On Determining the Real Output Xs by SAT-Based Reasoning

Embedded testing, built-in self-test and methods for test compression rely on efficient test response compaction. Often, a circuit under test contains sources of unknown values (X), uninitialized memories for instance. These X values propagate through the circuit and may spoil the response signatures. The standard way to overcome this problem is X-masking. Outputs which carry an X value are usually determined by logic simulation. In this paper, we show that the amount of Xs is significantly overestimated, and in consequence outputs are over masked, too. An efficient way for the exact computation of output Xs is presented for the first time. The resulting X-masking promises significant gains with respect to test time, test volume and fault coverage.