N. Pete Sedcole

Within-die delay variability in 90nm FPGAs and beyond

Semiconductor scaling causes increasing and unavoidable within-die parametric variability. This paper describes accurate measurement techniques for characterising both systematic and stochastic delay variability in FPGAs. Results and analysis are presented from measurements made on a sample of 90nm devices, showing that delay per logic element varies stochastically by plusmn3.54% on average over the set. The delay also varies by up to 3.66% across a single die from correlated sources of variability. The results are extrapolated to determine the impact at future technology nodes. The predicted significant performance degradation that variability will cause demonstrates the importance of new circuit or system design techniques to cope with variations in future FPGAs

Fault tolerant methods for reliability in FPGAs

Reliability and process variability are serious issues for FPGAs in the future. Fortunately FPGAs have the ability to reconfigure in the field and at runtime, thus providing opportunities to overcome some of these issues. This paper provides the first comprehensive survey of fault detection methods and fault tolerance schemes specifically for FPGAs, with the goal of laying a strong foundation for future research in this field. All methods and schemes are qualitatively compared and some particularly promising approaches highlighted.

On-FPGA Communication Architectures and Design Factors

The recent development of platform-FPGA or field-programmable system-on-chip architectures, with immersed coarse-grain processors, embedded memories and IP cores, offers the potential for immense computing power as well as opportunities for rapid system prototyping. These platforms require high-performance on-chip communication architectures for efficient and reliable inter-processor communication. However, as the number of embedded processors increases, communication bandwidth between embedded components becomes a limiting factor to overall system performance. In this paper, we survey the state-of-the-art on-FPGA communication architectures and methodologies. Salient factors, which include quantitative performance metrics and qualitative factors, relevant to design are identified and used to analyze and classify the on-FPGA communication architectures. This survey aims to facilitate innovation in and development of future on-FPGA communication architectures

Parametric yield in FPGAs due to within-die delay variations: a quantitative analysis

Variations in the semiconductor fabrication process results in variability in parameters between transistors on the same die, a problem exacerbated by lithographic scaling. The re-configurability of Field-Programmable Gate Arrays presents the opportunity to compensate for within-die delay variability. This paper presents three reconfiguration-based strategies for compensating within-die stochastic delay variability in FPGAs: reconfiguring the entire FPGA, relocating subcircuits within an FPGA, and reconfiguring signal paths within a design. The yield of each strategy is analysed and compared with worst-case design and statistical static timing analysis (SSTA). It is demonstrated that significant im-provements in circuit yield and timing are possible using SSTA alone, and these improvements can be enhanced by employing reconfiguration-based techniques.

Run-Time Integration of Reconfigurable Video Processing Systems

Embedded systems in field-programmable gate arrays (FPGAs) can be customized and adaptive if assembled from modular components at run time. This paper examines realizing run-time system assembly by extension of platform-based design. Two major challenges are addressed in this paper. First, the design of a reconfigurable platform architecture suitable for run-time system assembly is described. Different systems are constructed by integrating the platform architecture with different modular components, which employ the communication infrastructure supplied by the platform in order to interact. Second, where on-chip communications channels use shared media, we propose techniques for modeling the intermodule communication behavior based on statistical time-division multiplexing. The proposed techniques enable system designers to guarantee that logical communication requirements between the adjunct modules can be satisfied by the infrastructure. An in-depth analysis is presented and then verified with cycle-accurate simulations for an example reconfigurable platform for real-time video applications.

Fault tolerance and reliability in field-programmable gate arrays

Reduced device-level reliability and increased within-die process variability will become serious issues for future field-programmable gate arrays (FPGAs), and will result in faults developing dynamically during the lifetime of the integrated circuit. Fortunately, FPGAs have the ability to reconfigure in the field and at runtime, thus providing opportunities to overcome such degradation-induced faults. This study provides a comprehensive survey of fault detection methods and fault-tolerance schemes specifically for FPGAs and in the context of device degradation, with the goal of laying a strong foundation for future research in this field. All methods and schemes are quantitatively compared and some particularly promising approaches are highlighted.

Characterisation of FPGA Clock Variability

As integrated circuits are scaled down it becomes difficult to maintain uniformity in process parameters across each individual die. The resulting performance variation requires new design strategies to avoid pessimistic over-design. A quantified understanding of the contribution different circuit components make to performance variation is a necessary part of such strategies. This paper proposes a technique for quantifying variability in clock skew in FPGAs based on a novel differential delay measurement circuit. The technique is capable of isolating the effects on clock skew from different components in the clock network. Results from a 65 nm FPGA show that clock skew variation is significant, being comparable in magnitude to signal path delay variation.

Parametric Yield Modeling and Simulations of FPGA Circuits Considering Within-Die Delay Variations

Variations in the semiconductor fabrication process results in differences in parameters between transistors on the same die, a problem exacerbated by lithographic scaling. Field-Programmable Gate Arrays may be able to compensate for within-die delay variability, by judicious use of reconfigurability. This article presents two strategies for compensating within-die stochastic delay variability by using reconfiguration: reconfiguring the entire FPGA, and relocating subcircuits within an FPGA. Analytical models for the theoretical bounds on the achievable gains are derived for both strategies and compared to models for worst-case design as well as statistical static timing analysis (SSTA). All models are validated by comparison to circuit-level Monte Carlo simulations. It is demonstrated that significant improvements in circuit yield and timing are possible using SSTA alone, and these improvements can be enhanced by employing reconfiguration-based techniques.

A Hybrid Analog-Digital Routing Network for NoC Dynamic Routing

Dynamic routing can substantially enhance the quality of service for multiprocessor communication, and can provide intelligent adaptation of faulty links during run time. Implementing dynamic routing on a network-on-chip (NoC) platform requires a design that provides highly efficient optimal path computation coupled with reduced area and power consumption. In this paper, we present a hybrid analog-digital routing network design that enables efficient dynamic routing on an NoC architecture. The digital part provides accurate real-time traffic estimation using a temporal cost evaluation and adaptation scheme. The analog network, which is distributed within the digital communication network, provides an efficient implementation for the optimal routing algorithm with extremely low power consumption. Our results demonstrate the effectiveness of the hybrid analog-digital design, with a significant improvement in latency over the static routing for random hot spot traffics

A Structured Methodology for System-on-an-FPGA Design

Increasing logic resources coupled with a proliferation of integrated performance enhancing primitives in high-end FPGAs results in an increased design complexity which requires new methodologies to overcome. This paper presents a structured system based design methodology, centred around the concept of architecture reuse, which aims to increase productivity and exploit the reconfigurability of high-end FPGAs. The methodology is exemplified by the Sonic-on-a-Chip architecture. Preliminary experimental investigations reveal that while the proposed methodology is able to achieve the desired aims, its success would be enhanced if changes were made to existing FPGA fabrics in order to make them better suited to modular design.