Strongly fault-secure (SFS) circuits are known to achieve the totally self-checking (TSC) goal of producing a noncodeword as the first erroneous output due to a fault. Strongly code-disjoint (SCD) circuits are known to always map noncodeword inputs to noncodeword outputs, even in the presence of faults, as long as the faults remain undetected. The authors present a general design method for SFS and SCD combinational circuits for the previously proposed fault model that covers the broad classes of likely faults in VLSI. In the design, the input and output of a combinational circuit are encoded in systematic unordered codes whose check part is obtained by adding two extra bits to the check part of any known systematic unordered code. Thanks to the uniform input/output encoding and the SCD property for the proposed combinational circuits, a number of the circuits can be interconnected in cascade to construct a larger SFS combinational circuit if each interface is sufficiently exercised.
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