Embedded Systems Main Report

8/6/2019 Embedded Systems Main Report

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Contents

1]: Introduction …… 03

2]: History …… 06

3]: Examples of Embedded Systems …… 10

4]: Characteristics …… 14

4.1]: User Interfaces

4.2]: Simple Systems4.3]: In more complex systems

4.4]: CPU platforms

4.4.1]: Ready made computer

boards

4.4.2]: ASIC and FPGA solutions

4.5]: Peripherals

4.6]: Tools

4.7]: Debugging4.8]: Reliability

4.9]: High vs. Low Volume

5]: Computer Design Requirements …… 19

5.1]: Real time/reactive operation

5.2]: Small size, low weight

5.3]: Safe and reliable5.4]: Harsh environment

5.5]: Cost sensitivity

6]: System-level requirements …… 22

6.1]: End-product utility

6.2]: System safety & reliability

6.3]: Controlling physical systems6.4]: Power management

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7]: Life-cycle support …… 25

7.1]: Component acquisition

7.2]: System certification

7.3]: Logistics and repair

7.4]: Upgrades

7.5]: Long-term component availability

8]: Business Model …… 29

8.1]: Design vs. production costs

8.2]: Cycle time

8.3]: Product families

9]: Design culture …… 31

9.1]: Computer culture vs. other cultures

9.2]: Accounting for cost of engineering

9.3]: Inertia

10]: Application …… 34

11]: Conclusions …… 36

12]: Reference …… 37

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1. Introduction

We live in a world today in which software plays a critical part. The most critical software is not running on large systems and PCs. rather, it runs inside the infrastructureand in the devices that we use every day. Our transportation, communications, andenergy systems won't work if the embedded software contained in our cars, phones,routers and power plants crashes.

Evolution of a software system is a natural process. In most systems, evolution takesplace during the maintenance phase of their life cycles. Those systems that havereached their limit in evolution have usually reached their end of useful life and mayhave to be replaced. However, there are systems in which evolution occurs during theoperational phase of their life cycles. Such systems are designed to evolve while inuse or, in other words, be adaptable. Semantically adaptable systems are of particularinterest to industry as such systems often times adapt themselves to environment

change with little or no intervention from their developing or maintainingorganization. Since embedded systems usually have a restricted hardwareconfiguration, it is difficult to apply the techniques developed for non-embeddedsystems directly to embedded systems. This paper focuses on evolution throughadaptation and develops the concepts and techniques for semantic evolution inembedded systems. As the first step in the development of a software solution,architectures of software systems themselves have to be made semanticallyevolvable. In this paper we explore various architectural alternatives for the semanticevolution of embedded systems-- these architectures are based on four differenttechniques that we have identified for semantic evolution in embedded systems. Thedevelopment of these architectures follows the systematic, process provided by the

non-functional requirement (NFR) framework, which also permits the architectures tobe rated in terms of their evolvability. As the field of embedded systems is vast, thispaper concentrates on those embedded systems that can be remotely controlled. Inthis application domain the embedded system is connected to an external controllerby a communication link such as Ethernet, serial, radio frequency, etc., and receivescommands from and sends responses to the external controller via thecommunication link. The architectures developed in this paper have been partlyvalidated by applying them in a real embedded system--a test instrument used fortesting cell phones. These architectures and techniques for semantic evolution in thisapplication domain give a glimpse of what can be done in achieving semantic

evolution in software-implemented systems.

Approximately 3 billion embedded CPUs are sold each year, with smaller (4-, 8-, and16-bit) CPUs dominating by quantity and aggregate dollar amount [1]. Yet, mostresearch and tool development seems to be focused on the needs of high-end desktopand military/aerospace embedded computing. This paper seeks to expand the area of discussion to encompass a wide range of embedded systems.

The extreme diversity of embedded applications makes generalizations difficult.

Nonetheless, there is emerging interest in the entire range of embedded systems andthe related field of hardware/software codesign.

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This paper and the accompanying tutorial seek to identify significant areas in whichembedded computer design differs from more traditional desktop computer design.

They also present "design challenges" encountered in the course of designing severalreal systems. These challenges are both opportunities to improve methodology andtool support as well as impediments to deploying such support to embedded systemdesign teams. In some cases research and development has already begun in theseareas -- and in other cases it has not.

The observations in this paper come from the author's experience with commercial aswell as military applications, development methodologies, and life-cycle support. Allcharacterizations are implicitly qualified to indicate a typical, representative, orperhaps simply an anecdotal case rather than a definitive statement about allembedded systems. While it is understood that each embedded system has its ownset of unique requirements, it is hoped that the generalizations and examplespresented here will provide a broad-brush basis for discussion and evolution of CADtools and design methodologies.

An embedded system is a special-purpose computer system designed to performone or a few dedicated functions, often with real-time computing constraints. It isusually embedded as part of a complete device including hardware and mechanicalparts. In contrast, a general-purpose computer, such as a personal computer, can domany different tasks depending on programming. Embedded systems have becomevery important today as they control many of the common devices we use.

Since the embedded system is dedicated to specific tasks, design engineers canoptimize it, reducing the size and cost of the product, or increasing the reliability andperformance. Some embedded systems are mass-produced, benefiting from

economies of scale.

Physically, embedded systems range from portable devices such as digital watchesand MP3 players, to large stationary installations like traffic lights, factory controllers,or the systems controlling nuclear power plants. Complexity varies from low, with asingle microcontroller chip, to very high with multiple units, peripherals and networksmounted inside a large chassis or enclosure.

