Download Reliability Improving Approach with Opto

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
Yanagawa
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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.
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Our strategy for improving performance and reliability
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
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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
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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
FPGA
N bit
Txp
Serializer
Txn
N bit
Yanagawa
FPGA
/Other Devices
5
Rxp
Deserializer
Rxn
Rxp
N bit
Deserializer
Rxn
Txp
Serializer
N bit
Txn
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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
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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
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
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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)
Opto-Based voting
Electrical signal based voting
V1
V2
1
Vo
R
+
P2
R
Po
O->E
Vo
conversion
Vref
Vref
P
3
V3
Light waves
P
R
Vo=(V1+V2+V3)/3
Po=(P1+P2+P3)
Both types of voting are evaluated
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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
From
Serial Transmitter
(Differential Input)
An
Bn
Cn
X
X
X
X
Op
Z0
+
-
On
To
Serial Receiver
(Differential Output)
X
X
Voter
Outline of voltage averaging system for differential signal
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Electrical signal based Voting System
Experimental Setup
IN1
Ap
25
Bp
Cp
IN2
25
Op

An
LVDS
Receiver
Bn
Data
Generator
IN3
TTL-Input
Cn
LVDS
Transmitter
Voter
Overall Experiment system
LVDS transceiver spec.
•
•
TTLOutput
On
Magnified view of voter module
Transmission characteristic design
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
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Electrical signal based Voting System
Evaluation 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
Signal of TTL-Input (IN1,IN2,IN3) and
Logical signal of TTL-Input (IN1,IN2,IN3) and TTL-Output
TTL-Output (5V/Div, 50ns/Div, 2GSample)
Signal “IN2” simulates error
Signal “IN2” simulates error
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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 [], Z1,Z2=58.87 []
•
Glass epoxy board
–
Dielectric Constant e = 4.7
–
Hight h = 1.6 [mm]
–
Dielectric dissipation factor tan d = 0.0175
Microstrip Line
Zdiff=100Ω
π/2 phase
shift Output
50Ω
Termination
To Network
Analyzer
3dB
Coupler
Voter
π/2 phase
shift cable
Glass epoxy
board
6.4mm
50Ω
Termination
6.4mm
27Ω
Chip register
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Electrical signal based Voting System
Evaluation Result - Transmission Characteristic
•
Transmission Characteristic of the voting system
Loss in
3dB coupler
A
Loss in
SMA cable
+Microstrip line
+Connection
Loss in
Microstrip Line
+SMA cable
+Connection D
Loss in
Voter
B
C
Detail of the loss
Component
Loss [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
Transmission Characteristic (S21) of evaluation board
(Start:1GHz, Stop:3GHz,5dB/Div)
Yanagawa
Total loss is -13.933 [dB]
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Electrical signal based 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
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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.
Mixture of three light waves
•
Proper threshold is set at the photo detector to determine logical output
Threshold
・・・
1
0
1
1
・・・
Time
Module
A
Time
・・・
1
0
1
1
・・・
・・・ 1
0
1
1 ・・・
Module
B
Time
・・・
1
1
0
1
・・・
Photo
Detector
Time
Electrical Output
Module C
w/ Error
Time
Modulated Lights
Majority Vote by Amplitude Modulated Optical Signals
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Opto-Based Voting System
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
LAS
ER
D
3 LDs
iode
LASER Diode
R Dio
LASE
de
Photo
Diode
+9V
IV
Conversion
10k
2.2k
+
Vout
Experimental System
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PD
Opto-Based Voting System
Evaluation Result
•
Measure the output voltage of
photo detector with turning each
LD ON/OFF
Output Voltage of Photo Detector
•
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
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Our strategy for improving performance and reliability
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
–
Performance Comparison of each method
Electromagnetic Compatibility
(EMC)
The evaluation result shows that
optical voting system is feasible for
majority voting
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Electrical
Optical
Frequency
Response
Poor
Good
Circuit Design
Difficult
Easy
System
Complexity
Simple
Complex
P155/MAPLD 2004
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
CPU
A
Serial In
Serial Out
Optical
IN
CPU
B
LASER
Serial Out
Modulator
Serial
Memory
Optical
OUT
PD
Serial In
Serial Out
Optical
IN
Yanagawa
Optical
OUT
Serial In
Optical
IN
CPU
C
Modulator
Modulator
Optical connection
Optical
OUT
19
Electrical connection
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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
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
B=0
(1,0,0) (1,1,0)

(0,1,0)
(0,0,0)
B=1
C=0
(1,0,1)
r
(0,0,1)
A=0
(0,1,1)
C=1
A=1
(1,1,1)
Input Value from “A”
Input Value from “B”
Input Value from “C”
(1,0,1) Generated
Vector
Input
Vector
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Generated
Vector (r,)
Voted
Output
Error
module
(1, 0)
1
B
(1, p/3)
1
C
(1, 2p/3)
0
A
(1, p)
0
B
(1, 4p/3)
0
C
(1, 5p/3)
1
A
(2, 0)
1
-
(2, p)
0
-
P155/MAPLD 2004