Random pattern testing methods are known to result in poor fault coverage for most sequential circuits unless costly circuit modifications are made. In this paper, we propose a novel approach to improve the random pattern testability of sequential circuits. We introduce the concept of holding signals at primary inputs and scan flipflops of a partially scanned sequential circuit for a certain length of time, instead of applying a new random vector at each clock cycle. When a random vector is held at the primary inputs of the circuit under test or at the scan flip-flops, the system clock is applied and the primary outputs of the circuit are observed. Information obtained from a testability analysis or test generator is used to determine the number of clock cycles for which each random vector is to be held constant. The method is low cost and the results of our experiment on the benchmark circuits show that it is very effective in providing fault coverage close to the maximum obtainable fault coverage using random patterns with full scan.
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