Embedded Systems

2001. 4. 4최병주

http://home.ewha.ac.kr/~bjchoi

( 숭실대학교 컴퓨터 학부 양승민 교수

realtime.ssu.ac.kr)

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Table of ContentsEmbedded Systems and Characteristics

Real-time-ness in Embedded Systems

Robust-ness in Embedded Systems

Various-ness in Embedded Systems

Some Research Issues

Epilogue

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정보가전

셋탑박스 셋탑박스 또는또는

게이트웨이게이트웨이

선풍기선풍기감시용감시용카메라카메라

냉장고냉장고

오디오오디오

전기밥솥전기밥솥

TVTV

비디오비디오

조명조명조명조명

인터넷인터넷

정보가전정보가전원격제어원격제어

전자상거래전자상거래

멀티미디어 방송멀티미디어 방송

정보가전정보가전이란이란 ??유무선의 정보통신망에 연결되어 유무선의 정보통신망에 연결되어

데이터 송수신이 가능한 데이터 송수신이 가능한 차세대 네트워크 가전제품차세대 네트워크 가전제품

정보가전정보가전이란이란 ??유무선의 정보통신망에 연결되어 유무선의 정보통신망에 연결되어

데이터 송수신이 가능한 데이터 송수신이 가능한 차세대 네트워크 가전제품차세대 네트워크 가전제품

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PDA 와 이동통신 단말기 로드맵

20002000 년년20002000 년년 20012001 년년20012001 년년 20032003 년년20032003 년년 20042004 년년20042004 년년20022002 년년20022002 년년

물류용 물류용 PDAPDA바코드 스캐너바코드 스캐너+ Bluetooth+ Bluetooth

물류용 물류용 PDAPDA바코드 스캐너바코드 스캐너+ Bluetooth+ Bluetooth

PDAPDA(Bluetooth(Bluetooth

내장내장 ))

PDAPDA(Bluetooth(Bluetooth

내장내장 ))

복합무선단말기복합무선단말기(PDA+IMT2000+GPS(PDA+IMT2000+GPS

++ 고속 온라인 고속 온라인 동영상카메라동영상카메라

++ 화상회의화상회의 ++ 무선 무선 VODVOD++ 전자화폐전자화폐 ++ 온라인 게임온라인 게임++ 홈네트웍홈네트웍 ++ 방송수신기방송수신기 ))

복합무선단말기복합무선단말기(PDA+IMT2000+GPS(PDA+IMT2000+GPS

++ 고속 온라인 고속 온라인 동영상카메라동영상카메라

++ 화상회의화상회의 ++ 무선 무선 VODVOD++ 전자화폐전자화폐 ++ 온라인 게임온라인 게임++ 홈네트웍홈네트웍 ++ 방송수신기방송수신기 ))

복합무선단말기복합무선단말기(PDA+IMT2000(PDA+IMT2000+GPS++GPS+ 카메라카메라

++ 화상회의화상회의++ 전자화폐전자화폐

++ 온라인게임온라인게임 ))

복합무선단말기복합무선단말기(PDA+IMT2000(PDA+IMT2000+GPS++GPS+ 카메라카메라

++ 화상회의화상회의++ 전자화폐전자화폐

++ 온라인게임온라인게임 ))

CDMACDMA 스마트폰스마트폰(OS: Linux,Java (OS: Linux,Java

Web Web 가능가능 ))

CDMACDMA 스마트폰스마트폰(OS: Linux,Java (OS: Linux,Java

Web Web 가능가능 ))

무선인터넷단말기무선인터넷단말기(CDMA+Bluetooth)(CDMA+Bluetooth)무선인터넷단말기무선인터넷단말기

(CDMA+Bluetooth)(CDMA+Bluetooth)

GPSGPS네비게이션네비게이션

GPSGPS네비게이션네비게이션

온라인게임온라인게임PDAPDA

온라인게임온라인게임PDAPDA

군사 지령군사 지령시스템 단말기시스템 단말기

군사 지령군사 지령시스템 단말기시스템 단말기

IMT2000IMT2000단말기단말기

IMT2000IMT2000단말기단말기

복합무선단말기복합무선단말기(PDA+IMT2000)(PDA+IMT2000)복합무선단말기복합무선단말기

(PDA+IMT2000)(PDA+IMT2000)

홈네트워크홈네트워크연동 연동 PDAPDA홈네트워크홈네트워크연동 연동 PDAPDA

- - ㈜에이치엔티 단말기 사업계획서 참고㈜에이치엔티 단말기 사업계획서 참고- - ㈜에이치엔티 단말기 사업계획서 참고㈜에이치엔티 단말기 사업계획서 참고

MP3MP3 폰폰MP3MP3 폰폰

카메라폰카메라폰카메라폰카메라폰

TVTV 폰폰TVTV 폰폰

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Vending Machine : An Example

