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Addressing design for testability at the architectural level

The increasing use of hardware description languages (HDL's) in VLSI design and the emergence of high-level test generation programs has led to an interesting problem. There is a need for design for testability (DFT) techniques that can be applied early in the design phase to improve the effectiveness of ATPG programs on hard-to-test circuits. By an early identification of hard-to-test areas of a circuit, testability can be inserted prior to logic synthesis. In this paper, we first present a comparative study of a gate-level test generator and a high-level test generator by benchmarking them on a common suite of circuits. Based on an evaluation of the results, we propose techniques to automatically extract information from the high-level circuit description that could improve the performance of both ATPG tools. An automatic DFT tool that utilizes VHDL descriptions of the circuit to make an intelligent selection of flip-flops for partial scan is then described. Results on six hard-to-test circuits show that very high fault coverages can be obtained by both a gate-level and a high-level test generator on these circuits after scan. With this detailed study we demonstrate that a DFT tool can make a more efficient and effective selection of partial scan flip-flops by exploiting the high-level circuit information. It can accurately predict the hard-to-test areas of a circuit. Significant improvements in fault coverage and ATPG efficiency, and speedups in ATPG time, can be obtained by a gate-level and a high-level test generator after high-level scan selection. >