How to avoid random walks in hierarchical test path identification

Hierarchical test approaches address the complexity of test generation through symbolic reachability paths that provide access to the I/Os of each module in a hierarchical design. While transparency behavior suitable for symbolic design traversal can be utilized for datapath modules, control modules do not exhibit transparency, and therefore require exhaustive search algorithms or expensive DFT hardware. In this paper we introduce a fast hierarchical test path identification methodology for circuits with no DFT at the controller-datapath interface. We introduce the concept of influence tables, modeling the impact of control states on the datapath, based on which appropriate state sequences for accessing each module are identified. Imposition of such sequences on a hierarchical test path identification algorithm, in the form of constraints, results in significant speedup over alternative non-DFT based approaches.