Self-checking digital fault detector for modular redundant real time clock

Claim 1

A self-checking fault detector for a modular redundant clock of the type that includes a plurality of clock circuits, each producing a clock signal, the detector comprising a majority voter circuit for receiving the clock signals and for producing a voted majority output signal; comparison means for comparing each of the clock signals with the voted majority output signal and for providing a plurality of corresponding comparison signals, one for each clock signal, representative of the results of the comparisons; and means responsive to the plurality of comparison signals for providing a first failure signal indicating a failure of a clock circuit and for providing a second failure signal indicating a failure of the majority voter circuit; wherein the comparison means comprises a plurality of comparison circuit elements, each receiving a different one of the clock signals and the voted majority output, and each providing a separate comparison signal representing the results of the comparison between its received clock signal and the voted majority output signal, said comparison signal having a first state when the clock signal and the voted majority output signal agree, and a second state when the clock signal and the voted majority output signal disagree; and wherein the failure signal providing means comprises a first circuit for providing the first failure signal upon any of the plurality of comparison signals being in the second state, and comprises a second circuit for providing the second failure signal only upon all of the comparison signals being in the second state.