Embedded system

Anembedded systemon a plug-in card with processor, memory, power supply, and external interfaces

Anembedded systemis acomputer system—a combination of acomputer processor,computer memory,andinput/outputperipheral devices—that has a dedicated function within a larger mechanical orelectronicsystem.It is embedded as part of a complete device often including electrical or electronic hardware and mechanical parts. Because an embedded system typically controls physical operations of the machine that it is embedded within, it often hasreal-time computingconstraints. Embedded systems control many devices in common use.In 2009,it was estimated that ninety-eight percent of all microprocessors manufactured were used in embedded systems.

Modern embedded systems are often based onmicrocontrollers(i.e. microprocessors with integrated memory and peripheral interfaces), but ordinary microprocessors (using external chips for memory and peripheral interface circuits) are also common, especially in more complex systems. In either case, the processor(s) used may be types ranging from general purpose to those specialized in a certain class of computations, or even custom designed for the application at hand. A common standard class of dedicated processors is thedigital signal processor(DSP).

Since the embedded system is dedicated to specific tasks,design engineerscan optimize it to reduce the size and cost of the product and increase its reliability and performance. Some embedded systems are mass-produced, benefiting fromeconomies of scale.

Embedded systems range in size from portable personal devices such asdigital watchesandMP3 playersto bigger machines likehome appliances,industrialassembly lines,robots,transport vehicles,traffic light controllers,andmedical imagingsystems. Often they constitute subsystems of other machines likeavionicsinaircraftandastrionicsinspacecraft.Large installations likefactories,pipelines,andelectrical gridsrely on multiple embedded systems networked together. Generalized through software customization, embedded systems such asprogrammable logic controllersfrequently comprise their functional units.

Embedded systems range from those low in complexity, with a single microcontroller chip, to very high with multiple units,peripheralsand networks, which may reside inequipment racksor across large geographical areas connected via long-distance communications lines.

History

Background

See also:Microprocessor chronology

The origins of the microprocessor and the microcontroller can be traced back to theMOS integrated circuit,which is anintegrated circuitchipfabricatedfromMOSFETs(metal–oxide–semiconductorfield-effect transistors) and was developed in the early 1960s. By 1964, MOS chips had reached highertransistor densityand lower manufacturing costs thanbipolarchips. MOS chips further increased in complexity at a rate predicted byMoore's law,leading tolarge-scale integration(LSI) with hundreds oftransistorson a single MOS chip by the late 1960s. The application of MOS LSI chips tocomputingwas the basis for the first microprocessors, as engineers began recognizing that a completecomputer processorsystem could be contained on several MOS LSI chips.

The first multi-chip microprocessors, theFour-Phase Systems AL1in 1969 and theGarrett AiResearchMP944in 1970, were developed with multiple MOS LSI chips. The first single-chip microprocessor was theIntel 4004,released in 1971. It was developed byFederico Faggin,using hissilicon-gateMOS technology, along withIntelengineersMarcian HoffandStan Mazor,andBusicomengineerMasatoshi Shima.

Development

One of the first recognizably modern embedded systems was theApollo Guidance Computer,developed ca. 1965 byCharles Stark Draperat theMIT Instrumentation Laboratory.At the project's inception, the Apollo guidance computer was considered the riskiest item in the Apollo project as it employed the then newly developedmonolithic integrated circuitsto reduce the computer's size and weight.

An early mass-produced embedded system was theAutonetics D-17 guidance computerfor theMinuteman missile,released in 1961. When the Minuteman II went into production in 1966, the D-17 was replaced with a new computer that represented the first high-volume use of integrated circuits.

Since these early applications in the 1960s, embedded systems have come down in price and there has been a dramatic rise in processing power and functionality. An early microprocessor, theIntel 4004(released in 1971), was designed forcalculatorsand other small systems but still required external memory and support chips. By the early 1980s, memory, input and output system components had been integrated into the same chip as the processor forming a microcontroller. Microcontrollers find applications where a general-purpose computer would be too costly. As the cost of microprocessors and microcontrollers fell, the prevalence of embedded systems increased.

