IEEE TRANSACTIONS ON VERY LARGE SCALE INTEGRATION (VLSI) SYSTEMS, VOL. 21, NO. 6, JUNE 2013 1027 Self-Repairing Digital System With Unified Recovery Process Inspired by Endocrine Cellular Communication Isaak Yang, Sung Hoon Jung, and Kwang-Hyun Cho, Senior Member, IEEE Abstract— Self-repairing digital systems have recently emerged as the most promising alternative for fault-tolerant systems. However, such systems are still impractical in many cases, particularly due to the complex rerouting process that follows cell replacement. They lose efficiency when the circuit size increases, due to the extra hardware in addition to the functional circuit and the unutilization of normal operating hardware for fault recovery. In this paper, we propose a system inspired by endocrine cellular communication, which simplifies the rerouting process in two ways: 1) by lowering the hardware overhead along with the increasing size of the circuit and 2) by reducing the hardware unutilized for fault recovery while maintaining good fault-coverage. The proposed system is composed of a structural layer and a gene-control layer. The structural layer consists of novel modules and their interconnections. In each module of our system, the encoded data, called the genome, contains information about the function and the connection. Therefore, a faulty module can be replaced and the whole system’s functions and connections are maintained by simply assigning the same encoded data to a spare (stem) module. In existing systems, a huge amount of hardware, such as a dynamic routing system, is required for such an operation. The gene-control layer determines the neighboring spare module in the structural layer to replace the faulty module without collision. We verified the proposed mechanism by imple- menting the system with a field-programmable gate array with the application of a digital clock whose status can be monitored with light-emitting-diodes. In comparison with existing methods, the proposed architecture and mechanism are efficient enough for application with real fault-tolerant systems dealing with harsh and remote environments, such as outer space or deep sea. Index Terms— Bio-inspired engineering, dynamic routing, endocrine cellular communication, redundancy, self-repair, stem cell. I. I NTRODUCTION R ELIABILTY has always been an issue with electronic systems ever since the first electronic systems were designed. Different from biological systems, electronic Manuscript received October 7, 2011; revised April 23, 2012; accepted June 3, 2012. Date of publication July 12, 2012; date of current version May 20, 2013. This work was supported in part by the National Research Foundation of Korea (NRF) grants funded by the Korean Government, the Ministry of Education, Science and Technology (MEST) under Grant 2009- 0086964 and Grant 2010-0017662, and the World Class University Program under Grant R32-2008-000-10218-0 through the NRF funded by MEST. I. Yang and K.-H. Cho are with the Department of Bio and Brain Engineer- ing, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail: ckh@kaist.ac.kr). S. H. Jung is with the Department of Information and Communication Engineering, Hansung University, Seoul 136-792, Korea. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TVLSI.2012.2203618 systems are so fragile that even a single problem can render the entire system useless. Therefore, devising fault-tolerant systems that can deal with such delicate problems has been a considerable challenge. During the early stages of the development of fault-tolerant systems, dual modular redundancy (DMR) and triple modular redundancy (TMR) methods were introduced [1]. These techniques ran the same modules in parallel, and thus a faulty module could be distinguished by comparing outputs of the same modules and voting for the majority one (with TMR) or by using an additional device (with DMR). However, these methods have several problems. The size of the module is so huge that a large part of the circuit must be replaced even if a small part in the module is malfunctioning. Furthermore, redundancy has to be running all the time, and it can only cover the fault once. In the last 10 years, these conventional methods have proven to be rather inefficient, and scientists have consequently turned to biology to find inspiration for a more suitable self-repairing circuit that can resolve the aforementioned problems and faults with fault-tolerant systems [1]–[11], [12], [13]–[19]. A new approach called embryonics is the application of concepts inspired from the biological cell to the design of digital circuits [2]. As the biological cells carry the genetic code of the whole system and are differentiated according to the location of the cell in the system, an embryonic self-repairing circuit is organized with building blocks that have identical structures and that vary according to the expressed genetic code in each block [2]–[4]. These self-repairing circuits can also recover from a fault by isolating the faulty block and differentiating a spare (stem) block with the same genetic code previously held by the faulty block. With such biological inspiration, self-repairing systems make repairs on a fine-grained scale rather than the coarse-grained scale of conventional fault- tolerant systems. Thus, such systems only need to change a small part of the system, and the spare (stem) blocks do not need to operate constantly. As an additional advantage, spare (stem) blocks can become any kind of logic, such as a biological stem cell (SC). Moreover, these systems can recover a functional cell (module) several times, whereas conventional fault-tolerant systems deal with only a single fault. One of the representative methods of self-repairing digital systems is the MUXTREE method [4], [5], [12]. In this method, a digital circuit is converted into an array of MUXTREE cells and the initial connection information among the MUXTREEs is encoded as a gene in each MUXTREE cell. The system develops very large-scale integrated circuits 1063-8210/$31.00 © 2012 IEEE