Paul S. Graham

The reliability of FPGA circuit designs in the presence of radiation induced configuration upsets

FPGAs are an appealing solution for space-based remote sensing applications. However, in a low-Earth orbit, FPGAs (field programmable gate arrays) are susceptible to Single-Event Upsets (SEUs). In an effort to understand the effects of SEUs, an SEU simulator based on the SLAAC-1V computing board has been developed. This simulator artificially upsets the configuration memory of an FPGA and measures its impact on FPGA designs. The accuracy of this simulation environment has been verified using ground-based radiation testing. This simulation tool is being used to characterize the reliability of SEU mitigation techniques for FPGAs.

SEU-induced persistent error propagation in FPGAs

This paper introduces a new way to characterize the dynamic single-event upset (SEU) cross section of an FPGA design in terms of its persistent and nonpersistent components. An SEU in the persistent cross section results in a permanent interruption of service until reset. An SEU in the nonpersistent cross section causes a temporary interruption of service. These cross sections have been measured for several designs using fault-injection and proton testing. Some FPGA applications may realize increased reliability at lower costs by focusing SEU mitigation on just the persistent cross section.

SRAM FPGA Reliability Analysis for Harsh Radiation Environments

This paper investigates the viability of deploying SRAM-based FPGAs into harsh Earth-orbit environments. A reliability model is presented for estimating the MTTF of SRAM FPGA designs in specific orbits and orbit conditions. The model requires orbit- and condition-specific SEU rates and design-specific estimates of the probability of failure during a single scrubbing period. Probability of failure estimates are reported for several FPGA designs from both fault-injection and accelerator experiments. The model also includes a method for estimating composite mean time to failure (MTTF) that incorporates all orbit conditions over a solar cycle. Despite using pessimistic assumptions, the results from this model suggest that SRAM FPGA designs protected by TMR and scrubbing operate very reliably in a LEO orbit and surprisingly well in ¿harsh¿ orbits.

Domain Crossing Errors: Limitations on Single Device Triple-Modular Redundancy Circuits in Xilinx FPGAs

This paper discusses the limitations of single-FPGA triple-modular redundancy in the presence of multiple-bit upsets on Xilinx Virtex-II devices. This paper presents results from both fault injection and accelerated testing. From this study we have found that the configurable logic blocks routing network is vulnerable to domain crossing errors, or TMR defeats, by even 2-bit multiple-bit upsets.

Fine-Grain SEU Mitigation for FPGAs Using Partial TMR

The mitigation of single-event upsets (SEUs) in field-programmable gate arrays (FPGAs) is an increasingly important subject as FPGAs are used in radiation environments such as space. Triple modular redundancy (TMR) is the most frequently used SEU mitigation technique but is very expensive in terms of area and power costs. These costs can be reduced by sacrificing some reliability and applying TMR to only part of the FPGA design. Our partial TMR method focuses on the most critical sections of the design and increases reliability by applying TMR to continuous sections of the circuit. We introduce an automated software tool that uses the Partial TMR method to apply TMR incrementally at a very fine level until the available resources are utilized. Thus the tool aims to gives the maximum reliability gain for the specified area cost.

Static Proton and Heavy Ion Testing of the Xilinx Virtex-5 Device

This paper presents proton and heavy ion static results for the latest Xilinx field-programmable gate arrays (FPGAs). The paper analyzes static bit cross-sections, resources, multiple-bit upsets (MBUs) and angular effects.

Terrestrial-based radiation upsets: a cautionary tale

Problems with terrestrial-based neutron radiation from cosmic rays have become more commonplace. While the incident rate from neutron radiation is lower than space-based radiation, physics, system design and system locations have combined to make systems increasingly vulnerable to terrestrial radiation. FPGA systems are particularly sensitive to neutron radiation, as the FPGAs, microprocessors and memory are all sensitive to upsets. We are interested in reconfigurable supercomputers, which need to be highly reliable and highly available despite being very sensitive to radiation. In this paper, we estimate the error rate for FPGAs, memory, and microprocessors so that predictions for the sensitivity of the Cray XD1 reconfigurable supercomputer can be made. We also present possible mitigation methods that are appropriate for neutron radiation upset rates.

Using Duplication with Compare for On-line Error Detection in FPGA-based Designs

It is well known that SRAM-based FPGAs are susceptible to single-event upsets (SEUs) in radiation environments. A variety of mitigation strategies have been demonstrated to provide appropriate mitigation and correction of SEUs in these environments. While full mitigation of SEUs is appropriate for some situations, some systems may tolerate SEUs as long as these upsets are detected quickly and correctly. These systems require effective error detection techniques rather than costly error correction methods. This work leverages a well-known error detection technique for FPGAs called duplication with compare (DWC). This technique has been shown to be very effective at quickly and accurately detecting SEUs using fault injection and radiation testing.

SEU mitigation for half-latches in Xilinx Virtex FPGAs

The performance, in-system reprogrammability, flexibility, and reduced costs of SRAM-based field programmable gate arrays (FPGAs) make them very interesting for high-speed on-orbit data processing, but the current generation of radiation-tolerant SRAM-based FPGAs are based on commercial-off-the-shelf technologies and, consequently, are susceptible to single-event upset effects. In this paper, we discuss in detail the consequences of radiation-induced single-event upsets (SEUs) in the state of half-latch structures found in Xilinx Virtex FPGAs and describe methods for mitigating the effects of half-latch SEUs. One mitigation methods effectiveness is then illustrated through experimental data gathered through proton accelerator testing at Crocker Nuclear Laboratory, University of California-Davis. For the specific design and mitigation methodology tested, the mitigated design demonstrated more than an order of magnitude improvement in reliability over the unmitigated version of the design in regards to average proton fluence until circuit failure.

A Test Methodology for Determining Space Readiness of Xilinx SRAM-Based FPGA Devices and Designs

Using reconfigurable static random access memory (SRAM)-based field-programmable gate arrays (FPGAs) for space-based computation has been a very active area of research for the past decade. Because these commercially available devices are only radiation tolerant in terms of total ionizing dose and single-event latchup, these devices must be qualified for other types of single-event effects to be used in spacecraft. Furthermore, mission requirements often dictate the need to do radiation experiments on the FPGA user circuit. Because both the circuit and the circuits state are stored in memory that is susceptible to single-event upsets, both could be altered by the harsh space radiation environment. Both the circuit and the circuits state can be protected by triple-modular redundancy (TMR), but applying TMR to FPGA user designs is often an error-prone process. Faulty application of TMR could cause the FPGA user circuit to output incorrect data. This paper will describe both device-level static testing and user circuit dynamic testing, including a three-tiered methodology for testing FPGA user designs for space readiness.