Reducing the MISR size

Multiple-input signature registers (MISRs) are commonly used in built-in self-test (BIST) applications. The size of the MISR is dictated by the number of signals it has to compress. Normally a MISR includes a stage for every signal that it is sampling. In some applications this leads to very wide MISRs that may include several hundred stages. Large size MISRs pose problems in terms of hardware and wiring overhead. Shorter compressors are, therefore, needed. This paper investigates the problem of reducing the MISR so that it samples multiple signals at every stage. Issues like detection probability loss, test length penalty, fault coverage degradation, are some of the disadvantages that may arise from the MISR shrinkage. This paper analyzes all these issues; shows ways to reduce their negative effect, and compares the results to previously published proposals.