On self-healing digital system design

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Abstract

In recent years there has been a significant growth of interest in exploiting the principles of biological processes to create powerful methodologies for solving computational problems. This paper discusses how these features have been exploited in digital hardware design. It also introduces an architecture for implementing self-healing digital systems that is inspired by the antigen protection mechanism employed by the human immune system. In the proposed architecture, a spare cell is dedicated to replace one in a group of four functional cells. Once one of these four functional cells is found to be faulty, the spare cell is cloned as the faulty cell. This architecture is especially suitable for tolerating soft errors in functional cells or on interconnect lines. Another major advantage of this architecture is that the outputs of functional cells are connected to the inputs of other physically adjacent functional cells, thus making it appropriate for nanocomputing system design.

Introduction

Living organisms exhibit a unique range of complex characteristics such as evolution, adaptation and self-healing. The fundamental units of living organisms are cells; each cell contains the gene of the organism. In most biological systems a gene is a sequence of four nitrogen bases joined together in a chain. The names of the nitrogen bases (Adenine, Cytosine, Guanine, and Thymine) are usually shortened to A, C, G, and T. A chain of any number of A, C, G, and T bases in any order are known as a DNA. Thus, genes contain information encoded as sequences of A, C, G, and T bases. This information, called the genetic code, is decoded by special structures within the cells of an organism to determine the function of a cell. In recent years there has been significant growth of interest in exploiting the principles of biological processes to create powerful methodologies known as biologically-inspired approaches for solving computational problems.

This paper provides an overview of some of the work done so far in incorporating two very important characteristics of living organisms-self-replication and self-healing-in digital systems. It also proposes a new approach for designing self-healing systems. The concept of self-healing known as fault-tolerance to digital system designers is well known but the same is in general not true for self-replication. This paper is organized as follows. Section 2 discusses the classification of biologically-inspired systems and a model that has been proposed for such systems. Section 3 focuses on some earlier efforts in extending biological concepts to digital system design. Section 4 discusses the work done so far in imitating self-replication in silicon-based systems. Section 5 concentrates on how self-healing can be introduced in two-dimensional silicon structures. An architecture for implementing self-healing digital systems based on the concepts of human immune system, is proposed in Section 6. Section 7 contains the conclusions and suggestions for future work.

Section snippets

POE model

Sipper et al. [1] proposed a model known as the POE model to classify biologically-inspired systems as one of the following: Phylogenetic, Ontogenetic and Epigenetic. Fig. 1 shows the model. A phylogenetic system imitates the processes involved in the evolution of organisms through time. The process of evolution is based on alteration of genetic information through the mechanisms of crossover and mutation. Both of these contribute to the diversity of species; this diversity is indispensable for

Embryonics

A biologically inspired approach that utilizes the ontogenetic paradigm of the POE model has been proposed for designing arbitrary digital circuits by Menge et al. [9] and Marchal et al. [10]. This approach is called embryonics (embryology and electronics). Since living organisms are carbon-based, it is not possible to extend directly the concepts behind some of the mechanisms used by living organisms to digital circuits which are in general silicon-based. Thus, the objective of embryonics has

Self-replication

The possibility of a digital system creating copies of itself was first explored by Von Neumann. He proposed the idea of Cellular Automata (CA) to show that a universal computer, whose computational complexity is equivalent to that of a universal Turing machine [13], can be embedded in a CA. A CA consists of an n-dimensional (in general n is 1 or 2 but rarely 3) array of cells, each of which can be in one of a finite number of k possible states. Each cell is connected to m surrounding

Self-healing

As mentioned earlier one of the key features of a biological system is its ability to self-heal. This allows regrowth of a healthy cell at the site of a damaged cell, thus providing robustness to living organisms. Obviously true self-healing is possible only in carbon-based biological systems not in silicon-based digital systems. Mange et al. [10] have shown that if an FPGA system embryonic array has spare cells, then a damaged cell can be replaced by a healthy spare one. All cells in the array

Human-immune system inspired self-healing

The immune system consists of white blood cells called lymphocytes that detect the antigens and destroy them [3]. There are millions of lymphocytes in a human body that are distributed throughout the body, and there is no centralized control. The immune system recognizes specific antigens using a certain subset of lymphocytes, called B-cells [3], [19]. There is another subset of lymphocytes called killer T-cells, which help the B-cells in detecting and eliminating the antigens. The B-cell locks

Functional cell

A functional cell implements a logic function of three input variables and transfers its output to three other adjacent functional cells. The block diagram of the functional cell is shown in Fig. 6; it is a modified version of the functional cell proposed in Ref. [18]. It is composed of a 6-bit control register, tri-state buffers and a logic block. The contents of the control register in a functional cell may be considered as the genetic code of the functional cell. The code identifies the

Router cell

A router cell, as indicated earlier, is used to transfer the output of an internal cell to the input of another cell. The design of the router cell is shown in Fig. 11. It is composed of a buffer, a 3-to-8 decoder, and a programmable control register. The buffer output, identified as XYZ, corresponds to the contents of the control register and is fed to the decoder. The output of the decoder activates the appropriate output line. The content of the control register activates the output line

Spare cell

A spare cell is similar to a functional cell in its structure. Any available spare cell located in the NORTH or SOUTH or EAST or WEST of a functional cell can be selected as its spare. The control register in a spare cell, pre-selected to replace a functional cell, is enabled if the functional cell generates an error signal. The control register in the spare cell is then loaded with the contents of the control register of the faulty functional cell. In the human immune system a T-cell kills an

Conclusion

This paper presented a general discussion of how two important characteristics of biological organisms-self-propagation and self-healing-can be utilized in digital system design. As far as the authors are aware of currently there is no formal technique available for generating an automata capable of self-replication in a finite two-dimensional array.

True self-healing is possible only in carbon-based biological systems, not in silicon-based digital systems. However, self-diagnosis of damaged

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