Editing Embedded system

{{Short description|Computer system with a dedicated function}}
{{Use American English|date=December 2017}}

[[File:DHCOM Computer On Module - AM35x.jpg|thumb|right|An ''embedded system'' on a plug-in card with processor, memory, power supply, and external interfaces]]

An '''embedded system''' is a specialized [[computer system]]&mdash;a combination of a [[computer processor]], [[computer memory]], and [[input/output]] peripheral devices&mdash;that has a dedicated function within a larger mechanical or [[Electronics|electronic]] system.<ref name="Barr-glossary">{{cite web |author=Michael Barr |title=Embedded Systems Glossary |work=Neutrino Technical Library |access-date=2007-04-21 |url=http://www.netrino.com/Embedded-Systems/Glossary |author-link=Michael Barr (software engineer)}}</ref><ref>{{cite book |last=Heath |first=Steve |title=Embedded systems design|publisher=Newnes |year=2003 |edition=2 |series=EDN series for design engineers |page=[https://archive.org/details/embeddedsystemsd0000heat/page/2 2] |url=https://archive.org/details/embeddedsystemsd0000heat |url-access=registration|quote= An embedded system is a [[microprocessor]] based system that is built to control a function or a range of functions. | isbn=978-0-7506-5546-0}}</ref> 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 has [[real-time computing]] constraints.  Embedded systems control many devices in common use.<ref name=":0">{{cite book|last=Michael Barr|author2=Anthony J. Massa|title=Programming embedded systems: with C and GNU development tools|publisher=O'Reilly|year=2006|pages=1–2|chapter=Introduction|chapter-url=https://books.google.com/books?id=nPZaPJrw_L0C&pg=PA1 | isbn=978-0-596-00983-0}}</ref> {{as of|2009|alt=In 2009}}, it was estimated that ninety-eight percent of all microprocessors manufactured were used in embedded systems.<ref>{{cite web | title=Real men program in C | last=Barr |first=Michael | page=2 | date=1 August 2009 | work=Embedded Systems Design | publisher=TechInsights (United Business Media) | url=http://www.embedded.com/electronics-blogs/barr-code/4027479/Real-men-program-in-C | access-date=2009-12-23 }}</ref>{{Update inline|reason=Computing has changed a lot since 2009.|date=March 2022}}

Modern embedded systems are often based on [[microcontroller]]s (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 the [[digital signal processor]] (DSP).

Since the embedded system is dedicated to specific tasks, [[design engineer]]s can optimize it to reduce the size and cost of the product and increase its reliability and performance. Some embedded systems are mass-produced, benefiting from [[economies of scale]].

Embedded systems range in size from portable personal devices such as [[digital watch]]es and [[MP3 player]]s to bigger machines like [[home appliances]], industrial [[assembly lines]], [[robots]], transport vehicles, [[Traffic light control and coordination|traffic light controllers]], and [[medical imaging]] systems. Often they constitute subsystems of other machines like [[avionics]] in [[aircraft]] and [[astrionics]] in [[spacecraft]]. Large installations like [[factories]], [[pipelines]], and [[electrical grid]]s rely on multiple embedded systems networked together. Generalized through software customization, embedded systems such as [[programmable logic controller]]s frequently comprise their functional units.

Embedded systems range from those low in complexity, with a single microcontroller chip, to very high with multiple units, [[peripheral]]s and networks, which may reside in [[equipment rack]]s or 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 the [[MOS integrated circuit]], which is an [[integrated circuit]] chip [[semiconductor device fabrication|fabricated]] from [[MOSFET]]s (metal–oxide–semiconductor [[field-effect transistor]]s) and was developed in the early 1960s. By 1964, MOS chips had reached higher [[transistor density]] and lower manufacturing costs than [[bipolar junction transistor|bipolar]] chips. MOS chips further increased in complexity at a rate predicted by [[Moore's law]], leading to [[large-scale integration]] (LSI) with hundreds of [[transistors]] on a single MOS chip by the late 1960s. The application of MOS LSI chips to [[computing]] was the basis for the first microprocessors, as engineers began recognizing that a complete [[computer processor]] system could be contained on several MOS LSI chips.<ref name="ieee">{{cite journal |last1=Shirriff |first1=Ken |title=The Surprising Story of the First Microprocessors |journal=[[IEEE Spectrum]] |volume=53 |issue=9 |pages=48–54 |date=30 August 2016 |publisher=[[Institute of Electrical and Electronics Engineers]] |url=https://spectrum.ieee.org/the-surprising-story-of-the-first-microprocessors |access-date=13 October 2019|doi=10.1109/MSPEC.2016.7551353 |s2cid=32003640 |url-access=subscription }}</ref>