The design of this invisible, embedded software is crucial to all of us. Yet, there hasbeen a real shortage of good information as to effective design and implementation

practices specific to this very different world. Make no mistake, it is indeed differentand often more difficult to design embedded software than more traditional programs. Time, and the interaction of multiple tasks in real-time, must be managed.

In general, "embedded system" is not an exactly defined term, as many systems havesome element of programmability. For example, Handheld computers share someelements with embedded systems — such as the operating systems andmicroprocessors which power them — but are not truly embedded systems, becausethey allow different applications to be loaded and peripherals to be connected.

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2. HistoryIn the earliest years of computers in the 1930-40s, computers were sometimesdedicated to a single task, but were far too large and expensive for most kinds of

tasks performed by embedded computers of today. Over time however, the conceptof [[programmable controllers]] evolved from traditional [[electromechanical]]sequencers, via solid state devices, to the use of computer technology.One of the first recognizably modern embedded system was the [[Apollo GuidanceComputer]], developed by [[Charles Stark Draper]] at the MIT InstrumentationLaboratory. At the project's inception, the Apollo guidance computer was consideredthe riskiest item in the Apollo project as it employed the then newly developedmonolithic integrated circuits to reduce the size and weight. An early mass-producedembedded system was the [[Autonetics D-17]] guidance computer for the[[Minuteman (missile) |Minuteman missile]], released in 1961. It was built from[[transistor]] [[digital circuit logic]] and had a [[hard disk]] for main memory. When

the Minuteman II went into production in 1966, the D-17 was replaced with a newcomputer that was the first high-volume use of integrated circuits. This programalone reduced prices on quad [[Sheffer stroke#NAND gate|nand gate ICs]] from$1000/each to $3/each, permitting their use in commercial products.

Since these early applications in the 1960s, embedded systems have come down inprice and there has been a dramatic rise in processing power and functionality. Thefirst [[microprocessor]] for example, the [[Intel 4004]] was designed for [[calculator]]s and other small systems but still required many external memory and support chips.In 1978 National Engineering Manufacturers Association released a "standard" for

programmable microcontrollers, including almost any computer-based controllers,such as single board computers, numerical, and event-based controllers.

As the cost of microprocessors and microcontrollers fell it became feasible to replaceexpensive knob-based [[analog electronics analog]] components such as[[potentiometer]]s and [[variable capacitor]]s with up/down buttons or knobs read outby a microprocessor even in some consumer products. By the mid-1980s, most of thecommon previously external system components had been integrated into the samechip as the processor and this modern form of the [[microcontroller]] allowed an evenmore widespread use, which by the end of the decade was the norm rather than theexception for almost all electronics devices. Embedded systems are the applications

that fuel some of the microprocessors that play a hidden but crucial role in oureveryday lives. These are the tiny, quick, and smart microprocessors that live insideprinters, answering machines, elevators, cars, cash machines, refrigerators,thermostats, wristwatches, and even toasters. Embedded systems are on the cutting5 EMBEDDED SYSTEMS



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edge of consumer electronics, poised to revolutionize various technologies by makingthem "smarter." A branch of the embedded-systems industry wants to see some of this newly smart equipment hooked up to the Internet, so that networking capabilitiesbecome a ubiquitous feature of modern machines.

The history of embedded systems goes back at least to the sixties, but the expense

and limitations of the early systems limited their use. Embedded systems really took

off in 1992, when the PC/104 Consortium was founded by Ampro, RTD, and other

manufacturers. The group established a format for Intel microprocessors based on a

motherboard approximately four inches square, and just under an inch high. The

boards were stackable, allowing a very powerful computer to be assembled in a box

approximately four inches square, or even less.

The PC/104 was initially targeted at military and medical markets, where it became

widely used and respected. When the processor power increased enough to handle

multimedia applications, PC/104-based kiosks became possible, and eventually

common.

Today, there are estimated to be well over 100 different companies making PC/104

products. There are PC/104 cards to add Ethernet, FireWire, hard drives, RAM drives,

video cards, audio cards, general I/O, flash cards, modems, GPS, cellular telephone,

wireless Internet, and more, to the PC/104 motherboard of your choice. Some off-the-

shelf PC/104 cases can handle up to 13 or more cards, so your budget is your only

constraint.

Kiosk development software is also progressing. Amulet Technologies

(www.AmuletTechnologies.com) demonstrated a system that will allow LCD touch

screens to be programmed in HTML. Since it usually takes C++ programming and

several months to program a touch screen interface, the Amulet Technologies system

is a major breakthrough. In addition to saving time and money, the technology leaves

the interface design to an HTML layout artist, not an engineer. (We managed to

program a simple interface with it in less than fifteen minutes from the time we

opened the box.) It would be easy and economical to use this technology to make an

interface for a hot tub, or a table-top ordering system for customers at restaurants.

Interestingly, many of the exhibitors at the 2002 Embedded Systems Conference were

marketing systems much smaller than the PC/104 format. In fact, some of the

exhibitors were just selling chips, primarily to add Internet connectivity to products

ranging from cars to coffee makers.

Microchip design has advanced so far over the last few years that it's now possible to

build a complete computer system on a chip, including wireless Internet connectivity.

These chips can easily be added to thousands of products, and soon will be.

