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.