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200 VLSI Test Principles and Architectures with staggered or simultaneous clocking schemes, then a one-hot clocking scheme is used to detect any remaining faults [Wang 2003]. 4.6 UNTESTABLE FAULT IDENTIFICATION Untestable faults are faults for which there exists no test pattern that can both excite the fault and propagate its fault-effect to a primary output. Thus, a fault may be untestable for any of the following three reasons: The conditions necessary to excite the fault are not possible. The conditions necessary to propagate the fault-effect to a primary output are not possible. The conditions for fault excitation and fault propagation cannot be simulta- neously satisfied. In combinational circuits, untestable faults are due to redundancies in the circuit, while in sequential circuits untestable faults may also result from the presence of unreachable states or impossible state transitions. From an ATPG’s point of view, the presence of untestable faults in a design can degrade the performance of the ATPG tool. When considering untestable faults, an
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