SELF-TIMED IS SELF-CHECKING

Self-checking circuits detect (at least some of) their own faults. We describe self-timed circuits, including combinational logic and sequen tial machines, which either halt or generate illegal output if they in clude any single stuck-at faults. The self-timed circuits employ dual rail data encoding to implement ternary logic of 0, 1, and undefined s tates; the fourth state is used to signal illegal output and is shown to result only from certain circuit faults. The self-timed circuits al so employ four-phase signaling according to a well-defined protocol of communications between the circuit and its environment; failures due to certain faults prevent the circuit from communicating properly, thu s causing the circuit to halt. We show that any single stuck-at fault falls in either the first or the second category, thus providing compl ete fault coverage through self checking. No hardware needs to be adde d to our circuits to achieve the complete self-checking property; furt her, the circuit is guaranteed to never generate a legal but erroneous output if it contains a fault. Minimal hardware is needed to detect t hat a circuit has either halted or has generated an illegal output.