A comparatively low-cost microcontroller may be programmed to fulfill the same role as a large number of separate components. With microcontrollers, it became feasible to replace, even in consumer products, expensive knob-basedanalogcomponents such aspotentiometersandvariable capacitorswith up/down buttons or knobs read out by a microprocessor. Although in this context an embedded system is usually more complex than a traditional solution, most of the complexity is contained within the microcontroller itself. Very few additional components may be needed and most of the design effort is in the software. Software prototype and test can be quicker compared with the design and construction of a new circuit not using an embedded processor.

Applications

Embedded Computer Sub-Assembly for Accupoll Electronic Voting Machine

Embedded systems are commonly found in consumer, industrial,automotive,home appliances,medical, telecommunication, commercial, aerospace and military applications.

Telecommunicationssystems employ numerous embedded systems fromtelephone switchesfor the network tocell phonesat theend user.Computer networking uses dedicatedroutersandnetwork bridgesto route data.

Consumer electronicsincludeMP3 players,television sets,mobile phones,video game consoles,digital cameras,GPSreceivers, andprinters.Household appliances, such asmicrowave ovens,washing machinesanddishwashers,include embedded systems to provide flexibility, efficiency and features. Advancedheating, ventilation, and air conditioning(HVAC) systems use networkedthermostatsto more accurately and efficiently control temperature that can change by time of day andseason.Home automationuses wired- and wireless-networking that can be used to control lights, climate, security, audio/visual, surveillance, etc., all of which use embedded devices for sensing and controlling.

Transportation systems from flight to automobiles increasingly use embedded systems. New airplanes contain advancedavionicssuch asinertial guidance systemsandGPSreceivers that also have considerable safety requirements.Spacecraftrely onastrionicssystems for trajectory correction. Various electric motors —brushless DC motors,induction motorsandDC motors— use electronicmotor controllers.Automobiles,electric vehicles,andhybrid vehiclesincreasingly use embedded systems to maximize efficiency and reduce pollution. Other automotive safety systems using embedded systems includeanti-lock braking system(ABS),electronic stability control(ESC/ESP),traction control(TCS) and automaticfour-wheel drive.

Medical equipmentuses embedded systems formonitoring,and variousmedical imaging(positron emission tomography(PET),single-photon emission computed tomography(SPECT),computed tomography(CT), andmagnetic resonance imaging(MRI) for non-invasive internal inspections. Embedded systems within medical equipment are often powered by industrial computers.

Embedded systems are used forsafety-critical systemsin aerospace and defense industries. Unless connected to wired or wireless networks via on-chip 3G cellular or other methods for IoT monitoring and control purposes, these systems can be isolated from hacking and thus be more secure.For fire safety, the systems can be designed to have a greater ability to handle higher temperatures and continue to operate. In dealing with security, the embedded systems can be self-sufficient and be able to deal with cut electrical and communication systems.

Miniature wireless devices calledmotesare networked wireless sensors.Wireless sensor networkingmakes use of miniaturization made possible by advancedintegrated circuit(IC) design to couple full wireless subsystems to sophisticated sensors, enabling people and companies to measure a myriad of things in the physical world and act on this information through monitoring and control systems. These motes are completely self-contained and will typically run off a battery source for years before the batteries need to be changed or charged.

Characteristics

Embedded systems are designed to perform a specific task, in contrast with general-purpose computers designed for multiple tasks. Some havereal-timeperformance constraints that must be met, for reasons such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs.

Embedded systems are not always standalone devices. Many embedded systems are a small part within a larger device that serves a more general purpose. For example, theGibson Robot Guitarfeatures an embedded system for tuning the strings, but the overall purpose of the Robot Guitar is to play music.Similarly, an embedded system in anautomobileprovides a specific function as a subsystem of the car itself.

e-con Systems eSOM270 & eSOM300 Computer on Modules

The program instructions written for embedded systems are referred to asfirmware,and are stored in read-only memory orflash memorychips. They run with limited computer hardware resources: little memory, small or non-existent keyboard or screen.

