Transient error mitigation by means of approximate logic circuits

Mención Internacional en el título de doctorThe technological advances in the manufacturing of electronic circuits have allowed to greatly improve their performance, but they have also increased the sensitivity of electronic devices to radiation-induced errors. Among them, the most common effects are the SEEs, i.e., electrical perturbations provoked by the strike of high-energy particles, which may modify the internal state of a memory element (SEU) or generate erroneous transient pulses (SET), among other effects. These events pose a threat for the reliability of electronic circuits, and therefore fault-tolerance techniques must be applied to deal with them. The most common fault-tolerance techniques are based in full replication (DWC or TMR). These techniques are able to cover a wide range of failure mechanisms present in electronic circuits. However, they suffer from high overheads in terms of area and power consumption. For this reason, lighter alternatives are often sought at the expense of slightly reducing reliability for the least critical circuit sections. In this context a new paradigm of electronic design is emerging, known as approximate computing, which is based on improving the circuit performance in change of slight modifications of the intended functionality. This is an interesting approach for the design of lightweight fault-tolerant solutions, which has not been yet studied in depth. The main goal of this thesis consists in developing new lightweight fault-tolerant techniques with partial replication, by means of approximate logic circuits. These circuits can be designed with great flexibility. This way, the level of protection as well as the overheads can be adjusted at will depending on the necessities of each application. However, finding optimal approximate circuits for a given application is still a challenge. In this thesis a method for approximate circuit generation is proposed, denoted as fault approximation, which consists in assigning constant logic values to specific circuit lines. On the other hand, several criteria are developed to generate the most suitable approximate circuits for each application, by using this fault approximation mechanism. These criteria are based on the idea of approximating the least testable sections of circuits, which allows reducing overheads while minimising the loss of reliability. Therefore, in this thesis the selection of approximations is linked to testability measures. The first criterion for fault selection developed in this thesis uses static testability measures. The approximations are generated from the results of a fault simulation of the target circuit, and from a user-specified testability threshold. The amount of approximated faults depends on the chosen threshold, which allows to generate approximate circuits with different performances. Although this approach was initially intended for combinational circuits, an extension to sequential circuits has been performed as well, by considering the flip-flops as both inputs and outputs of the combinational part of the circuit. The experimental results show that this technique achieves a wide scalability, and an acceptable trade-off between reliability versus overheads. In addition, its computational complexity is very low. However, the selection criterion based in static testability measures has some drawbacks. Adjusting the performance of the generated approximate circuits by means of the approximation threshold is not intuitive, and the static testability measures do not take into account the changes as long as faults are approximated. Therefore, an alternative criterion is proposed, which is based on dynamic testability measures. With this criterion, the testability of each fault is computed by means of an implication-based probability analysis. The probabilities are updated with each new approximated fault, in such a way that on each iteration the most beneficial approximation is chosen, that is, the fault with the lowest probability. In addition, the computed probabilities allow to estimate the level of protection against faults that the generated approximate circuits provide. Therefore, it is possible to generate circuits which stick to a target error rate. By modifying this target, circuits with different performances can be obtained. The experimental results show that this new approach is able to stick to the target error rate with reasonably good precision. In addition, the approximate circuits generated with this technique show better performance than with the approach based in static testability measures. In addition, the fault implications have been reused too in order to implement a new type of logic transformation, which consists in substituting functionally similar nodes. Once the fault selection criteria have been developed, they are applied to different scenarios. First, an extension of the proposed techniques to FPGAs is performed, taking into account the particularities of this kind of circuits. This approach has been validated by means of radiation experiments, which show that a partial replication with approximate circuits can be even more robust than a full replication approach, because a smaller area reduces the probability of SEE occurrence. Besides, the proposed techniques have been applied to a real application circuit as well, in particular to the microprocessor ARM Cortex M0. A set of software benchmarks is used to generate the required testability measures. Finally, a comparative study of the proposed approaches with approximate circuit generation by means of evolutive techniques have been performed. These approaches make use of a high computational capacity to generate multiple circuits by trial-and-error, thus reducing the possibility of falling into local minima. The experimental results demonstrate that the circuits generated with evolutive approaches are slightly better in performance than the circuits generated with the techniques here proposed, although with a much higher computational effort. In summary, several original fault mitigation techniques with approximate logic circuits are proposed. These approaches are demonstrated in various scenarios, showing that the scalability and adaptability to the requirements of each application are their main virtuesLos avances tecnológicos en la fabricación de circuitos electrónicos han