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It is unavoidable that computer will continue to become cheaper, smaller and more

powerful, and that eventually they will be inexpensive enough to put in nearly every

product, including soda straws and matchbooks. In addition, nearly all computer

equipped products will have some kind of access to either local networks, or the

Internet.

Imagine a bathroom shower that will maintain exactly whatever water temperature

you tell it to maintain. Aside from meaning no child would ever be accidentally

scalded in a bathtub again, this would also mean more efficient and comfortable

showers. A computer equipped bathtub would also be smart enough to turn it self off

before it overflowed, and send you e-mail to let you know when your bath was ready.

Over the next decade, many common household items will be given embedded

systems, reinventing them, and changing forever how we interface with them.

Like desktop publishing, and later the Internet, embedded systems is a technology

that will fundamentally, and permanently, change the way advertising and marketing

works. It will also permanently change the kind of products that are made, and how

they are made.

It's about time, too. For all the benefits industrialization has brought, it has also

brought a host of problems as well. The problems go beyond VCR's and microwaves

that take an engineering degree to program.

For several hundred years, man has increasingly adjusted life to fit the needs of

industrialization and the corporate structure that has grown to support it. In the

process, we've become slaves of the machines built to sustain us, and some would

argue, slaves to the system itself.

The development of intelligent products, and intelligent product marketing, made

possible by embedded systems, will at least offer the possibility of a world where

machines exist for the convenience of people, and not the reverse. Given the tools we

have now, this kind of dream is no longer impossible.

The rise of embedded systems marks a new phase in industrialization. We've

mechanized civilization. Now we have to civilize mechanization.

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3. Examples of Embedded SystemsFigure 1 shows one possible organization for an embedded system.

Figure 1. An embedded system encompasses the CPU as well as many otherresources.

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In addition to the CPU and memory hierarchy, there are a variety of interfaces thatenable the system to measure, manipulate, and otherwise interact with the externalenvironment. Some differences with desktop computing may be:

• The human interface may be as simple as a flashing light or as complicated asreal-time robotic vision.

• The diagnostic port may be used for diagnosing the system that is beingcontrolled -- not just for diagnosing the computer.

• Special-purpose field programmable (FPGA), application specific (ASIC), or evennon-digital hardware may be used to increase performance or safety.

• Software often has a fixed function, and is specific to the application.

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Table 1. Four example embedded systems with approximate attributes.

In order to make the discussion more concrete, we shall discuss four examplesystems (Table 1). Each example portrays a real system in current production,but has been slightly generalized to represent a broader cross-section of

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applications as well as protect proprietary interests. The four examples are aSignal Processing system, a Mission Critical control system, a Distributed controlsystem, and a Small consumer electronic system. The Signal Processing andMission Critical systems are representative of traditional military/aerospaceembedded systems, but in fact are becoming more applicable to generalcommercial applications over time.

Using these four examples to illustrate points, the following sections describethe different areas of concern for embedded system design: computer design,system-level design, life-cycle support, business model support, and designculture adaptation.

Desktop computing design methodology and tool support is to a large degreeconcerned with initial design of the digital system itself. To be sure, experienceddesigners are cognizant of other aspects, but with the recent emphasis onquantitative design life-cycle issues that aren't readily quantified could be left

out of the optimization process. However, such an approach is insufficient tocreate embedded systems that can effectively compete in the marketplace. Thisis because in many cases the issue is not whether design of an immenselycomplex system is feasible, but rather whether a relatively modest system canbe highly optimized for life-cycle cost and effectiveness.

While traditional digital design CAD tools can make a computer designer moreefficient, they may not deal with the central issue -- embedded design is aboutthe system, not about the computer. In desktop computing, design oftenfocuses on building the fastest CPU, then supporting it as required for maximumcomputing speed. In embedded systems the combination of the external

interfaces (sensors, actuators) and the control or sequencing algorithms is orprimary importance. The CPU simply exists as a way to implement thosefunctions. The following experiment should serve to illustrate this point: ask aroomful of people what kind of CPU is in the personal computer or workstationthey use. Then ask the same people which CPU is used for the engine controllerin their car (and whether the CPU type influenced the purchasing decision).

In high-end embedded systems, the tools used for desktop computer design areinvaluable. However, many embedded systems both large and small must meetadditional requirements that are beyond the scope of what is typically handled

by design automation. These additional needs fall into the categories of specialcomputer design requirements, system-level requirements, life-cycle supportissues, business model compatibility, and design culture issues.

Embedded systems span all aspects of modern life and there are many examples of their use.

Telecommunications systems employ numerous embedded systems from telephoneswitches for the network to mobile phones at the end-user. Computer networking usesdedicated routers and network bridges to route data.

Consumer electronics include personal digital assistants (PDAs), mp3 players, mobilephones, videogame consoles, digital cameras, DVD players, GPS receivers, andprinters. Many household appliances, such as microwave ovens, washing machinesand dishwashers, are including embedded systems to provide flexibility, efficiency11 EMBEDDED SYSTEMS



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and features. Advanced HVAC systems use networked thermostats to more accuratelyand efficiently control temperature that can change by time of day and season. Homeautomation uses wired- and wireless-networking that can be used to control lights,climate, security, audio/visual, etc., all of which use embedded devices for sensingand controlling.

Transportation systems from flight to automobiles increasingly use embeddedsystems. New airplanes contain advanced avionics such as inertial guidance systemsand GPS receivers that also have considerable safety requirements. Various electricmotors — brushless DC motors, induction motors and DC motors — are usingelectric/electronic motor controllers. Automobiles, electric vehicles. and hybridvehicles are increasingly using embedded systems to maximize efficiency and reducepollution. Other automotive safety systems such as anti-lock braking system (ABS),Electronic Stability Control (ESC/ESP), and automatic four-wheel drive.

