Classification of faults in synchronous sequential circuits

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Abstract
Undetectable and redundant faults in synchronous sequential circuits are analyzed. A distinction is drawn between undetectable faults and faults that are never manifested as output errors. The latter are classified as redundant. It is shown that there are faults for which a test sequence does not exist; however, under certain initial conditions (or initial states) of the circuit, faulty behavior may be observed. Such faults are called partially detectable faults. A partially detectable fault is undetectable, but is not redundant, as it affects circuit operation under some conditions. We observe that the notion of redundancy cannot be separated from the mode of operation of the circuit. Two modes of operation are considered, representative of common modes, called the synchronization mode and the free mode. Accordingly, the identification of redundant faults calls for different test generation strategies. Two test strategies to generate tests for detectable faults and partial tests for partially detectable faults are defined, called the restricted test strategy and the unrestricted test strategy. Each test strategy is related to a different operation mode, and is suitable for generating tests as well as partial tests under the corresponding mode of operation, and identifying the redundant faults under that mode. We present a complete test procedure for each operation mode, present experimental evidence that partially detectable faults exist, and propose a new definition of a fault coverage measure that accounts for the existence of such faults.

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