实时嵌入式系统：调度与分析的综合指南

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"Real-Time Embedded Systems 1st edition 2017" 是一本专注于实时嵌入式系统设计与开发的书籍，属于Quantitative Software Engineering Series系列，由Lawrence Bernstein编辑。该系列书籍关注实时嵌入式系统的调度、资源访问控制、软件设计与开发以及高级系统建模、分析和验证的融合。在实时嵌入式系统中，关键的知识点包括： 1. **实时性**：实时系统是指其输出必须在特定时间限制内完成的系统。这通常涉及严格的时序要求，对于例如航空航天、医疗设备或工业自动化等领域的应用至关重要。 2. **嵌入式系统**：这些是集成到更大型设备或环境中的专用计算机系统，负责特定功能。它们可能包括硬件和软件，并且在设计时通常考虑功耗、体积和成本。 3. **调度**：实时系统中的任务调度涉及到确定任务执行的优先级和时间安排，以确保满足截止期限。这可能涉及到抢占式调度、非抢占式调度或其他策略。 4. **资源访问控制**：在实时嵌入式系统中，如何管理和分配有限的硬件资源，如处理器、内存和I/O端口，是确保系统性能和可靠性的关键。这可能涉及多任务环境下并发控制和同步机制。 5. **软件设计与开发**：实时系统软件的开发需要特别关注模块化、可维护性和可扩展性。这通常包括采用面向对象设计、设计模式和特定的编程范式。 6. **高级系统建模**：系统模型可以帮助分析和预测系统的性能，这可能涉及到状态机、Petri网或形式化建模语言（如UML）。 7. **分析与验证**：对实时系统进行定量分析以评估其性能、响应时间和可靠性。这可能涉及数学模型、仿真和代码静态分析。验证确保系统满足预定的规格和标准。 8. **工程权衡分析**：在设计实时嵌入式系统时，软件工程师和系统工程师需要考虑各种因素，如成本、性能、功耗和安全性，进行定量的工程权衡分析。 9. **案例研究**：书中通过实际案例帮助读者理解理论如何应用于实际项目，这是学习和掌握这些概念的重要途径。 10. **量化的工程方法**：强调使用量化方法来评估和优化软件工程过程，如性能测试、基准测试和容量规划。这本书籍适合软件开发者、软件工程师、系统工程师和研究生阅读，旨在通过将软件工程案例历史、定量分析和技术结合，提升他们开发有用且精心设计的软件系统的能力。

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Introduction to Real-Time Embedded Systems

Real-time embedded systems have become pervasive. ey are in your cars,

cell phones, Personal Digital Assistants (PDAs), watches, televisions, and

home electrical appliances. ere are also larger and more complex real-time

embedded systems, such as air-traﬃc control systems, industrial process

control systems, networked multimedia systems, and real-time database

applications. It is reported that in the Lexus LS-460 released in September

2006, there are more than 100 microprocessors embedded when all optional

features are installed. It is also estimated that 98% of all microprocessors are

manufactured as components of embedded systems. In fact, our daily life has

become more and more dependent on real-time embedded applications. is

chapter explains the concepts of embedded systems and real-time systems,

introduces the fundamental characteristics of real-time embedded systems,

and deﬁnes hard and soft real-time systems. e automotive antilock braking

system (ABS) is used as an example to show a real-world embedded system.

1.1 Real-Time Embedded Systems

An embedde d system is a microcomputer system embedded in a larger system

and designed for one or two dedicated services. It is embedded as part of

a complete device that often has hardware and mechanical parts. Examples

include the controllers built inside our home electrical appliances. Most

embedded systems have real-time computing constraints. erefore, they are

also called real-time embedded systems. Compared with general-purpose

computing systems that have multiple functionalities, embedded systems are

often dedicated to speciﬁc tasks. For example, the embedded airbag control

system is only responsible for detecting collision and inﬂating the airbag

when necessary, and the embedded controller in an air conditioner is only

responsible for monitoring and regulating the temperature of a room.

Real-Time Embedded Systems, First Edition. Jiacun Wang.

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2 1 Introduction to Real-Time Embedded Systems

Another noteworthy diﬀerence between a general-purpose computing

system and an embedded system is that a general-purpose system has full-scale

operating system support, while embedded systems may or may not have oper-

ating system support at all. Many small-sized embedded systems are designed

to perform simple tasks and thus do not need operating system support.

