Method for identifying untestable faults in logic circuits

Details Method for identifying untestable faults in logic circuits Method for identifying untestable faults in logic circuits

1994-09-14

1996-10-15

Beausoliel, Jr., Robert W.

Excavating

371 27, 39518301, G01R 3128, G06F 1100

Patent

active

055661874

ABSTRACT:
A method of identifying untestable faults in a logic circuit. A lead in the circuit is selected and the circuit is analyzed to determine which faults would be untestable if the selected circuit lead were unable to assume a logic 0 and which faults would be untestable if the selected circuit lead were unable to assume a logic 1. Faults that would be untestable in both (hypothetical) cases are identified as untestable faults. Faults which would be untestable if the selected lead were unable to assume a given value may be determined based on an implication procedure. The implication procedure comprises the forward propagation of uncontrollability indicators and the backward propagation of unobservability indicators. An uncontrollability indicator for the given value is assigned to the selected circuit lead and propagated forward through the circuit according to a set of well-defined propagation rules. In addition, unobservability indicators are generated in the circuit based on the propagation of uncontrollability indicators. These unobservability indicators are then propagated backward through the circuit. The (hypothetically) untestable faults are then determined based on the resultant indicators and their corresponding circuit leads. Untestable faults may be identified in a sequential circuit by generating an equivalent combinational iterative array circuit model for a fixed number of time frames. Faults that would be untestable in both (hypothetical) cases and which are located in the last (i.e., latest-in-time) time frame are identified as untestable faults.

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