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10 VLSI Test Principles and Architectures produced by a defective circuit. A circuit defect may lead to a fault, a fault can cause a circuit error, and a circuit error can result in a system failure. To test a circuit with n inputs and m outputs, a set of input patterns is applied to the circuit under test (CUT), and its responses are compared to the known good responses of a fault-free circuit. Each input pattern is called a test vector . In order to completely test a circuit, many test patterns are required; however, it is difficult to know how many test vectors are needed to guarantee a satisfactory reject rate. If the CUT is an n -input combinational logic circuit, we can apply all 2 n possible input patterns for testing stuck-at faults; this approach is called exhaustive testing . If a circuit passes exhaustive testing, we might assume that the circuit does not contain functional faults, regardless of its internal structure. Unfortunately, exhaustive testing is not practical when n
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