Medical equipment is continuing to advance with more embedded systems for vital

signs monitoring, electronic stethoscopes for amplifying sounds, and various medicalimaging (PET, SPECT, CT, MRI) for non-invasive internal inspections.

4. Characteristics

Embedded systems are designed to do some specific task, rather than be a general-purpose computer for multiple tasks. Some also have [[Real-time computing real-time]] performance constraints that must be met, for reason such as safety andusability; others may have low or no performance requirements, allowing the systemhardware to be simplified to reduce costs.# Embedded systems are not always separate devices. Most often they are physicallybuilt-in to the devices they control. {{Fact date=September 2007}}.# The software written for embedded systems is often called [[firmware]], and isstored in read-only memory or [[Flash memory]] chips rather than a disk drive. It often

runs with limited computer hardware resources: small or no keyboard, screen, andlittle memory.

4.1 User Interfaces:

Embedded systems range from no user interface at all — dedicated only to onetask — to full user interfaces similar to desktop operating systems in devicessuch as PDAs.

4.2 Simple Systems:

Simple embedded devices use buttons, [[LED]] s, and small character- or digit-onlydisplays, often with a simple [[Menu (computing) |menu system]].

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4.3 In More Complex Systems:

A full graphical screen, with [[touch screen touch]] sensing or screen-edge buttons

provides flexibility while minimizing space used: the meaning of the buttons canchange with the screen, and selection involves the natural behavior of pointing atwhat's desired.

Handheld systems often have a screen with a "joystick button" for a pointing device.

The rise of the [[World Wide Web]] has given embedded designers another quitedifferent option: providing a web page interface over a network connection. Thisavoids the cost of a sophisticated display, yet provides complex input and displaycapabilities when needed, on another computer. This is successful for remote,permanently installed equipment such as Pan-Tilt-Zoom cameras and network routers.

4.4 CPU Platforms:

Embedded processors can be broken into two broad categories: ordinarymicroprocessors (μP) and microcontrollers (μC), which have many more peripherals onchip, reducing cost and size. Contrasting to the personal computer and servermarkets, a fairly large number of basic [[CPU architecture]]s are used; there are [[VonNeumann architecture|Von Neumann]] as well as various degrees of [[Harvardarchitecture]]s, [[RISC]] as well as non-RISC and [[VLIW]]; word lengths vary from 4-bitto 64-bits and beyond (mainly in [[Digital signal processor|DSP]] processors) although

the most typical remain 8/16-bit. Most architectures come in a large number of different variants and shapes, many of which are also manufactured by severaldifferent companies.

A long but still not exhaustive list of common architectures are: [[65816]], [[65C02]],[[68HC08]], [[68HC11]], [[68k]], [[8051]], [[ARM architecture ARM]], [[Atmel AVR|AVR]], [[Blackfin]], [[C167 family|C167]], [[Coldfire]], [[COP8]], [[Zilog Z8|eZ8]],[[eZ80]], [[FR-V]], [[Renesas H8|H8]], [[HT48FXX Flash I/O type series|HT48]],[[M16C]], [[M32C]], [[MIPS architecture|MIPS]], [[MSP430]], [[PIC microcontroller|PIC]],[[PowerPC]], [[R8C]], [[Super Harvard Architecture Single-Chip Computer|SHARC]],[[ST6]], [[SuperH]], [[TLCS-47]], [[TLCS-870]], [[TLCS-900]], [[Tricore]], [[V850]], [[x86architecture|x86]], [[XE8000]], [[Z80]], etc.

4.4.1 Ready Made Computer Boards:

[[PC/104]] and PC/104+ are examples of available ''ready made'' computer boardsintended for small, low-volume embedded and ruggedized systems. These often use[[DOS]], [[Linux]], [[NetBSD]], or an embedded [[real-time operating system]] such as[[MicroC/OS-II]], [[QNX]] or [[VxWorks]].

4.4.2 ASIC and FPGA Solutions:A common configuration for very-high-volume embedded systems is the [[system on achip]] (SoC), an [[application-specific integrated circuit]] (ASIC), for which the CPUcore was purchased and added as part of the chip design. A related scheme is to use

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4.7 Debugging:

Embedded [[Debugging]] may be performed at different levels, depending on thefacilities available. From simplest to most sophisticated they can be roughly grouped

into the following areas:* Interactive resident debugging, using the simple shell provided by the embeddedoperating system (e.g. Forth and Basic)* External debugging using logging or serial port output to trace operation usingeither a monitor in flash or using a debug server like the [[Remedy Debugger]] whicheven works for heterogeneous [[multicore]] systems.* An in-circuit debugger (ICD), a hardware device that connects to the microprocessorvia a [[JTAG]] or NEXUS interface. This allows the operation of the microprocessor tobe controlled externally, but is typically restricted to specific debugging capabilities inthe processor.* An [[in-circuit emulator]] replaces the microprocessor with a simulated equivalent,providing full control over all aspects of the microprocessor.* A complete [[emulator]] provides a simulation of all aspects of the hardware,allowing all of it to be controlled and modified and allowing debugging on a normalPC.

Unless restricted to external debugging, the programmer can typically load and runsoftware through the tools, view the code running in the processor, and start or stopits operation. The view of the code may be as [[assembly code]] or [[source-code]].

