FOF: Functionally Observable Fault and its ATPG techniques

Due to advanced and finer technology, faults in devices can become much more complicated and traditional stuck-at faults may miss important defects. In this paper, we discuss about general functional faults, called “Functionally Observable Fault” (FOF), where the faulty sub-circuit may change its input-output logic functions to any ones. That is, if the faulty sub-circuit has N inputs, faulty behavior can realize as many as (2^2N - 1) functions. Also, we take care of observability of the functional faults in the sense that non-observable faults from primary outputs are considered as not-faulty. We define FOF and discuss about its Automatic Test Pattern Generation Techniques (ATPG). Although numbers of FOFs can be much more than multiple stuck-at faults for the same sub-circuit, the required numbers of input patterns to detect FOF are not so many compared with multiple stuck-at faults, which suggests practical values of FOF. We show experimental results on ATPG for FOF using ISCAS85 circuits as well as word-level circuits. As our ATPG algorithm processes FOFs in implicit ways, the numbers of test vectors for many multiple FOFs remain small as experimentally shown.