Embedded systems are reactive systems in nature. ey are basically designed

to regulate a physical variable in response to the input signal provided by the

end users or sensors, which are connected to the input ports. For example, the

goal of a grain-roasting embedded system is regulating the temperature of a

furnace by adjusting the amount of fuel being injected into the furnace. e

regulation or control is performed based on the diﬀerence between the desired

temperature and the real temperature detected by temperature sensors.

Embedded systems can be classiﬁed based on their complexity and perfor-

mance into small-scale, medium-scale, and large-scale. Small-scale systems

perform simple functions and are usually built around low-end 8- or 16-bit

microprocessors or microcontrollers. For developing embedded software for

small-scale embedded systems, the main programming tools are an editor,

assembler, cross-assembler, and integrated development environment (IDE).

Examples of small-scale embedded systems are mouse and TV remote control.

ey typically operate on battery. Normally, no operating system is found in

such systems.

Medium-scale systems have both hardware and software complexities. ey

use 16- or 32-bit microprocessors or microcontrollers. For developing embed-

ded software for medium-scale embedded systems, the main programming

tools are C, C++,JAVA,VisualC++, debugger, source-code engineering tool,

simulator, and IDE. ey typically have operating system support. Examples

of medium-scale embedded systems are vending machines and washing

machines.

Large-scale or sophisticated embedded systems have enormous hardware

and software complexities, which are built around 32- or 64-bit microproces-

sors or microcontrollers, along with a range of other high-speed integrated

circuits. ey are used for cutting-edge applications that need hardware and

software codesign techniques. Examples of large-scale embedded systems are

ﬂight-landing gear systems, car braking systems, and military applications.

Embedded systems can be non-real-time or real-time. For a non-real-time

system, we say that it is correctly designed and developed if it delivers the

desired functions upon receiving external stimuli or internal triggers, with a

satisﬁed degree of QoS (Quality of Service). Examples are TV remote controls

and calculators.

Real-time systems, however, are required to compute and deliver correct

results within a speciﬁed period of time. In other words, a job of a real-time

system has a deadline, being it hard or soft. If a hard deadline is missed, then

the result is useless, even if it is correct. Consider the airbag control system in

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1.2 Example: Automobile Antilock Braking System 3

automobiles. Airbags are generally designed to inﬂate in the cases of frontal

impacts in automobiles. Because vehicles change speed so quickly in a crash,

airbags must inﬂate rapidly to reduce the risk of the occupant hitting the

vehicle’s interior. Normally, from the onset of the crash, the entire deployment

and inﬂation process is about 0.04 seconds, while the limit is 0.1 seconds.

Non-real-time embedded systems may have time constraints as well.

Imagining if it takes more than 5 seconds for your TV remote control to send

a control signal to your TV and then the embedded device inside the TV takes

another 5 seconds to change the channel for you, you will certainly complain.

It is reasonable that consumers expect a TV to respond to remote control

event within 1 second. However, this kind of constraints is only a measure of

system performance.

Traditional application domains of real-time embedded systems include

automotive, avionics, industrial process control, digital signal processing,

multimedia, and real-time databases. However, with the continuing rapid

advance in information and communication technology and the emergence of

Internet of things and pervasive computing, real-time embedded applications

will be found in any objects that can be made smart.

1.2 Example: Automobile Antilock Braking System

A distinguished application area of real-time embedded systems is automo-

biles. Automotive embedded systems are designed and developed to control

engine, automatic transmission, steering, brake, suspension, exhaustion, and

so on. ey are also used in body electronics, such as instrument panel, key,

door, window, lighting, air bag, and seat bag. is section introduces the ABS.

An ABS is an automobile safety system. It is designed to prevent the wheels of

a vehicle from locking when brake pedal pressure is applied, which may occur

all of a sudden in case of emergency or short stopping distance. A sudden lock of

wheels will cause moving vehicles to lose tractive contact with the road surface

and skid uncontrollably. For this reason, ABS also stands for antiskid braking

system. e main beneﬁt from ABS operation is retaining the directional con-

trol of the vehicle during heavy braking in rare circumstances.

