Partially Invariant Patterns for LFSR -Based Generation of Close-to-Functional Broadside Tests

Abstract

Close-to-functional scan-based tests are expected to create close-to-functional operation conditions in order to avoid overtesting of delay faults. Existing metrics for the proximity to functional operation conditions are based on the scan-in state. For example, they consider the distance between the scan-in state and a reachable state (a state that the circuit can visit during functional operation). However, the deviation from functional operation conditions can increase during a test beyond the deviation that is measured by the scan-in state. To ensure that the deviation does not increase, this article introduces the concept of a partially invariant pattern. The article describes a procedure for extracting partially invariant patterns from functional broadside tests whose scan-in states are reachable states. Being partially specified, partially invariant patterns are suitable for test data compression. The article studies the use of partially invariant patterns for linear-feedback shift-register ( LFSR ) based test data compression. Noting that a seed may not exist for a given partially invariant pattern with a given LFSR , the procedure described in this article uses an iterative process that not only matches a seed to a partially invariant pattern, but also adjusts the partially invariant pattern based on the test that the seed produces. The article also addresses the selection of LFSR s for the generation of close-to-functional broadside tests based on partially invariant patterns. Experimental results are presented to demonstrate the feasibility of the procedure.