Using Limited Dependence Sequential Expansion for Decompressing Test Vectors

Existing techniques that incorporate decompressor constraints in the ATPG search/backtrace (e.g., Illinois scan) are based on combinational expansion in which each scan slice must be encoded using only the free-variables arriving from the tester in the current clock cycle. Sequential expansion is more powerful as it allows free-variables across multiple clock cycles to be used, however conventional approaches for sequential expansion that are based on linear finite state machines (LFSRs) and ring generators are not amenable to including the constraints in the ATPG backtrace because the constraints are too complex. This paper investigates the use of limited dependence sequential expansion to combine the benefits of sequential decompression with the benefits of incorporating the decompressor constraints in the ATPG backtrace. Analytical and experimental results are presented showing the benefits of the proposed approach