Reliability Improving Approach with Opto-Based Voting System

Yanagawa 1 P155/MAPLD 2004

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


Our strategy for improving performance and reliability

Abstract

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.



Outline

• 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


Trend of Computer Systems

High 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


FPGA Interface

High 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

Txp

Txn

Rxp

Rxn

Txp

Txn

Rxp

RxnSerializer

Deserializer

Deserializer

Serializer

N bit N bit

N bitN bit

FPGA/ Other DevicesFPGA


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


Our Approach for improving Reliability and Performance

Reliability 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 re

dundant system

• System Requirement– Compared to traditional parallel bus (eg. 32bit 100MHz), more than 3Gbps d

ata 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


Our Approach for improving Reliability and Performance

Performance Improving Approach

• Improving approach

– Electrical signal based voting system (Voltage averaging)

– Opto-Based voting system (Newly proposed in this paper)

R

R

R

Vo

V1

V3

V2

Vref

Vo=(V1+V2+V3)/ 3

P2

Light waves

Po+O->E

conversionVo

VrefPo=(P1+P2+P3)

Electrical signal based voting Opto-Based voting

Both types of voting are evaluated


Electrical signal based Voting System

Scheme 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

+

-

Ap

Bp

Cp

An

Bn

Cn

Z0

X

X

X

X

X

X

Op

On

ToSerial Receiver

(Differential Output)

Voter

FromSerial Transmitter(Differential Input)

Outline of voltage averaging system for differential signal



Experimental 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

DataGenerator

LVDSTransmitter

Ap

Bp

Cp

An

Bn

Cn

25

Op

On

Voter

25

LVDSReceiver

IN1

IN2

IN3TTL- Input

TTL-Output

LVDS transceiver spec.

Magnified view of voter moduleOverall Experiment system

Transmission characteristic design



Evaluation Result

• The signals are voted properly at 60MHz (120Mbps)

– Observe signal TTL-Input and TTL-Output by digital storage oscilloscope and logic analyzer

Signal of TTL-Input (IN1,IN2,IN3) and

TTL-Output (5V/Div, 50ns/Div, 2GSample)

Signal “IN2” simulates error

Logical signal of TTL-Input (IN1,IN2,IN3) and TTL-Output

Signal “IN2” simulates error

IN1

IN2

IN3

OUT



Evaluation of transmission characteristic at high frequency

• Transmission characteristic is evaluated by the evaluation board below.

• Target frequency 2GHz( = 15 [cm] )• Voting region is designed as concentrated const

ant circuit

(6.4[mm] x 6.4[mm] < /20)• Microstrip Line is designed

• Zdiff=100.7 [, Z1,Z2=58.87 [

• Glass epoxy board– Dielectric Constant = 4.7– Hight h = 1.6 [mm]– Dielectric dissipation factor tan = 0.0175

To NetworkAnalyzer

Microstrip LineZdiff=100Ω

Voter

50ΩTermination

50ΩTermination

3dBCoupler

π / 2 phaseshift Output

π / 2 phaseshift cable

6.4mm

6.4m

m

27ΩChip register

Glass epoxyboard


Loss in3dB coupler

Loss inSMA cable

+Microstrip line+Connection

Loss inVoter

Loss inMicrostrip Line+SMA cable+ConnectionA B C D


Evaluation Result - Transmission Characteristic

ComponentLoss [dB]

Note

A : 3dB coupler -3.437 Measured value

B+D : Transmission path -0.956 -13.933 – (A + C)

C : Voter -9.54 Theoretical value

Total (A + B + C + D) -13.933

Total loss is -13.933 [dB]

Detail of the loss

Transmission Characteristic (S21) of evaluation board (Start:1GHz, Stop:3GHz,5dB/Div)

• Transmission Characteristic of the voting system



Evaluation 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


Opto-Based Voting System

Scheme 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

Time

1 0 1 1

Time

1 0 1 1

Time

ModuleA

1011

Modulated Lights

・・・・・・

・・・・・・

・・・・・・

PhotoDetector Time

1 0 1 1 ・・・・・・Module

B

Module Cw/ Error

Electrical Output

Time

Threshold

Mixture of three light waves



Fundamental 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

LASER DiodePhotoDiode

I VConversion

+

-

2.2k10k+9V

Vout

3 LDs PD

Experimental System



Evaluation 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



Summary

• 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

Electrical Optical

Frequency Response

Poor Good

Circuit Design

Difficult Easy

System Complexity

Simple Complex

Performance Comparison of each method


Future work

Triple 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

CPUA

LASER PD

ModulatorOpticalIN

OpticalOUT

CPUB

CPU

ModulatorOpticalIN

OpticalOUT

CPUC

CPU

ModulatorOpticalIN

OpticalOUT

Serial Out

Serial Out

Serial Out

SerialMemory

Serial In

Serial In

Serial In

Optical connection

Electrical connection


Future work

Vector 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 modulati

on• These optical signals are mixed at the detector• Error module detection and majority voting can be achieved all at once by checking vectorial sum

mation of the mixed optical signals

InputVector

GeneratedVector

(1,0,1)

Input Value from “A”Input Value from “B”

Input Value from “C”(0,1,1)

(1,0,1)

(1,1,0)

(0,1,0)

(1,0,0)

(0,0,1)

(1,1,1)(0,0,0) A = 1

B = 1

C = 1

C = 0

A = 0

B = 0

r

Generated Vector (r

VotedOutput

Error module

(1, 0) 1 B

(1, /3) 1 C

(1, 2/3) 0 A

(1, ) 0 B

(1, 4/3) 0 C

(1, 5/3) 1 A

(2, 0) 1 -

(2, ) 0 -

Correspondence between vector and voting result

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Reliability Improving Approach with Opto-Based Voting System

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