4.8 Reliability:

Embedded systems often reside in machines that are expected to run continuously foryears without errors and in some cases recover by themselves if an error occurs.

Therefore the software is usually developed and tested more carefully than that forpersonal computers, and unreliable mechanical moving parts such as disk drives,switches or buttons are avoided.

Specific reliability issues may include:#The system cannot safely be shut down for repair, or it is too inaccessible to repair.Examples include space systems, undersea cables, navigational beacons, bore-hole

systems, and automobiles.#The system must be kept running for safety reasons. "Limp modes" are lesstolerable. Often backups are selected by an operator. Examples include aircraftnavigation, reactor control systems, safety-critical chemical factory controls, trainsignals, engines on single-engine aircraft.#The system will lose large amounts of money when shut down: Telephone switches,factory controls, bridge and elevator controls, funds transfer and market making,automated sales and service.

A variety of techniques are used, sometimes in combination, to recover from errors --both software bugs such as memory leaks, and also [[soft error]] s in the hardware:

* [[watchdog timer]] that resets the computer unless the software periodically notifiesthe watchdog

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* Subsystems with redundant spares that can be switched over to* Software "limp modes" that provide partial function* [[Immunity Aware Programming]]

4.9 High vs. Low Volume:

For high volume systems such as [[Digital audio player portable music players]] or[[mobile phone]]s, minimizing cost is usually the primary design consideration.Engineers typically select hardware that is just “good enough” to implement thenecessary functions.

For low-volume or prototype embedded systems, general purpose computers may beadapted by limiting the programs or by replacing the operating system with a [[real-time operating system

5. Computer Design RequirementsEmbedded computers typically have tight constraints on both functionality andimplementation. In particular, they must guarantee real time operation reactive toexternal events, conform to size and weight limits, budget power and coolingconsumption, satisfy safety and reliability requirements, and meet tight cost targets.

5.1. Real time/reactive operation

Real time system operation means that the correctness of a computation depends, in

part, on the time at which it is delivered. In many cases the system design must takeinto account worst case performance. Predicting the worst case may be difficult oncomplicated architectures, leading to overly pessimistic estimates erring on the sideof caution. The Signal Processing and Mission Critical example systems have asignificant requirement for real time operation in order to meet external I/O andcontrol stability requirements.

Reactive computation means that the software executes in response to externalevents. These events may be periodic, in which case scheduling of events toguarantee performance may be possible. On the other hand, many events may be

aperiodic, in which case the maximum event arrival rate must be estimated in orderto accommodate worst case situations. Most embedded systems have a significantreactive component.

Design challenge:

• Worst case design analyses without undue pessimism in the face of hardwarewith statistical performance characteristics.

5.2. Small size, low weight

Many embedded computers are physically located within some larger artifact. Therefore, their form factor may be dictated by aesthetics, form factors existing inpre-electronic versions, or having to fit into interstices among mechanical

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components. In transportation and portable systems, weight may be critical for fueleconomy or human endurance. Among the examples, the Mission Critical system hasmuch more stringent size and weight requirements than the others because of its usein a flight vehicle, although all examples have restrictions of this type.

Design challenges:

• Non-rectangular, non-planar geometries.• Packaging and integration of digital, analog, and power circuits to reduce size.

5.3. Safe and reliable

Some systems have obvious risks associated with failure. In mission-criticalapplications such as aircraft flight control, severe personal injury or equipmentdamage could result from a failure of the embedded computer. Traditionally, suchsystems have employed multiply-redundant computers or distributed consensusprotocols in order to ensure continued operation after an equipment failure.

However, many embedded systems that could cause personal or property damagecannot tolerate the added cost of redundancy in hardware or processing capacityneeded for traditional fault tolerance techniques. This vulnerability is often resolved atthe system level as discussed later.

Design challenge:

• Low-cost reliability with minimal redundancy.

5.4. Harsh environment

Many embedded systems do not operate in a controlled environment. Excessive heatis often a problem, especially in applications involving combustion (e.g., manytransportation applications). Additional problems can be caused for embeddedcomputing by a need for protection from vibration, shock, lightning, power supplyfluctuations, water, corrosion, fire, and general physical abuse. For example, in theMission Critical example application the computer must function for a guaranteed, butbrief, period of time even under non-survivable fire conditions.

Design challenges:

• Accurate thermal modeling.• De-rating components differently for each design, depending on operating

environment.

5.5. Cost sensitivity

Even though embedded computers have stringent requirements, cost is almostalways an issue (even increasingly for military systems). Although designers of systems large and small may talk about the importance of cost with equal urgency,their sensitivity to cost changes can vary dramatically. A reason for this may be thatthe effect of computer costs on profitability is more a function of the proportion of

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cost changes compared to the total system cost, rather than compared to the digitalelectronics cost alone. For example, in the Signal Processing system cost sensitivitycan be estimated at approximately $1000 (i.e., a designer can make decisions at the$1000 level without undue management scrutiny). However, with in the Small systemdecisions increasing costs by even a few cents attract management attention due tothe huge multiplier of production quantity combined with the higher percentage of total system cost it represents. Embedded computers typically have tight constraintson both functionality and implementation. In particular, they must guarantee real timeoperation reactive to external events, conform to size and weight limits, budget powerand cooling consumption, satisfy safety and reliability requirements, and meet tightcost targets.

Design challenge:

• Variable "design margin" to permit tradeoff between product robustness andaggressive cost optimization.

