. All of them require periodic, high-quality tests to assure required test coverage and short test time in designs that must test themselves during system operations. In order for deterministic tests to be in-system applicable, they should compact multi-million-bit test responses with unknowns (X) to small signatures. This, in turn, allows for a much faster input-only streaming and reduction of the stored test data volume, a system memory, and test time. Typically, the unknown states, whose sources vary from uninitialized memories to unpredictable last-minute timing violations, render signatures unusable. Hence, test response compaction requires some form of protection. This paper presents a user-tunable X-masking scheme that employs compressed data to completely filter out unknown values that otherwise might reach a test response compactor such as a MISR or test result sticky-bits used by the on-chip compare framework. Experimental results obtained for several industrial cores show feasibility and efficiency of the proposed scheme altogether with actual impact of X-mask-in2 on various test-related statistics.