User interfaces

Embedded systemtext user interfaceusing MicroVGA

Embedded systems range fromno user interfaceat all, in systems dedicated to one task, to complexgraphical user interfacesthat resemble modern computer desktop operating systems. Simple embedded devices usebuttons,light-emitting diodes(LED), graphic or characterliquid-crystal displays(LCD) with a simplemenu system.More sophisticated devices that use a graphical screen withtouch sensingor screen-edgesoft keysprovide flexibility while minimizing space used: the meaning of the buttons can change with the screen, and selection involves the natural behavior of pointing at what is desired.

Some systems provide user interface remotely with the help of a serial (e.g.RS-232) or network (e.g.Ethernet) connection. This approach extends the capabilities of the embedded system, avoids the cost of a display, simplifies theboard support package(BSP) and allows designers to build a rich user interface on the PC. A good example of this is the combination of anembedded HTTP serverrunning on an embedded device (such as anIP cameraor anetwork router). The user interface is displayed in aweb browseron a PC connected to the device.

Processors in embedded systems

Examples of properties of typical embedded computers when compared with general-purpose counterparts, are low power consumption, small size, rugged operating ranges, and low per-unit cost. This comes at the expense of limited processing resources.

Numerous microcontrollershave been developed for embedded systems use. General-purpose microprocessors are also used in embedded systems, but generally, require more support circuitry than microcontrollers.

Ready-made computer boards

PC/104and PC/104+ are examples of standards for ready-made computer boards intended for small, low-volume embedded and ruggedized systems. These are mostly x86-based and often physically small compared to a standard PC, although still quite large compared to most simple (8/16-bit) embedded systems. They may useDOS,FreeBSD,Linux,NetBSD,OpenHarmonyor an embeddedreal-time operating system(RTOS) such asMicroC/OS-II,QNXorVxWorks.

In certain applications, where small size or power efficiency are not primary concerns, the components used may be compatible with those used in general-purpose x86 personal computers. Boards such as the VIAEPIArange help to bridge the gap by being PC-compatible but highly integrated, physically smaller or have other attributes making them attractive to embedded engineers. The advantage of this approach is that low-cost commodity components may be used along with the same software development tools used for general software development. Systems built in this way are still regarded as embedded since they are integrated into larger devices and fulfill a single role. Examples of devices that may adopt this approach areautomated teller machines(ATM) andarcade machines,which contain code specific to the application.

However, most ready-made embedded systems boards are not PC-centered and do not use theISAorPCIbusses. When asystem-on-a-chipprocessor is involved, there may be little benefit to having a standardized bus connecting discrete components, and the environment for both hardware and software tools may be very different.

One common design style uses a small system module, perhaps the size of a business card, holding high densityBGAchips such as anARM-basedsystem-on-a-chipprocessor and peripherals, externalflash memoryfor storage, andDRAMfor runtime memory. The module vendor will usually provide boot software and make sure there is a selection of operating systems, usually includingLinuxand some real-time choices. These modules can be manufactured in high volume, by organizations familiar with their specialized testing issues, and combined with much lower volume custom mainboards with application-specific external peripherals. Prominent examples of this approach includeArduinoandRaspberry Pi.

ASIC and FPGA SoC solutions

Asystem on a chip(SoC) contains a complete system - consisting of multiple processors, multipliers, caches, even different types of memory and commonly various peripherals like interfaces for wired or wireless communication on a single chip. Often graphics processing units (GPU) and DSPs are included such chips. SoCs can be implemented as anapplication-specific integrated circuit(ASIC) or using afield-programmable gate array(FPGA) which typically can be reconfigured.

ASIC implementations are common for very-high-volume embedded systems likemobile phonesandsmartphones.ASIC or FPGA implementations may be used for not-so-high-volume embedded systems with special needs in kind of signal processing performance, interfaces and reliability, like in avionics.

