An Iterative Cell Switch Design for Hybrid Redundancy

A marriage of N-modular redundancy (NMR) and standby sparing has resulted in a promising redundancy technique for protecting those portions of a fault-tolerant system whose continuous real-time operation is essential. This technique, known as hybrid redundancy, consists of N identical modules connected to a majority voter to form an NMR core. Disagreement detectors instruct a switch to replace any of the N modules that disagree with the majority consensus by a standby spare. The switch is essential to the operation of the hybrid redundancy scheme. The system reliability is a product of the reliabilities of the switch and the hybrid redundancy scheme assuming a perfect switch. To realize the demonstrated potential of the latter a highly reliable, thus simple, switch is required. An iterative cell switch is proposed and demonstrated to save at least 25 percent, and more than 80 percent in some instances, of the complexity of a switch design presented elsewhere in the literature. The use of threshold, rather than majority, voters is considered and shown to yield a simpler design in some cases. Three techniques for decreasing the propagation delay through the iterative cell switch are presented as well as a scheme to implement retry of failed modules. Finally, five different switch designs are compared on a cost and complexity basis.