Deviation-Based LFSR Reseeding for Test-Data Compression

Linear feedback shift register (LFSR) reseeding forms the basis for many test-compression solutions. A seed can be computed for each test cube by solving a system of linear equations based on the feedback polynomial of the LFSR. Despite the availability of numerous LFSR-reseeding-based compression methods in the literature, relatively little is known about the effectiveness of these seeds for unmodeled defects, particularly since there are often several candidate seeds for a test cube. We use the recently proposed output deviation measure of the resulting patterns as a metric to select appropriate LFSR seeds. Experimental results are reported using test patterns for stuck-at and transition faults derived from selected seeds for the ISCAS-89 and the IWLS-05 benchmark circuits. These patterns achieve higher coverage for transition and stuck-open faults than patterns obtained using other seed-generation methods for LFSR reseeding. Given a pattern pair (p ₁, p ₂) for transition faults, we also examine the transition-fault coverage for launch on capture by using p ₁ and p ₂ to separately compute output deviations. Results show that p ₁ tends to be better when there is a high proportion of do-not-care bits in the test cubes, while p ₂ is a more appropriate choice when the transition-fault coverage is high.