6. System-level requirementsIn order to be competitive in the marketplace, embedded systems require that thedesigners take into account the entire system when making design decisions.

6.1. End-product utility

The utility of the end product is the goal when designing an embedded system, notthe capability of the embedded computer itself. Embedded products are typically soldon the basis of capabilities, features, and system cost rather than which CPU is usedin them or cost/performance of that CPU.

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One way of looking at an embedded system is that the mechanisms and theirassociated I/O are largely defined by the application. Then, software is used tocoordinate the mechanisms and define their functionality, often at the level of controlsystem equations or finite state machines. Finally, computer hardware is madeavailable as infrastructure to execute the software and interface it to the externalworld. While this may not be an exciting way for a hardware engineer to look atthings, it does emphasize that the total functionality delivered by the system is whatis paramount.

Design challenge:

• Software- and I/O-driven hardware synthesis (as opposed to hardware-drivensoftware compilation/synthesis).

6.2. System safety & reliability

An earlier section discussed the safety and reliability of the computing hardware itself.But, it is the safety and reliability of the total embedded system that really matters.

The Distributed system example is mission critical, but does not employ computerredundancy. Instead, mechanical safety backups are activated when the computersystem loses control in order to safely shut down system operation.

A bigger and more difficult issue at the system level is software safety and reliability.While software doesn't normally "break" in the sense of hardware, it may be socomplex that a set of unexpected circumstances can cause software failures leadingto unsafe situations. This is a difficult problem that will take many years to address,

and may not be properly appreciated by non-computer engineers and managersinvolved in system design decisions (discusses the role of computers in systemsafety).

Design challenges:

• Reliable software.• Cheap, available systems using unreliable components.• Electronic vs. non-electronic design tradeoffs.

6.3. Controlling physical systems The usual reason for embedding a computer is to interact with the environment, oftenby monitoring and controlling external machinery. In order to do this, analog inputsand outputs must be transformed to and from digital signal levels. Additionally,significant current loads may need to be switched in order to operate motors, lightfixtures, and other actuators. All these requirements can lead to a large computercircuit board dominated by non-digital components.

In some systems "smart" sensors and actuators (that contain their own analoginterfaces, power switches, and small CPUS) may be used to off-load interface

hardware from the central embedded computer. This brings the additional advantageof reducing the amount of system wiring and number of connector contacts byemploying an embedded network rather than a bundle of analog wires. However, thischange brings with it an additional computer design problem of partitioning the

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computations among distributed computers in the face of an inexpensive networkwith modest bandwidth capabilities.

Design challenge:

• Distributed system tradeoffs among analog, power, mechanical, network, anddigital hardware plus software.

6.4. Power management

A less pervasive system-level issue, but one that is still common, is a need for powermanagement to either minimize heat production or conserve battery power. While thepush to laptop computing

has produced "low-power" variants of popular CPUs, significantly lower power isneeded in order to run from inexpensive batteries for 30 days in some applications,and up to 5 years in others.

Design challenge:

• Ultra-low power design for long-term battery operation.

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7. Life-cycle support

Figure 2 shows one view of a product life-cycle.

Figure 2. An embedded system lifecycle.

First a need or opportunity to deploy new technology is identified. Then a productconcept is developed. This is followed by concurrent product and manufacturingprocess design, production, and deployment. But in many embedded systems, the

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designer must see past deployment and take into account support, maintenance,upgrades, and system retirement issues in order to actually create a profitable design.Some of the issues affecting this life-cycle profitability are discussed below.

7.1. Component acquisition

Because an embedded system may be more application-driven than a typicaltechnology-driven desktop computer design, there may be more leeway in componentselection. Thus, component acquisition costs can be taken into account whenoptimizing system life-cycle cost. For example, the cost of a component generallydecreases with quantity, so design decisions for multiple designs should becoordinated to share common components to the benefit of all.

Design challenge:

• Life-cycle, cross-design component cost models and optimization rather thansimple per-unit cost.

7.2. System certification

Embedded computers can affect the safety as well as the performance the system. Therefore, rigorous qualification procedures are necessary in some systems after any design change in order to assess and reduce the risk of malfunction or unanticipatedsystem failure. This additional cost can negate any savings that might have otherwisebeen realized by a design improvement in the embedded computer or its software.

This point in particular hinders use of new technology by resynthesizing hardware

components -- the redesigned components cannot be used without incurring the costof system recertification.

One strategy to minimize the cost of system recertification is to delay all designchanges until major system upgrades occur. As distributed embedded systems comeinto more widespread use, another likely strategy is to partition the system in such away as to minimize the number of subsystems that need to be recertified whenchanges occur. This is a partitioning problem affected by potential design changes,technology insertion strategies, and regulatory requirements.

Design challenge:

• Partitioning/synthesis to minimize recertification costs.

7.3. Logistics and repair

Whenever an embedded computer design is created or changed, it affects thedownstream maintenance of the product. A failure of the computer can cause the

entire system to be unusable until the computer is repaired. In many cases embeddedsystems must be repairable in a few minutes to a few hours, which implies that sparecomponents and maintenance personnel must be located close to the system. A fastrepair time may also imply that extensive diagnosis and data collection capabilities

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must be built into the system, which may be at odds with keeping production costslow.

Because of the long system lifetimes of many embedded systems, proliferation of design variations can cause significant logistics expenses. For example, if acomponent design is changed it can force changes in spare component inventory,maintenance test equipment, maintenance procedures, and maintenance training.Furthermore, each design change should be tested for compatibility with varioussystem configurations, and accommodated by the configuration managementdatabase.

