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Design for Testability 45 To observe an input of the AND gate at d requires setting the other input to 1; therefore, the combinational and sequential observability measures for both inputs a and b are: CO a = CO d + CC1 b + 1 SO a = SO d + SC1 b CO b = CO d + CC1 a + 1 SO b = SO d + SC1 a It is important to note that controllability and observability measures calculated using SCOAP are heuristics and only approximate the actual testability of a logic circuit. When scan design is used, testability analysis can assume that all scan cells are directly controllable and observable. It was also shown in [Agrawal 1982] that SCOAP may overestimate testability measures for circuits containing many reconvergent fanouts; however, by being able to perform testability analysis in O ( n ) computational complexity for n signals in a circuit, SCOAP provides a quick estimate of the circuit’s testability that can be used to guide testability enhancement and test generation.
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