Peripherals

A close-up of the SMSC LAN91C110 (SMSC 91x) chip, an embeddedEthernetchip

Embedded systems talk with the outside world viaperipherals,such as:

• Serial communication interfaces(SCI):RS-232,RS-422,RS-485,etc.
• Synchronous Serial Interface:I2C,SPI,SSC and ESSI (Enhanced Synchronous Serial Interface)
• Universal Serial Bus(USB)
• Media cards (SD cards,CompactFlash,etc.)
• Network interface controller:Ethernet,WiFi,etc.
• Fieldbuses:CAN bus,LIN-Bus,PROFIBUS,etc.
• Timers:Phase-locked loops,programmable interval timers
• General Purpose Input/Output(GPIO)
• Analog-to-digitalanddigital-to-analog converters
• Debugging:JTAG,In-system programming,background debug mode interfaceport, BITP, and DB9 ports.

Tools

As with other software, embedded system designers usecompilers,assemblers,anddebuggersto develop embedded system software. However, they may also use more specific tools:

• In circuit debuggers or emulators (seenext section).
• Utilities to add a checksum orCRCto a program, so the embedded system can check if the program is valid.
• For systems usingdigital signal processing,developers may use acomputational notebookto simulate the mathematics.
• System-level modeling and simulation tools help designers to construct simulation models of a system with hardware components such asprocessors,memories,DMA,interfaces,buses and software behavior flow as a state diagram or flow diagram using configurable library blocks. Simulation is conducted to select the right components by performing power vs. performance trade-offs, reliability analysis and bottleneck analysis. Typical reports that help a designer to make architecture decisions include application latency, device throughput, device utilization, power consumption of the full system as well as device-level power consumption.
• A model-based development tool creates and simulates graphical data flow and UML state chart diagrams of components like digital filters, motor controllers, communication protocol decoding and multi-rate tasks.
• Custom compilers and linkers may be used to optimize specialized hardware.
• An embedded system may have its own special language or design tool, or add enhancements to an existing language such asForthorBasic.
• Another alternative is to add a RTOS orembedded operating system
• Modeling and code generatingtoolsoften based onstate machines

Software tools can come from several sources:

• Software companies that specialize in the embedded market
• Ported from theGNUsoftware development tools
• Sometimes, development tools for a personal computer can be used if the embedded processor is a close relative to a common PC processor

As the complexity of embedded systems grows, higher-level tools and operating systems are migrating into machinery where it makes sense. For example,cellphones,personal digital assistantsand other consumer computers often need significant software that is purchased or provided by a person other than the manufacturer of the electronics. In these systems, an open programming environment such asLinux,NetBSD,FreeBSD,OSGiorEmbedded Javais required so that the third-party software provider can sell to a large market.

Debugging

Embeddeddebuggingmay be performed at different levels, depending on the facilities available. Considerations include: does it slow down the main application, how close is the debugged system or application to the actual system or application, how expressive are the triggers that can be set for debugging (e.g., inspecting the memory when a particularprogram countervalue is reached), and what can be inspected in the debugging process (such as, only memory, or memory and registers, etc.).

From simplest to most sophisticated debugging techniques and systems be roughly grouped into the following areas:

• Interactive resident debugging, using the simple shell provided by the embedded operating system (e.g. Forth and Basic)
• Software-only debuggers have the benefit that they do not need any hardware modification but have to carefully control what they record in order to conserve time and storage space.
• External debugging using logging or serial port output to trace operation using either a monitor in flash or using a debug server like theRemedy Debuggerthat even works for heterogeneousmulticoresystems.
• An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via aJTAGorNexusinterface.This allows the operation of the microprocessor to be controlled externally, but is typically restricted to specific debugging capabilities in the processor.
• Anin-circuit emulator(ICE) replaces the microprocessor with a simulated equivalent, providing full control over all aspects of the microprocessor.
• A completeemulatorprovides a simulation of all aspects of the hardware, allowing all of it to be controlled and modified, and allowing debugging on a normal PC. The downsides are expense and slow operation, in some cases up to 100 times slower than the final system.
• For SoC designs, the typical approach is to verify and debug the design on an FPGA prototype board. Tools such as Certusare used to insert probes in the FPGA implementation that make signals available for observation. This is used to debug hardware, firmware and software interactions across multiple FPGAs in an implementation with capabilities similar to a logic analyzer.