Design challenge:

• Designs optimized to minimize spares inventory.• High-coverage diagnosis and self-test at system level, not just digital

component level.

7.4. Upgrades

Because of the long life of many embedded systems, upgrades to electroniccomponents and software may be used to update functionality and extend the life of the embedded system with respect to competing with replacement equipment. Whileit may often be the case that an electronics upgrade involves completely replacingcircuit boards, it is important to realize that the rest of the system will remainunchanged. Therefore, any special behaviors, interfaces, and undocumented featuresmust be taken into account when performing the upgrade. Also, upgrades may be

subject to recertification requirements.

Of special concern is software in an upgraded system. Legacy software may not beexecutable on upgraded replacement hardware, and may not be readily cross-compiled to the new target CPU. Even worse, timing behavior is likely to be differenton newer hardware, but may be both undocumented and critical to system operation.

Design challenge:

• Ensuring complete interface, timing, and functionality compatibility whenupgrading designs.

7.5. Long-term component availability

When embedded systems are more than a few years old, some electronic componentsmay no longer be available for production of new equipment or replacements. Thisproblem can be especially troublesome with obsolete processors and small-sizeddynamic memory chips.

When a product does reach a point at which spare components are no longereconomically available, the entire embedded computer must sometimes be

redesigned or upgraded. This redesign might need to take place even if the system isno longer in production, depending on the availability of a replacement system. Thisproblem is a significant concern on the Distributed example system.

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Design challenge:

• Cost-effectively update old designs to incorporate new components.

8. Business model

The business models under which embedded systems are developed can vary aswidely as the applications themselves. Costs, cycle time, and the role of productfamilies are all crucial business issues that affect design decisions.

8.1. Design vs. production costs

Design costs, also called Non-Recurring Engineering costs (NRE), are of majorimportance when few of a particular embedded system are being built. Conversely,production costs are important in high-volume production. Embedded systems varyfrom single units to millions of units, and so span the range of tradeoffs between

designs versus production costs.

At the low-volume end of the spectrum, CAD tools can help designers complete theirwork with a minimum of effort. However, at the high-volume end of the spectrum thedesigns may be simple enough and engineering cost such a small fraction of totalsystem cost that extensive hand-optimization is performed in order to reduceproduction costs.

CAD tools may be able to outperform an average engineer at all times, and a superiorengineer on very large designs (because of the limits of human capacity to deal withcomplexity and repetition). However, in small designs some embedded computerdesigners believe that a superior human engineer can outperform CAD tools. In theSmall system example a programmer squeezed software into a few hundred bytes of memory by hand when the compiler produced overly large output that needed more

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memory than was available. It can readily be debated whether CAD tools or humansare "better" designers, but CAD tools face skepticism in areas that requireextraordinary optimization for size, performance, or cost.

Design challenge:

• Intelligently trade off design time versus production cost.

8.2. Cycle time

The cycle time between identification of a product opportunity and productdeployment (also called Time to Market) can be quite long for embedded systems. Inmany cases the electronics are not the driving force; instead, product schedules aredriven by concerns such as tooling for mechanical components and manufacturingprocess design. Superficially, this would seem to imply that design time for theelectronics is not an overriding concern, but this is only partially true.

Because the computer system may have the most malleable design, it may absorbthe brunt of changes. For example, redesign of hardware was required on the MissionCritical example system when it was found that additional sensors and actuators wereneeded to meet system performance goals. On the Small example system, delays inmaking masked ROM changes in order to revise software dominate concerns aboutmodifications (and programmable memory is too expensive). So, although the initialdesign is often not in the critical path to product deployment, redesign of thecomputer system may need to be done quickly to resolve problems.

Design challenge:

• Rapid redesign to accommodate changing form factors, control algorithms, andfunctionality requirements.

8.3. Product families

In many cases embedded system designs are not unique, and there are a variety of systems of various prices and capabilities forming a product family. To the extent thatsystem designers can reuse components, they lower the total cost of all systems in

the product family.

However, there is a dynamic tension between overly general solutions that satisfy alarge number of niche requirements, and specifically optimized designs for each pointin a product family space. Also, there may be cases in which contradictoryrequirements between similar systems prevent the use of a single subsystem design.In the Mission Critical and Small examples different customers require differentinterfaces between the embedded system and their equipment. In the Distributedexample regulatory agencies impose different safety-critical behavior requirementsdepending on the geographic area in which the system is deployed.

Design challenge:

• Customize designs while minimizing component variant proliferation.

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9. Design cultureDesign is a social activity as well as a technical activity. The design of desktopcomputers and CPUs in particular, has matured in terms of becoming morequantitative in recent years. With this new maturity has come an emphasis onsimulation and CAD tools to provide engineering tradeoffs based on accurateperformance and cost predictions.

Computer designers venturing into the embedded arena must realize that theirculture (and the underlying tool infrastructure) is unlike what is commonly practiced

in some other engineering disciplines. But, because embedded system designrequires a confluence of engineering skills, successful computer designers and designmethodologies must find a harmonious compromise with the techniques andmethodologies of other disciplines as well as company management. Also, in manycases the engineers building embedded computer systems are not actually trained incomputer engineering (or, perhaps not even electrical engineering), and so are notattuned to the culture and methodologies of desktop computer design.

