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Reliability Improving Approach with Opto-Based Voting System

Reliability Improving Approach with Opto-Based Voting System. Y.Yanagawa A , T.Takahara B , T.Mizuno BC , H.Saito C A The University of Tokyo B The Graduate University for Advanced Studies C The Institute of Space and Astronautical Science, JAXA.

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Reliability Improving Approach with Opto-Based Voting System

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  1. Reliability Improving Approach with Opto-Based Voting System Y.Yanagawa A , T.Takahara B, T.Mizuno BC, H.Saito C A The University of Tokyo B The Graduate University for Advanced Studies C The Institute of Space and Astronautical Science, JAXA

  2. Our strategy for improving performance and reliabilityAbstract We propose new high-speed voting system based on optical signal. Majority voting is major approach to boost system reliability in computers. It has been used for many space applications. In such redundant system, signals are usually voted by logical synchronous circuits. However, such circuits often limits operating frequency of the whole system because voter should run at least twice as fast as system. System operating frequency is naturally limited by the voting speed. On the other hand, in these days, traditional parallel buses in commercial computers are going to be serial. This is because, the faster bus operating speed becomes to increase band width,the more difficult to decrease clock or signal skew to synchronize signals in them. The same thing can be said to FPGA. Many FPGAs provide ultra high-speed serial port to achieve both high speed data rate and simple wiring. In general, serial connection requires to be operated at very high frequency to keep bandwidth wide enough. In this paper, we introduce two types of analog voting scheme: electrical signal based voting system and optical signal based one. The latter one is new type of voting system based on optical signal, that is fast enough to be used in redundant high-speed serial connection. Then, these two voting system is evaluated their feasibility.

  3. Our strategy for improving performance and reliabilityOutline • Backgrounds • Trend of computer systems • FPGA Interface • Redundant computing system • Our Approach for Improving Reliability and Performance • Electrical signal based Voting System • Opto-Based Voting System • Discussion • Summary and Future Work

  4. Trend of Computer SystemsHigh Speed Serial Bus in COTS Computers • COTS (Commercial Off The Shelf) Computers are going to use serial bus • Difficult to synchronize signals in the parallel bus • To improve bandwidth, every data bus is going to be serial • PCI Express • Serial ATA • Serial memory • Now several Gbps per channel is achieved on the serial bus

  5. FPGA InterfaceHigh Speed Serial interfaces in FPGAs • Some FPGAs begin to provide embedded high speed serial interface • Save many I/O pins with keeping high I/O bandwidth • Built-in high speed serial interface • 10Gbps ether net • Fiber channel • Etc… • Low jitter • Easy to implement

  6. Redundant computing system Triple Voting System • Improve system reliability • Utilize multiple module redundancy • Mitigate radiation effect for space application • Performance is limited by the speed of voting module Triple Voting System Overview

  7. Our Approach for improving Reliability and PerformanceReliability Improving Approach • Motivation • Improving system performance and memory access band width • Simplify system configuration for resource reduction • Serial interface is already available in some FPGAs • Easier to manage signal synchronization • Remove performance limitation caused by traditional voting circuits in the redundant system • System Requirement • Compared to traditional parallel bus (eg. 32bit 100MHz), more than 3Gbps data rate is desirable for serial bus • Strategy • Propose very high speed voting circuit • Fast enough to be used for memory bus • Improving voter performance up to several GHz

  8. Our Approach for improving Reliability and PerformancePerformance Improving Approach • Improving approach • Electrical signal based voting system (Voltage averaging) • Opto-Based voting system (Newly proposed in this paper) Opto-Based voting Electrical signal based voting Both types of voting are evaluated

  9. Electrical signal based Voting SystemScheme of Electrical signal based Voting System • Design high-speed voting system with traditional voltage voting scheme to estimate its speed limit • Serial communication often uses differential signal • Voltage Averaging System for differential signal • Positive signals and negative signals are averaged using registers, respectively Outline of voltage averaging system for differential signal

