Transient error mitigation by means of approximate logic circuits

Created by W.Langdon from gp-bibliography.bib Revision:1.4524

  author =       "Antonio Jose {Sanchez Clemente}",
  title =        "Transient error mitigation by means of approximate
                 logic circuits",
  school =       "Universidad Carlos III de Madrid",
  year =         "2017",
  address =      "Spain",
  keywords =     "genetic algorithms, genetic programming, EHW",
  unstable_url = "",
  size =         "257 pages",
  abstract =     "The 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 circuit performance in exchange for
                 slight modifications of the intended functionality.
                 This is an interesting approach for the design of
                 lightweight fault-tolerant solutions, which has not
                 been studied in depth yet.

                 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 for
                 different tradeoffs. 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 tradeoff between
                 reliability and overheads. In addition, its
                 computational complexity is very low.

                 However, the selection criterion based in static
                 testability measures has some drawbacks. Adjusting the
                 trade-off 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 in 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 for different trade-offs 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
                 characteristics than with the approach based in static
                 testability measures. Finally, 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 specificities 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 evolutionary techniques have
                 been performed. These approaches are able 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 evolutionary approaches present slightly
                 better trade-offs than the circuits generated with the
                 techniques here proposed, although with a much higher
                 computational effort.

                 In summary, several original error 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 virtues.",
  notes =        "supervisor Luis Entrena Arrontes",

Genetic Programming entries for Antonio Jose Sanchez-Clemente