On full path delay fault testability of combinational circuits

We show that robust tests for all path delay faults in a combinational circuit are not necessary in order to avoid test invalidation due to undesired hazards. Further extension leads to the formulation of the necessary and sufficient conditions for any path delay fault in a multi-level combinational circuit to be testable without potential invalidation by undesired hazards. We prove that all algebraic transformations and constrained resubstitution with complement are testability-preserving for the tests chosen.<>