Hypothesized performance improvements of the subscripted D-Algorithm have now been confirmed by a series of benchmark comparisons with the Roth's DALG. The major concept exploited by the subscripted algorithm is the acquisition of control over a submodule by back-propagation of multiple simultaneously sensitized D /sub j/ paths. The existence of these simultaneously sensitizable control paths has now been verified for many circuits of very large size. The run time required to establsih such control paths is usually about the same as the time that DALG takes to find a single test. Hence, for high fan-in submodules the performance enhancements are impressive. The algorithm readily finds multiple control paths in circuits with high fan-out and reconvergence. The algorithm even backpropagates readily through Exclusive-Or trees. Therefore, apprehension that multiple control paths would suffer mutual interference in practical circuits is not completely justified.

Measured Performance of a Programmed Implementation of the Subscripted D-Algorithm

The principles of Controllability and Observability have been known for Linear Systems for some time. In this thesis similar concepts are shown to be very useful in Digital Test Set Generation. In effect the control of a gate (or submodule) can be achieved by exciting a behavioral mode of this subunit from the primary inputs. Also to insure fault detection, we must guarantee that the response to this excitation is observable at a circuit's output. The method presented in this thesis is an extension of Roth's D-Algorithm which we call the Subscripted D-Algorithm. This algorithm has the property that it can generate many test patterns simultaneously for multiple input submodules. In effect the main idea is to establish the existence of multiple control paths simultaneously in order to excite more than one behavioral mode at the target submodule using the same paths repeatedly. Under the most favorable circumstances all of these modes are checked at once. This is due to the fact that the Subscripted D-Algorithm uses a new kind of D-cube called Subscripted D-Cube. The net effect is felt not only at the submodule under test but also throughout the circuit thereby affecting fault coverage computations. Most faults located on or near the established control and observation paths are detectable without effort. It is also shown that a relatively high number of faults can in general be detected by every test pattern derived using this new method. The Subscripted D-Algorithm reaches a high percentage of fault coverage much faster than the D-Algorithm. Another advantage is that fewer test patterns are needed than the number of test patterns required by the D-Algorithm. Results are shown for circuits with a large number of gates and high fanout.

Cherif Benmehrez

Papers

Controllability/observability concepts in the subscripted d-algorithm