The COSMOS symbolic fault simulator generates test sets for combinational and sequential MOS circuits represented at the switch level. All aspects of switch-level networks including bidirectional transistors, stored charge, different signal strengths, and indeterminate (X) logic values are captured. To generate tests for a circuit, the program derives Boolean functions representing the behavior of the good and faulty circuits over a sequence of symbolic input patterns. It then determines a set of assignments to the input variables that will detect all faults. Symbolic simulation provides a natural framework for the user to supply an overall test strategy, letting the program determine the detailed conditions to detect a set of faults. Symbolic preprocessing of switch-level networks, combined with efficient Boolean manipulation makes this approach feasible.
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