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Flows in Embedded Systems

동전입력동전입력동전입력동전입력

품목선택품목선택품목선택품목선택

금액확인 및 금액확인 및 구매 가능 품목 결정구매 가능 품목 결정

금액확인 및 금액확인 및 구매 가능 품목 결정구매 가능 품목 결정

물품 재고 확인물품 재고 확인물품 재고 확인물품 재고 확인

거스름돈 계산거스름돈 계산거스름돈 계산거스름돈 계산

물품 수량 물품 수량 DBDB물품 수량 물품 수량 DBDB

물품출력물품출력물품출력물품출력

거스름돈 출력거스름돈 출력거스름돈 출력거스름돈 출력

물품 보충 요청물품 보충 요청물품 보충 요청물품 보충 요청

구매가능구매가능품목표시품목표시

구매가능구매가능품목표시품목표시

Sensors Analysis & Decision

Actuators

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Which One is Correct ?

1. Embedded System Real-Time System

2. Embedded System is Real-Time System

3. Real-Time System is Embedded System

4. Embedded System <> Real-Time System

5. None of the Above

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Embedded Real-Time Applications

Embedded system“… a standard programming language for embedded-system applications, in which one or more computers are part of a larger system such as an aircraft, ship, or a communications system.”

Real-time system“the correctness of the system depends not only on the logical result of the computations but also on time at which the results are produced”

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Embedded (Real-time) System

Characteristics and Related Issues Kernel Kernel

SupportSupport

Software Software EngineeringEngineering

System System ArchitecturArchitecturee

CharacteristicsCharacteristics• Real-time-ness (Real-time-ness ( 실시간성실시간성 ))

• Robust-ness (Robust-ness ( 견고성견고성 ))

• Various-ness (Various-ness ( 다양성다양성 , , 특수성특수성 ) )

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Real-time-ness in Embedded System

Hard Real-Time vs. Soft Real-TimeTime Specification and VerificationSchedulability

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Hard Real-Time vs. Soft Real-Time

Time Value Function“Reward value” of the system when the task terminates

Reward Value

HRT task

SRT task

Time

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Times in Real-Time Systems

Times associated with DevicesFrequency

Tardiness

Times associated with TasksInvocation time

Execution deadline

Execution period (or interval)

Sleep and wake-up time

Time associated with Messages and DataDelivery deadline (event message)

Validity time (state message)

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Specification and Verification of Times

In real-time systems, time are associated with devices (i.e., sensors and actuators), tasks (i.e., analysis and decision), data and messages (i.e., interactions between device and task, and between tasks)

Need mechanisms to specify(or represent) and verify times throughout the life cycle of real-time system development

• Requirement specification level, e.g., Real-Time Logic• Design description level, e.g., Timed Petri Nets• Programming language level, e.g., Ada

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Time Specification with RTL : An example

Producer-Consumer with a single bufferActions

• S: Sending device which puts a message into the buffer every T seconds

• P: Processing task which gets a message from the buffer every T seconds with C seconds processing time

i @(S, i ) = i * T i @(P, i ) i * T ^ @( P, i ) (i+1) * T i @(P, i ) @( P, i ) + C Where X : beginning of action X X : completion of action X @(E, i) : time of the i-th occurrence of event E

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Time Verification with RTL an ExampleSafety Assertion (SA)

i @(P, i) @(S, i+1) (P should consume a message before S overwrites it)

Verification (by contradiction) Step 1 : Remove quantifier from the specification @(S, i ) = i * T

@(P, i ) i * T @(P, i ) (i+1) * T @(P, i ) @(P, i ) + C ¬ SA : @(P, i ) > @(S, i+1)

Step 2 : @(S, i+1) = (i+1) * T

(i+1) * T @(P, i)

@(P, i ) @(P, i ) + C @(P, i ) > @(S, i+1) Step 3 : Consequently we can derive the following formula which is false. @(S, i+1) > @(S, i+1) + C

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Two Aspects of Verification

Check if the specified times reflect all the original requirements correctly

Completeness: All necessary timing constraints are specified

Consistency: All specified times are not conflict

Check if the specified timing requirements are feasible for implementation on a given environment (Validation)

Task allocation / Task scheduling

Execution time estimation

Real-time protocol (or real-time message scheduling)

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Schedulability AnalysisGiven properties of processes and target architecture

( including execution time, request interval and deadline )

Scheduling discipline chosen• Can tasks be scheduled ? (Can all tasks be completed within deadlines ?) OR • How can we maximize the reward value ? (How can we minimize the penalty ?)

Performance Criteria• Optimality• Complexity• Run-time overhead

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What Makes Schedulability Analysis Difficul

t ? Precedence among tasksResource sharingCommunication delay (distributed systems)Sporadic tasks (or aperiodic tasks)Task allocation (multiprocessor systems)

Optimal scheduling strategies are known mostly for a single processor system with independent tasks

Are estimated execution times of tasks correct ?

