Selection of the state variables for partial enhanced scan techniques

Structural scan based delay testing is used to detect delay faults. Because of the architectural limitations not each test pair v1, v2 can be applied by scan delay testing. That declines test coverage. Partial enhanced scan approach based on selection of flip-flops was suggested to permit using arbitrary test pairs v1, v2. The problem of selection of flip-flops may be solved with applying estimations of controllability and observability of the state variables corresponding to the flip-flops. Calculation of controllability and observability estimations is based on 2-length combinational equivalent analyses and PDF testing.