Unless restricted to external debugging, the programmer can typically load and run software through the tools, view the code running in the processor, and start or stop its operation. The view of the code may be ashigh-level programming language,assembly codeor mixture of both.

Tracing

Real-time operating systems often supporttracingof operating system events. A graphical view is presented by a host PC tool, based on a recording of the system behavior. The trace recording can be performed in software, by the RTOS, or by special tracing hardware. RTOS tracing allows developers to understand timing and performance issues of the software system and gives a good understanding of the high-level system behaviors. Trace recording in embedded systems can be achieved using hardware or software solutions. Software-based trace recording does not require specialized debugging hardware and can be used to record traces in deployed devices, but it can have an impact on CPU and RAM usage.One example of a software-based tracing method used in RTOS environments is the use of emptymacroswhich are invoked by the operating system at strategic places in the code, and can be implemented to serve ashooks.

Reliability

Embedded systems often reside in machines that are expected to run continuously for years without error, and in some cases recover by themselves if an error occurs. Therefore, the software is usually developed and tested more carefully than that for personal 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. Reduced functionality in the event of failure may be intolerable. Often backups are selected by an operator. Examples include aircraft navigation, reactor control systems, safety-critical chemical factory controls, train signals.
• 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 asmemory leaks,and alsosoft errorsin the hardware:

• watchdog timerthat resets and restarts the system unless the software periodically notifies the watchdog subsystems
• Designing with atrusted computing base(TCB) architecture ensures a highly secure and reliable system environment
• Ahypervisordesigned for embedded systems is able to provide secure encapsulation for any subsystem component so that a compromised software component cannot interfere with other subsystems, or privileged-level system software.This encapsulation keeps faults from propagating from one subsystem to another, thereby improving reliability. This may also allow a subsystem to be automatically shut down and restarted on fault detection.
• Immunity-aware programming can help engineers produce more reliable embedded systems code.Guidelines and coding rules such asMISRA C/C++aim to assist developers produce reliable, portable firmware in a number of different ways: typically by advising or mandating against coding practices which may lead to run-time errors (memory leaks, invalid pointer uses), use of run-time checks and exception handling (range/sanity checks, divide-by-zero and buffer index validity checks, default cases in logic checks), loop bounding, production of human-readable, well commented and well structured code, and avoiding language ambiguities which may lead to compiler-induced inconsistencies or side-effects (expression evaluation ordering, recursion, certain types of macro). These rules can often be used in conjunction with codestatic checkersor boundedmodel checkingfor functional verification purposes, and also assist in determination of codetiming properties.

High vs. low volume

For high-volume systems such asmobile phones,minimizing cost is usually the primary design consideration. Engineers typically select hardware that is just good enough to implement the necessary functions.

For low-volume or prototype embedded systems, general-purpose computers may be adapted by limiting the programs or by replacing the operating system with an RTOS.

Embedded software architectures

Main article:Embedded software

In 1978National Electrical Manufacturers Associationreleased ICS 3-1978, a standard for programmable microcontrollers,including almost any computer-based controllers, such assingle-board computers,numerical, and event-based controllers.

There are several different types of software architecture in common use.

Simple control loop

In this design, the software simply has aloopwhich monitors the input devices. The loop callssubroutines,each of which manages a part of the hardware or software. Hence it is called a simple control loop or programmed input-output.

Interrupt-controlled system

Some embedded systems are predominantly controlled byinterrupts.This means that tasks performed by the system are triggered by different kinds of events; an interrupt could be generated, for example, by a timer at a predefined interval, or by a serial port controller receiving data.

This architecture is used if event handlers need low latency, and the event handlers are short and simple. These systems run a simple task in a main loop also, but this task is not very sensitive to unexpected delays. Sometimes the interrupt handler will add longer tasks to a queue structure. Later, after the interrupt handler has finished, these tasks are executed by the main loop. This method brings the system close to a multitasking kernel with discrete processes.

Cooperative multitasking

Cooperative multitaskingis very similar to the simple control loop scheme, except that the loop is hidden in anAPI.The programmer defines a series of tasks, and each task gets its own environment to run in. When a task is idle, it calls an idle routine which passes control to another task.