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Robust-ness in Embedded Systems

Definitions of Faults

Redundancy

Software Fault Tolerance

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Faults, Errors, Failures

Not all faults manifest (i.e., become error)Not all errors lead to failures or system malfunctioning

System FailureSystem FailureSystem FailureSystem Failure

FaultFaultFaultFault

ErrorErrorErrorError

A physical defect - a gateway always produces 0 or 1 - mistakes within software

Manifestation of faults - deviation from required operation

Incorrect performance of system(or System malfunctioning)

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Fault Avoidance vs. Fault Tolerance

Fault avoidanceBetter specification/verification

Better quality component• Hardware components

• Programmers

• Debugging, testing

PremiseIt is not possible to completely avoid faults

Fault tolerance is needed

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Software Errors (1)(by Robert L. Glass )

Thoughts

Not all software errors can be found by known error-

removal techniques

Not all software errors are found EVER

Not all software error-finders are equal

Not all software errors are equal

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Software Errors (2)Conclusions

There is not now a process answer to the problem of software errors

Software in critical systems must take precautions beyond error removal for defense against software errors

The most important element in software error removal is not the nature of the process, but the choice of people

Both error removal and fault tolerance must concentrate on the worst of the errors-the ones that can result in unsafe systems

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RedundancyFault tolerance requires redundancy

Hardware redundancy

Software redundancy

Information redundancy

Time redundancy

Forms of redundancy Multiple identical copies

Multiple versions

DiversitySpecification diversity

Design diversity

Implementation diversity

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Information Flow in Embedded Systems

Communication Communication SystemSystem

Communication Communication SystemSystem

HardwareHardwareHardwareHardware

SoftwareSoftwareSoftwareSoftware

SensorSensorSensorSensorInput

ActuatorActuatorActuatorActuator Output

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SystemSystemFailureFailureSystemSystemFailureFailure

Software Fault Tolerance

Software Fault Tolerance

SoftwareSoftwareErrorError

SoftwareSoftwareErrorError

WrongWrongInformationInformation

WrongWrongInformationInformation

CommunicationCommunicationFaultFault

CommunicationCommunicationFaultFault

SoftwareSoftwareFaultFault

SoftwareSoftwareFaultFault

HardwareHardwareFaultFault

HardwareHardwareFaultFault

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Software Fault ToleranceHistorical View

70’s : Concepts in redundancy and atomic actions; NVP, RB, Conversation scheme, PTC Scheme, etc : “Pseudo” parallel systems80’s : Theoretical study and empirical study ; Reliability modeling and performance modeling; Design and implementation approach; “limited” parallel system90’s : Clinical study, I.e., FT in “real” systems, esp. for real-time distributed/parallel systems; “True” parallel systems.Encouraging Factors

Computers will be used more and more for control of critical embedded real-time applications.Not all software errors are found EVER!“True” parallel systems are available.

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Software Fault Tolerance Technique

RedundancyMultiple identical copiesMultiple versions

Fault by ageing(or by hardware fault) Identical copy may work

Design/Implementation fault Identical copy does not work

N-version Programming, Recovery Block, and Conversation Scheme

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Design and Implementation Problem

Cost

Design diversityAny systematic approach ?

Error detectionApproval(e.g., Acceptance Test)

Consensus(e.g., Comparison)

Totally application dependant

Timing constraints and timeout, esp. in distributed systems

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Various-nessin Embedded Software

Various Applications

Various Requirements / Equipment / Services

20012001 년의 년의 어느날어느날

20012001 년의 년의 어느날어느날

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Embedded (Real-time) System

Characteristics and Related Issues Kernel Kernel

SupportSupport

Software Software EngineeringEngineering

System System ArchitecturArchitecturee

CharacteristicsCharacteristics• Real-time-ness (Real-time-ness ( 실시간성실시간성 ))

• Robust-ness (Robust-ness ( 견고성견고성 ))

• Various-ness (Various-ness ( 다양성다양성 , , 특수성특수성 ) )

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Some Research Issues

시간 요구 사항의 명세 및 표현 Good Languages실시간 보장 Time analysis and schedulingOverload 상황에 대한 처리 “ Best effort” scheduling시스템 감시 Monitoring and diagnosis결함 처리 Fault tolerance techniques and implementation다양성 처리 Polymorphic Kernel and componentEmbedded System 을 위한 개발 방법론 및 Testing 방법

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EpilogueComputer systems will be used more and more for “intelligent” embedded systems

From “computer-aided” to “computer-controlled” societyDependability is the Key Factor

• Temporal as well as logical correctness

• Robust

기반 기술 포기할 것인가 ?• 실시간 커널 개발 시급