An Efficient Deterministic Test Pattern Compaction Scheme Using Modified IC Scan Chain

In this paper, we propose a new scheme for Built-In Self-Test (BIST) that uses an LFSR obtained by adding feedback loops to the IC boundary scan chain. This LFSR first generates random patterns to cover easy-to-test faults and after the random testing phase it is partially loaded with seeds to generate deterministic vectors for hard-to-test faults. The seeds are obtained by solving systems of linear equations for seed variables, respecting the positions where the test patterns have specified values. The scan chain based LFSR due to its length produces sequences with very low probability of linear dependence of generated patterns. This would hardly be achievable using an added external LFSR, whose length has to be minimized due to the cost limitations. The probability of encoding a test pattern into a seed is the same as in the case of using multiple polynomial LFSR, but the hardware overhead is lower. Fault coverage for the ISCAS benchmark circuits was simulated and an optimized testing strategy for fully scanned ICs was proposed on the basis of the obtained results. The test patterns in mixed-mode testing are obtained partially from a hardware TPG and partially from a software ATPG tool, where the hardware TPG tests the easy-to-test faults and the ATPG generated patterns detect the hard-to-test faults. We propose a scheme that instead of using a MP LFSR [1] for compaction of deterministic test patterns uses longer LFSRs partially or completely embedded into the boundary scan chain. One of possible implementations of this approach is shown in Fig.1. The scan chain is divided into three parts. The first part which corresponds to the input boundary scan cells is converted into an LFSR. It is used for pseudorandom test pattern generation and deterministic test pattern compaction. The conversion is done by adding linear feedback loops to the respective part of the scan chain, according to a primitive or non-primitive polynomial. The feedback taps can be disconnected by the multiplexer and an arbitrarily big part at the beginning of the scan chain can be seeded from a FIFO memory. The second part of the chain which corresponds to the internal CUT flip-flops is a standard shift register, which receives the values from the LFSR. Finally, the third (output) part of the scan chain forms a data compactor. In the random pattern generation phase the patterns are generated in the LFSR and simultaneously shifted into the rest of the chain. Then the functional clock cycle is performed and the responses are stored either in the data compactor or in the internal CUT flip-flops. Next shifts accumulate signatures of all the