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<p><b>Abstract</b>—A circuit may produce unknown output values during simulation of a test set, e.g., due to an unknown initial state or due to the existence of tristate elements. Unknown output values in the output response of a circuit make it impossible to determine a single unique signature for the fault-free circuit when built-in self-test is used for testing the circuit. We consider the problem of synthesizing a logic block that replaces unknown output values in the output response of a circuit with a known constant. The logic block is constructed from building blocks called comparison units. The synthesis procedure ensures that the built-in self-test scheme will be able to detect all the faults detectable by the test set applied to the circuit while allowing a single unique signature to be computed. Two variations of the synthesis procedure are considered, a two-dimensional version suitable for synchronous sequential circuits without scan and for scan circuits with multiple scan chains and a one-dimensional version suitable for scan circuits with a single scan chain.</p>
Built-in self-test, output response compression, scan design.

I. Pomeranz, S. M. Reddy and S. Kundu, "Masking of Unknown Output Values during Output Response Compression byUsing Comparison Units," in IEEE Transactions on Computers, vol. 53, no. , pp. 83-88, 2004.
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