The paper proposes a new hierarchical untestable stuck-at fault identification method for non-scan sequential circuits containing feedback loops. The method is based on deriving, minimizing and solving test path activation constraints for modules embedded into Register-Transfer Level (RTL) designs. First, an RTL test pattern generator is applied in order to extract the set of all possible test path activation constraints for a module under test. Then, the constraints are minimized and a constraint-driven deterministic test pattern generator is run providing hierarchical test generation and untestability proof in sequential circuits. We show by experiments that the tool is capable of quickly proving a large number of untestable faults obtaining high fault efficiency. As a side effect, our study shows that traditional bottom-up test generation based on symbolic test environment generation at RTL is too optimistic due to the fact that propagation constraints are ignored.
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