9.1. Computer culture vs. other cultures

A specific problem is that computer design tools have progressed to the point thatmany believe it is more cost-effective to do extensive simulation than build successiveprototypes. However, in the mechanical arena much existing practice strongly favorsprototyping with less exhaustive up-front analysis. Thus, it may be difficult toconvince project managers (who may be application area specialists rather thancomputer specialists) to spend limited capital budgets on CAD tools and defer thegratification of early prototype development in favor of simulation.

Design challenge:

• Make simulation-based computer design accessible to non-specialists.

9.2. Accounting for cost of engineering design

One area of common concern is the effectiveness of using engineers in any designdiscipline. But, some computer design CAD tools are very expensive, and in generalorganizations have difficulty trading off capital and tool costs against engineeringtime. This means that computer designers may be deprived of CAD tools that wouldreduce the total cost of designing a system.

Also, in high-volume applications engineering costs can be relatively small when

compared to production costs. Often, the number of engineers is fixed, and book-keptas a constant expense that is decoupled from the profitability of any particular systemdesign, as is the case in all four example systems. This can be referred to as the"Engineers Are Free" syndrome. But, while the cost of engineering time may have a

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small impact on product costs, the unavailability of enough engineers to do work onall the products being designed can have a significant opportunity cost (which is, ingeneral, unmeasured).

Design challenge:

• Improved productivity via using tools and methodologies may be betterreceived by managers if it is perceived to increase the number of products thatcan be designed, rather than merely the efficiency of engineers on any givenproduct design effort. This is a subtle but, in practice, important distinction.

9.3. Inertia

In general, the cost of change in an organization is high both in terms of money andorganizational disruption. The computer industry can be thought of as being forced tochange by inexorable exponential growth in hardware capabilities. However, theimpact of this growth seems to have been delayed in embedded system development.In part this is because of the long time that elapses between new technologyintroduction and wide-scale use in inexpensive systems. Thus, it may simply be thatcomplex designs will force updated CAD tools and design methodologies to beadopted for embedded systems in the near future.

On the other hand, the latest computer design technologies may not have beenadopted by many embedded system makers because they aren't necessary. Tooldevelopment that concentrates on the ability to handle millions of transistors maysimply not be relevant to designers of systems using 4- and 8-bit microprocessors that

constitute the bulk of the embedded CPU market. And, even if they are useful, theneed for them may not be compelling enough to justify the pain and up-front expenseof change so long as older techniques work.

That is not to say that new tools aren't needed, but rather that the force of culturalinertia will only permit adoption of low-cost tools with significant advantages to the problem at hand .

However, the impact of this growth seems to have been delayed in embedded system

development. In part this is because of the long time that elapses between newtechnology introduction and wide-scale use in inexpensive systems. Tool developmentthat concentrates on the ability to handle millions of transistors may simply not berelevant to designers of systems using 4- and 8-bit microprocessors that constitutethe bulk of the embedded CPU market. On the other hand, the latest computer designtechnologies may not have been adopted by many embedded system makersbecause they aren't necessary.

Design challenge:

• Find/create design tools and methodologies that provide unique, compellingadvantages for embedded design.

• Rapid redesign to accommodate changing form factors, control algorithms, andfunctionality requirements.

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Embedded systems already play an important role not only in consumer electronics

but also in many important and safety-critical systems in applications such asavionics, space, railway and transport, process control and medical systems. Thereare, for instance, already many embedded systems in cars with critical controlfunctions (e.g. ABS braking systems, airbags), and these will become much morewidely used in the automotive industry once they can be delivered at pricesacceptable to the automotive market.

Hybrid system techniques can provide significant contributions to the improvement of the design flow for embedded systems in the automotive industry, since they allow toclearly represent the complex combination of time and event-based behaviors as wellas the interactions between continuous and discrete phenomena. Hybrid formalismsand methodologies proved to be very effective in handling several critical issues of the design flow such as:

• formalization of system specifications;• representation of embedded controller inputs/outputs;• plant and environment modeling;• representation of multirate asynchronous control loops;• description of the control-flow and data-flow components of control algorithms;• validation and verification of control algorithms and their implementations;• description of the HW/SW implementation requirements.

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11. Conclusions

Many embedded systems have requirements that differ significantly both in detailsand in scope from desktop computers. In particular, the demands of the specificapplication and the interface with external equipment may dominate the systemdesign. Also, long life-cycles and in some cases extreme cost sensitivity require more

attention to optimization based on these goals rather than maximizing thecomputational throughput.

The business and cultural climates in many embedded system design situations aresuch that traditional simulation-based computer design techniques may not be viablein their current form. Such methodologies may not be cost-effective given constraintson categories of expenditures, may not be seen as worthwhile by non-computer-trained professionals, or may simply be solving the wrong problems.

Recent interest in hardware/software codesign is a step in the right direction, as itpermits tradeoffs between hardware and software that are critical for more cost-

effective embedded systems. However, to be successful future tools may well need toincrease scope even further to include life-cycle issues and business issues.

12. References

1]. www.wikipedia.org

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http://www.wikipedia.org/

http://www.wikipedia.org/



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2]. www.embeddedstar.org

3]. www.embeddedindia.com

4]. www.zdnet.com

5]. Real time concepts for embedded systems by Qing LI.

http://www.embeddedstar.org/

http://www.embeddedindia.com/

http://www.zdnet.com/

http://www.embeddedstar.org/

http://www.embeddedindia.com/

http://www.zdnet.com/

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