The first multi-chip microprocessors, the [[Four-Phase Systems AL1]] in 1969 and the [[Garrett AiResearch]] [[MP944]] in 1970, were developed with multiple MOS LSI chips. The first single-chip microprocessor was the [[Intel 4004]], released in 1971. It was developed by [[Federico Faggin]], using his [[silicon-gate]] MOS technology, along with [[Intel]] engineers [[Marcian Hoff]] and [[Stan Mazor]], and [[Busicom]] engineer [[Masatoshi Shima]].<ref>{{cite web |title=1971: Microprocessor Integrates CPU Function onto a Single Chip |website=The Silicon Engine |url=https://www.computerhistory.org/siliconengine/microprocessor-integrates-cpu-function-onto-a-single-chip/ |publisher=[[Computer History Museum]] |access-date=22 July 2019}}</ref>

=== Development ===
One of the first recognizably modern embedded systems was the [[Apollo Guidance Computer]],{{citation needed|reason=linked article claims it is the first IC-based computer. nothing there supports it being the first embedded system.|date=August 2018}} developed ca. 1965 by [[Charles Stark Draper]] at the [[MIT 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 developed [[monolithic integrated circuit]]s to reduce the computer's size and weight.

An early mass-produced embedded system was the [[D-17B|Autonetics D-17 guidance computer]] for the [[Minuteman 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, the [[Intel 4004]] (released in 1971), was designed for [[calculator]]s and 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-based [[Analogue electronics|analog]] components such as [[potentiometer]]s and [[variable capacitor]]s with 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==
[[File:Accupoll-embedded-computer.jpg|thumb|Embedded Computer Sub-Assembly for Accupoll Electronic Voting Machine<ref>{{cite web|url=https://www.eff.org/|title=Electronic Frontier Foundation|website=Electronic Frontier Foundation}}</ref>]]

Embedded systems are commonly found in consumer, industrial, [[automotive]], [[home appliance]]s, medical, telecommunication, commercial, aerospace and military applications.

[[Telecommunications system]]s employ numerous embedded systems from [[telephone switch]]es for the network to [[cell phone]]s at the [[end user]]. Computer networking uses dedicated [[Router (computing)|routers]] and [[network bridge]]s to route data.

[[Consumer electronics]] include [[MP3 player]]s, [[television set]]s, [[mobile phone]]s, [[video game console]]s, [[digital camera]]s, [[GPS receiver]]s, and [[computer printer|printers]]. Household appliances, such as [[microwave oven]]s, [[washing machine]]s and [[dishwashers]], include embedded systems to provide flexibility, efficiency and features. Advanced [[heating, ventilation, and air conditioning]] (HVAC) systems use networked [[thermostat]]s to more accurately and efficiently control temperature that can change by time of day and [[season]]. [[Home automation]] uses 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 advanced [[avionics]] such as [[inertial guidance system]]s and GPS receivers that also have considerable safety requirements. [[Spacecraft]] rely on [[astrionics]] systems for trajectory correction. Various electric motors — [[brushless DC motor]]s, [[induction motor]]s and [[DC motor]]s — use electronic [[motor controller]]s. [[Automobile]]s, [[electric vehicle]]s, and [[hybrid vehicle]]s increasingly use embedded systems to maximize efficiency and reduce pollution. Other automotive safety systems using embedded systems include [[anti-lock braking system]] (ABS), [[electronic stability control]] (ESC/ESP), [[traction control]] (TCS) and automatic [[four-wheel drive]].