permitido mejorar en gran medida sus prestaciones, pero también han incrementado la sensibilidad de los mismos a los errores provocados por la radiación. Entre ellos, los más comunes son los SEEs, perturbaciones eléctricas causadas por el impacto de partículas de alta energía, que entre otros efectos pueden modificar el estado de los elementos de memoria (SEU) o generar pulsos transitorios de valor erróneo (SET). Estos eventos suponen un riesgo para la fiabilidad de los circuitos electrónicos, por lo que deben ser tratados mediante técnicas de tolerancia a fallos. Las técnicas de tolerancia a fallos más comunes se basan en la replicación completa del circuito (DWC o TMR). Estas técnicas son capaces de cubrir una amplia variedad de modos de fallo presentes en los circuitos electrónicos. Sin embargo, presentan un elevado sobrecoste en área y consumo. Por ello, a menudo se buscan alternativas más ligeras, aunque no tan efectivas, basadas en una replicación parcial. En este contexto surge una nueva filosofía de diseño electrónico, conocida como computación aproximada, basada en mejorar las prestaciones de un diseño a cambio de ligeras modificaciones de la funcionalidad prevista. Es un enfoque atractivo y poco explorado para el diseño de soluciones ligeras de tolerancia a fallos. El objetivo de esta tesis consiste en desarrollar nuevas técnicas ligeras de tolerancia a fallos por replicación parcial, mediante el uso de circuitos lógicos aproximados. Estos circuitos se pueden diseñar con una gran flexibilidad. De este forma, tanto el nivel de protección como el sobrecoste se pueden regular libremente en función de los requisitos de cada aplicación. Sin embargo, encontrar los circuitos aproximados óptimos para cada aplicación es actualmente un reto. En la presente tesis se propone un método para generar circuitos aproximados, denominado aproximación de fallos, consistente en asignar constantes lógicas a ciertas líneas del circuito. Por otro lado, se desarrollan varios criterios de selección para, mediante este mecanismo, generar los circuitos aproximados más adecuados para cada aplicación. Estos criterios se basan en la idea de aproximar las secciones menos testables del circuito, lo que permite reducir los sobrecostes minimizando la perdida de fiabilidad. Por tanto, en esta tesis la selección de aproximaciones se realiza a partir de medidas de testabilidad. El primer criterio de selección de fallos desarrollado en la presente tesis hace uso de medidas de testabilidad estáticas. Las aproximaciones se generan a partir de los resultados de una simulación de fallos del circuito objetivo, y de un umbral de testabilidad especificado por el usuario. La cantidad de fallos aproximados depende del umbral escogido, lo que permite generar circuitos aproximados con diferentes prestaciones. Aunque inicialmente este método ha sido concebido para circuitos combinacionales, también se ha realizado una extensión a circuitos secuenciales, considerando los biestables como entradas y salidas de la parte combinacional del circuito. Los resultados experimentales demuestran que esta técnica consigue una buena escalabilidad, y unas prestaciones de coste frente a fiabilidad aceptables. Además, tiene un coste computacional muy bajo. Sin embargo, el criterio de selección basado en medidas estáticas presenta algunos inconvenientes. No resulta intuitivo ajustar las prestaciones de los circuitos aproximados a partir de un umbral de testabilidad, y las medidas estáticas no tienen en cuenta los cambios producidos a medida que se van aproximando fallos. Por ello, se propone un criterio alternativo de selección de fallos, basado en medidas de testabilidad dinámicas. Con este criterio, la testabilidad de cada fallo se calcula mediante un análisis de probabilidades basado en implicaciones. Las probabilidades se actualizan con cada nuevo fallo aproximado, de forma que en cada iteración se elige la aproximación más favorable, es decir, el fallo con menor probabilidad. Además, las probabilidades calculadas permiten estimar la protección frente a fallos que ofrecen los circuitos aproximados generados, por lo que es posible generar circuitos que se ajusten a una tasa de fallos objetivo. Modificando esta tasa se obtienen circuitos aproximados con diferentes prestaciones. Los resultados experimentales muestran que este método es capaz de ajustarse razonablemente bien a la tasa de fallos objetivo. Además, los circuitos generados con esta técnica muestran mejores prestaciones que con el método basado en medidas estáticas. También se han aprovechado las implicaciones de fallos para implementar un nuevo tipo de transformación lógica, consistente en sustituir nodos funcionalmente similares. Una vez desarrollados los criterios de selección de fallos, se aplican a distintos campos. En primer lugar, se hace una extensión de las técnicas propuestas para FPGAs, teniendo en cuenta las particularidades de este tipo de circuitos. Esta técnica se ha validado mediante experimentos de radiación, los cuales demuestran que una replicación parcial con circuitos aproximados puede ser incluso más robusta que una replicación completa, ya que un área más pequeña reduce la probabilidad de SEEs. Por otro lado, también se han aplicado las técnicas propuestas en esta tesis a un circuito de aplicación real, el microprocesador ARM Cortex M0, utilizando un conjunto de benchmarks software para generar las medidas de testabilidad necesarias. Por ´último, se realiza un estudio comparativo de las técnicas desarrolladas con la generación de circuitos aproximados mediante técnicas evolutivas. Estas técnicas hacen uso de una gran capacidad de cálculo para generar múltiples circuitos mediante ensayo y error, reduciendo la posibilidad de caer en algún mínimo local. Los resultados confirman que, en efecto, los circuitos generados mediante técnicas evolutivas son ligeramente mejores en prestaciones que con las técnicas aquí propuestas, pero con un coste computacional mucho mayor. En definitiva, se proponen varias técnicas originales de mitigación de fallos mediante circuitos aproximados. Se demuestra que estas técnicas tienen diversas aplicaciones, haciendo de la flexibilidad y adaptabilidad a los requisitos de cada aplicación sus principales virtudes.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Raoul Velazco.- Secretario: Almudena Lindoso Muñoz.- Vocal: Jaume Segura Fuste

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