Modeling Custom Digital Circuits for Test
Journal of Electronic Testing: Theory and Applications
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Production VLSI circuits typically consist of primitives like tristate buffers, bidirectional buffers, and bus configurations that assume non-Boolean values like the high-impedance state. We describe a systematic methodology for extending test generation algorithms that work on combinational circuits with only Boolean primitives to full-scan production circuits. Key features of the methodology are illustrated using the energy minimization based test generation algorithm for combinational circuits. The main features of our methodology that make the test generation algorithm practical for large production circuits are: (1) only one Boolean variable is used to represent the value on a signal and all signals assume only Boolean values during the test generation procedure; (2) the function of non-Boolean primitives is separated into Boolean and non-Boolean components with energy functions required only for the Boolean component; and (3) non-Boolean components are implicitly considered in the energy minimization procedure. In this process, no new energy functions other than the normal Boolean gate energy functions are needed. We give a method for identifying and removing redundancies in production circuits using energy minimization. The formulation is also applicable to Boolean satisfactorily and BDD methods. We first use the test generation algorithm for identifying undetectable faults and then relax specific constraints in the original test generation problem by ignoring the non-Boolean components. We show that undetectability in the relaxed formulation implies redundancy. We report redundancy removal results for production VLSI circuits, ISCAS 85, and full-scan versions of the ISCAS 89 benchmark circuits