  10. Electrical signal based Voting SystemExperimental Setup • Voltage Averaging System for differential signal is built with chip register and microstrip line on the glass epoxy board • Validate system functionality by using low frequency LVDS signal (120Mbps) and evaluate transmission characteristic at high frequency Magnified view of voter module Overall Experiment system LVDS transceiver spec. Transmission characteristic design

  11. Electrical signal based Voting SystemEvaluation Result • The signals are voted properly at 60MHz (120Mbps) • Observe signal TTL-Input and TTL-Output by digital storage oscilloscope and logic analyzer IN1 IN2 IN3 OUT Logical signal of TTL-Input (IN1,IN2,IN3) and TTL-Output Signal “IN2” simulates error Signal of TTL-Input (IN1,IN2,IN3) and TTL-Output (5V/Div, 50ns/Div, 2GSample) Signal “IN2” simulates error

  12. Electrical signal based Voting System Evaluation of transmission characteristic at high frequency • Transmission characteristic is evaluated by the evaluation board below. • Target frequency 2GHz(l = 15 [cm] ) • Voting region is designed as concentrated constant circuit (6.4[mm] x 6.4[mm] < l/20) • Microstrip Line is designed • Zdiff=100.7 [W], Z1,Z2=58.87 [W] • Glass epoxy board • Dielectric Constant e = 4.7 • Hight h = 1.6 [mm] • Dielectric dissipation factor tan d = 0.0175

  13. Electrical signal based Voting SystemEvaluation Result - Transmission Characteristic • Transmission Characteristic of the voting system Detail of the loss Transmission Characteristic (S21) of evaluation board (Start:1GHz, Stop:3GHz,5dB/Div) Total loss is -13.933 [dB]

  14. Electrical signal based Voting SystemEvaluation Result - Discussion • As the digital signal has wide frequency spectrum, the voting circuit is built with chip register (lumped parameter circuit ) and microstrip line • This design results in large transmission loss of -13.9[dB] • -13.9[dB] loss is critical to the signal transmission • Increase noise susceptibility • On the other hand, if it is built with pure microstrip line (distributed constant circuit) , its characteristic varies largely with the input signal frequency • Unable to operate with wide spectrum signal Electrical signal based voting system is not suitable for high frequency signal

  15. Opto-Based Voting SystemScheme of Opto-Based Voting System • This system utilizes optical power to decide majority • Electrical signals are modulated into AM optical signals. • Each signals are mixed together to sum up the power of every optical signal at photo detector. • Proper threshold is set at the photo detector to determine logical output Majority Vote by Amplitude Modulated Optical Signals

  16. 3 LDs PD Opto-Based Voting SystemFundamental evaluation • Many optical transceivers use laser diode to modulate optical wave by high speed input signal • Check if such “coherent” optical signals can be summed up without interfering each other for majority voting Experimental System

  17. Opto-Based Voting SystemEvaluation Result • Measure the output voltage of photo detector with turning each LD ON/OFF • Output voltage increases linearly as the number of ON LD increases • This result shows that even coherent light wave can be summed up • It is possible to determine how many modules are ON by comparing the output voltage Output Voltage of Photo Detector

  18. Our strategy for improving performance and reliabilitySummary • Optical voting method have many advantages • No parasitic capacitance on the transmission line • Easy to transmit high speed signal • Many high speed optical devices are available on the market • Electromagnetic Compatibility (EMC) • The evaluation result shows that optical voting system is feasible for majority voting Performance Comparison of each method

  19. Future workTriple Voting System with Serial Memory • Application of this voting system • Triple voting system with serial memory • Simple wiring, Save resource for voter, Easy to synchronize

  20. Future workVector based Optical Error Detection • Vector based optical error detection and error correction system • Map each input signal from redundant modules to optical wave vectors by using phase modulation • These optical signals are mixed at the detector • Error module detection and majority voting can be achieved all at once by checking vectorial summation of the mixed optical signals Correspondence between vector and voting result

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