Fault Tolerant Quadded Logic Cell Structure with Built-in Adaptive Time Redundancy

Abstract

This paper describes research carried out using a quadded logic cell (QLC) structure with the purpose of creating a fault tolerant strategy for stuck-at faults. In order to create the tolerant built-in behaviour, the basic logic elements must have resilience against transistor level stuck-at failures. To achieve this, we add fine-grained redundancy to the transistor structure of the individual logic gates. In our research NAND gates which are been used throughout the QLC design. Simulation data shows that the chosen enhanced NAND gate structure can cope with single random stuck-at fault and if not indicates it through a distinct current indication. The QLC design contains four individual logic units which can be configured to perform four different types of two input logic functions. The QLC contains an interconnection structure that links three logic units to form a logic structure with four inputs and one output. This fixed internal structure revolves clockwise in four steps in a “round-robin” time redundancy scheme to create a set number of results. Through a majority voting a combined overall output result gets generated. Individual comparison of each clock cycle result against the voted result reveals the cycle and logic unit combination in which the faulty result has been generated. In this case alteration of the individual logic unit configuration has been used to generate another set of results for pattern mapping to identify the single logic unit within the QLC. After identification a self-initiated logic unit replacement with a spare unit happens. An additional detection method based on power rail grading of the individual logic units is devised to enable built-in classification of the stuck-at fault occurring within the unit and subsequently to trigger self-repair. These features are intended to be self-coordinated without requiring outside influence, making this resulting design capable of autonomous self-healing under specific failure conditions.