Observation Points on State Variables for the Compaction of Multicycle Tests

Multicycle tests support test compaction by including several functional capture cycles in every test. However, even highly compacted multicycle test sets typically contain single-cycle or two-cycle tests for faults that cannot be detected using tests with larger numbers of clock cycles. An analysis of such tests reveals that the tests can be extended to include larger numbers of functional capture cycles if observation points are placed in the circuit. Moreover, the analysis shows that it is sufficient to place observation points on selected present-state or next-state variables. This is advantageous for the detection of delay faults without changing the paths they traverse. Based on this analysis, this brief describes a test compaction procedure for multicycle tests that places observation points on present-state variables in order to enable additional test compaction. Experimental results demonstrate the existence of cases where the observation points are effective in reducing the number of tests in a test set that is already highly compacted.