Evolving Static Hardware Redundancy for Defect Tolerant FPGAs

Abstract

Integrated circuits have been in constant progression since the first prototype in 1958. The semiconductor industry has maintained a constant rate of miniaturisation of transistors and wires, resulting in ever increasing speed, size and complexity of circuits. One challenge that has always been present is reduced yield due to production defects. A certain amount of chips must be scrapped because production defects have rendered the chips unusable. Recent predictions suggest that the average number of production defects per chip will rise drastically in the future as CMOS scaling approaches the physical limits of what is possible to manufacture. If these predictions are true, circuits should exhibit some level of tolerance to defects so to keep yield at acceptable levels.

The main contribution of the thesis is to the field of defect tolerance, with a focus on FPGAs. Apart from the widespread employment of FPGAs, two technical reasons make the FPGA especially suited for inclusion of defect tolerance techniques. The regular structure of the FPGA can be exploited for efficient redundancy techniques. In addition, the FPGA can be seen as a bridge between production and the application designer. Through defect tolerance techniques incorporated transparently in the FPGA, a fully functioning gate array can be provided to the application designer despite defects from production.

The approach taken in this thesis is to search for new ways of introducing static hardware redundancy in a circuit through the application of artiffcial evolution. However, the challenge of applying evolutionary techniques provided a secondary contribution. The work provides a contribution to the field of artifficial evolution

and the subfield evolvable hardware (EHW) by addressing ways in which such techniques may be applied to search for non-specifiable structures. The work is also bridging the fields of EHW and traditional hardware design and reliability metrics have been investigated for the purpose of comparing evolved and traditionally designed circuits.

Redundant structures are first evolved for gate level circuits where both voter based solutions and more intricate non-voter based solutions are achieved. Transistor level redundancy structures are targeted next to approach the main goal of defect tolerance for FPGAs. A defect tolerant inverter is evolved which forms the basis of a general defect tolerance technique, termed the Multiple Short-Open (MSO) technique. The FPGA look-up table (LUT) is one of the essential components of the FPGA and a defect tolerant LUT is, therefore, constructed applying the MSO technique. An evolutionary experiment is also conducted where a defect tolerant 1-input LUT is evolved directly.