Dynamically Determined Preferred Values and a Design-for-Testability Approach for Multiplexer Select Inputs under Functional Test Sequences

Abstract

Earlier works observed that certain primary inputs have preferred values, which help increase the gate-level fault coverage when they appear in a functional test sequence. This article observes that multiplexers present additional opportunities for increasing the fault coverage of a functional test sequence, which are not captured by preferred primary input values. Because multiplexers are prevalent, their effect on the fault coverage can be significant. A static analysis that is independent of any functional test sequence is performed in this article to identify preferred values for the outputs of multiplexers. This is followed by a dynamic analysis that adjusts the select inputs of the multiplexers for a given functional test sequence to ensure that the preferred values appear on the outputs of the multiplexers more often. The analysis yields design-for-testability logic for the select inputs of the multiplexers that have preferred values. The logic is independent of the functional test sequence, and it allows the fault coverage to be increased when the select inputs are not primary inputs, or when the same select inputs are used for different multiplexers. Experimental results are presented to demonstrate that this approach has a significant effect on the fault coverage of functional test sequences.