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

摘要

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 LFSRs 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.