1.2.1 Slip Rate and Brake Force

When the brake pedal is depressed during driving, the wheel velocity (the

tangential speed of the tire surface) decreases, and so does the vehicle velocity.

e decrease in the vehicle velocity, however, is not always synchronized with

the wheel velocity. When the maximum friction between a tire and the road

surface is reached, further increase in brake pressure will not increase the

braking force,whichistheproductoftheweightofthevehicleandthefriction

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4 1 Introduction to Real-Time Embedded Systems

0 20406080100

Slip rate (%)

Braking force

Cornering force

Braking force

Cornering force

ABS ideal control region

Figure 1.1 Relationship among the slip rate,

braking force, and cornering force.

coeﬃcient. As a consequence, the vehicle velocity is greater than the wheel

speed, and the wheel starts to skid. e wheel slip rate, s, is deﬁned as

s =

V − 𝜔R

where V , 𝜔,andR denote the vehicle speed, wheel angular velocity, and wheel

rolling radius, respectively. Under normal driving conditions, V = 𝜔R and

thus, s = 0. During severe braking, it is common to have 𝜔 = 0andV > 0, which

translates to s = 1, a case of wheel lockup.

Figure 1.1 describes the relationship among the slip rate, braking force, and

cornering force. e continuous line in the ﬁgure represents the relationship

between the slip rate and braking force. It shows that the braking force is the

largest when the slip rate is between 10% and 20%. e braking distance is

the shortest at this rate. With further increase in slip rate, the braking force

is decreased, which results in a longer braking distance. e dashed line depicts

therelationshipbetweenthesliprateandcornering force. e cornering force

is generated by a vehicle tire during cornering. It works on the front wheels as

steering force and on the rear wheels to keep the vehicle stable. It decreases as

thesliprateincreases.Incaseofalockup,thecorneringforcebecomes0and

steering is disabled.

1.2.2 ABS Components

e ABS is composed of four components. ey are speed sensors, valves,

pumps, and an electrical control unit (ECU). Valves and pumps are often

housed in hydraulic control units (HCUs).

1.2.2.1 Sensors

ere are multiple types of sensors used in the ABS. A wheel speed sensor is

a sender device used for reading a vehicle’s wheel rotation rate. It is an elec-

tromagnet illustrated in Figure 1.2. As the sensor rotor rotates, it induces AC

voltage in the coil of the electromagnet. When the rotation of the sensor rotor

increases, the magnitude and frequency of induced voltage increase as well.

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1.2 Example: Automobile Antilock Braking System 5

Figure 1.2 Wheel speed sensor.

To ECU

Wheel speed sensor

Sensor rotor

Air gap

A deceleration sensor measures the vehicle’s rate of deceleration. It is a

switch type of sensor. It uses phototransistors that can be activated by light. In

a deceleration sensor, two Light-Emitting Diode (LEDs) aim at two phototran-

sistors that are separated by a slit plate. When the vehicle’s rate of deceleration

changes, the slit plate swings in the vehicle’s rear-to-front direction. e slits

in the slit plate act to expose the light from the LEDs to the phototransistors.

is movement of the slit plate switches the phototransistors ON and OFF.

e combinations formed by the two phototransistors switching ON and OFF

distinguish the rate of deceleration into four levels, which are sent as signals to

the ABS ECU.

A steering angle sensor (SAS) measures the steering wheel position angle

and rate of turn. e SAS is located in a sensor cluster in the steering column.

e cluster always has more than one steering position sensor for redundancy

and to conﬁrm data. e ECU module must receive two signals to conﬁrm the

steering wheel position. ese signals are often out of phase with each other.

e SAS tells the ABS control module where the driver is steering the vehicle

while the body motion sensors tell it how the body is responding.

A yaw-rate sensor isagyroscopicdevicethatmeasuresavehicle’sangular

velocity around its vertical axis. e angle between the vehicle’s heading and

vehicle actual movement direction is called slip angle, which is related to the

yaw rate.

A brakepressuresensorcaptures the dynamic pressure distribution between

a brake pad and the rotor surfaces during actual braking.

1.2.2.2 Valves and Pumps

Auto brakes typically work with hydraulic ﬂuid. e brake’s master cylinder

supplies ﬂuid pressure when the pedal is applied. In a standard ABS system,

the HCU houses electrically operated hydraulic control solenoid valves that

control the brake pressure to speciﬁc wheel brake circuits. A solenoid valve is

a plunger valve that is opened and closed electrically. When power is applied

to the solenoid, a magnetic coil is energized, which moves the plunger.

ere are multiple hydraulic circuits within the HCU, and each hydraulic

circuit controls a pair of solenoid valves: an isolation valve and a dump valve.

e valves have three operation modes: apply, hold,andrelease. In the apply

mode, both valves are open and allow the brake ﬂuid to freely ﬂow through the

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