The advantages and disadvantages are similar to that of the control loop, except that adding new software is easier, by simply writing a new task, or adding to the queue.

Preemptive multitasking or multi-threading

In this type of system, a low-level piece of code switches between tasks or threads based on a timer invoking an interrupt. This is the level at which the system is generally considered to have an operating system kernel. Depending on how much functionality is required, it introduces more or less of the complexities of managing multiple tasks running conceptually in parallel.

As any code can potentially damage the data of another task (except in systems using amemory management unit) programs must be carefully designed and tested, and access to shared data must be controlled by some synchronization strategy such asmessage queues,semaphoresor anon-blocking synchronizationscheme.

Because of these complexities, it is common for organizations to use an off-the-shelf RTOS, allowing the application programmers to concentrate on device functionality rather than operating system services. The choice to include an RTOS brings in its own issues, however, as the selection must be made prior to starting the application development process. This timing forces developers to choose the embedded operating system for their device based on current requirements and so restricts future options to a large extent.

The level of complexity in embedded systems is continuously growing as devices are required to manage peripherals and tasks such as serial, USB, TCP/IP,Bluetooth,Wireless LAN,trunk radio, multiple channels, data and voice, enhanced graphics, multiple states, multiple threads, numerous wait states and so on. These trends are leading to the uptake ofembedded middlewarein addition to an RTOS.

Microkernels and exokernels

Amicrokernelallocates memory and switches the CPU to different threads of execution. User-mode processes implement major functions such as file systems, network interfaces, etc.

Exokernelscommunicate efficiently by normal subroutine calls. The hardware and all the software in the system are available to and extensible by application programmers.

Monolithic kernels

Amonolithic kernelis a relatively large kernel with sophisticated capabilities adapted to suit an embedded environment. This gives programmers an environment similar to a desktop operating system likeLinuxorMicrosoft Windows,and is therefore very productive for development. On the downside, it requires considerably more hardware resources, is often more expensive, and, because of the complexity of these kernels, can be less predictable and reliable.

Common examples of embedded monolithic kernels areembedded Linux,VXWorksandWindows CE.

Despite the increased cost in hardware, this type of embedded system is increasing in popularity, especially on the more powerful embedded devices such aswireless routersandGPS navigation systems.

Additional software components

In addition to the core operating system, many embedded systems have additional upper-layer software components. These components include networking protocol stacks likeCAN,TCP/IP,FTP,HTTP,andHTTPS,and storage capabilities likeFATand flash memory management systems. If the embedded device has audio and video capabilities, then the appropriate drivers and codecs will be present in the system. In the case of the monolithic kernels, many of these software layers may be included in the kernel. In the RTOS category, the availability of additional software components depends upon the commercial offering.

Domain-specific architectures

In the automotive sector,AUTOSARis a standard architecture for embedded software.

See also

• Communications server
• Cyber-physical system
• Electronic control unit
• Information appliance
• Integrated development environment
• Photonically Optimized Embedded Microprocessors
• Silicon compiler
• Software engineering
• System on module
• Ubiquitous computing

Further reading

• John Catsoulis (May 2005).Designing Embedded Hardware, 2nd Edition.O'Reilly.ISBN0-596-00755-8.
• James M. Conrad; Alexander G. Dean (September 2011).Embedded Systems, An Introduction Using the Renesas RX62N Microcontroller.Micrium.ISBN978-1935-7729-96.
• Klaus Elk (August 2016).Embedded Software Development for the Internet Of Things, The Basics, The Technologies and Best Practices.CreateSpace Independent Publishing Platform.ISBN978-1534602533.

External links

• Embedded Systems course with mbedYouTube, ongoing from 2015
• Trends in Cyber Security and Embedded SystemsDan Geer, November 2013
• Modern Embedded Systems Programming Video CourseYouTube, ongoing from 2013
• Embedded Systems Week (ESWEEK)yearly event with conferences, workshops and tutorials covering all aspects of embedded systems and software
• Workshop on Embedded and Cyber-Physical Systems Educationat theWayback Machine(archived 2018-02-11), workshop covering educational aspects of embedded systems