[[Medical equipment]] uses embedded systems for [[Monitoring (medicine)|monitoring]], and various [[medical imaging]] ([[positron emission tomography]] (PET), [[single-photon emission computed tomography]] (SPECT), [[computed tomography]] (CT), and [[magnetic resonance imaging]] (MRI) for non-invasive internal inspections. Embedded systems within medical equipment are often powered by industrial computers.<ref>[http://content.dell.com/us/en/enterprise/oem-industry-solutions-build-your-product-with-dell Embedded Systems Dell OEM Solutions | Dell] {{Webarchive|url=https://web.archive.org/web/20130127080734/http://content.dell.com/us/en/enterprise/oem-industry-solutions-build-your-product-with-dell |date=2013-01-27 }}. Content.dell.com (2011-01-04). Retrieved on 2013-02-06.</ref>

Embedded systems are used for [[safety-critical system]]s in 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.{{citation needed|date=April 2021}} 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 called [[mote (sensor)|motes]] are networked wireless sensors. [[Wireless sensor networking]] makes use of miniaturization made possible by advanced [[integrated 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 have [[Real-time computing|real-time]] performance 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, the [[Gibson Robot Guitar]] features an embedded system for tuning the strings, but the overall purpose of the Robot Guitar is to play music.<ref>{{cite magazine |url=http://www.embedded.com/underthehood/207401418 |title=Under the Hood: Robot Guitar embeds autotuning |archive-url=https://web.archive.org/web/20080708195311/http://embedded.com/underthehood/207401418 |archive-date=2008-07-08 |author=David Carey |date=2008-04-22 |magazine=Embedded Systems Design}}</ref> Similarly, an embedded system in an [[automobile]] provides a specific function as a subsystem of the car itself.

[[File:ESOM270 eSOM300 Computer on Modules.jpg|thumb|e-con Systems eSOM270 & eSOM300 Computer on Modules]]
The program instructions written for embedded systems are referred to as [[firmware]], and are stored in read-only memory or [[flash memory]] chips. They run with limited computer hardware resources: little memory, small or non-existent keyboard or screen.

===User interfaces===
[[File:MicroVGA TUI demoapp.jpg|thumb|Embedded system [[text user interface]] using MicroVGA<ref group="nb" name="MicroVGA">For more details of MicroVGA see this [http://www.microvga.com/pdf/uvga-text-ds.pdf PDF].</ref>]]

Embedded systems range from [[Headless computer|no user interface]] at all, in systems dedicated to one task, to complex [[graphical user interface]]s that resemble modern computer desktop operating systems. Simple embedded devices use [[Push-button|buttons]], [[light-emitting diode]]s (LED), graphic or character [[liquid-crystal display]]s (LCD) with a simple [[menu (computing)|menu system]]. More sophisticated devices that use a graphical screen with [[touch screen|touch sensing]] or screen-edge [[soft key]]s provide 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 the [[board support package]] (BSP) and allows designers to build a rich user interface on the PC. A good example of this is the combination of an [[embedded HTTP server]] running on an embedded device (such as an [[IP camera]] or a [[network router]]). The user interface is displayed in a [[web browser]] on 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.

[[List of common microcontrollers|Numerous microcontrollers]] have 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/104]] and 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 use [[DOS]], [[FreeBSD]], [[Linux]], [[NetBSD]], [[OpenHarmony]] or an embedded [[real-time operating system]] (RTOS) such as [[MicroC/OS-II]], [[QNX]] or [[VxWorks]].

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 VIA [[EPIA]] range 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 are [[automated teller machine]]s (ATM) and [[arcade machines]], which contain code specific to the application.

However, most ready-made embedded systems boards are not PC-centered and do not use the [[Industry Standard Architecture|ISA]] or [[Peripheral Component Interconnect|PCI]] busses. When a [[system-on-a-chip]] processor 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 density [[Ball grid array|BGA]] chips such as an [[ARM architecture|ARM]]-based [[system-on-a-chip]] processor and peripherals, external [[flash memory]] for storage, and [[DRAM]] for runtime memory. The module vendor will usually provide boot software and make sure there is a selection of operating systems, usually including [[Linux]] and 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 include [[Arduino]] and [[Raspberry Pi]].

====ASIC and FPGA SoC solutions====
A [[system 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 an [[application-specific integrated circuit]] (ASIC) or using a [[field-programmable gate array]] (FPGA) which typically can be reconfigured.

ASIC implementations are common for very-high-volume embedded systems like [[mobile phone]]s and [[smartphone]]s. 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===
[[File:SMSC LAN91C110 ethernet chip.jpg|thumb|A close-up of the SMSC LAN91C110 (SMSC 91x) chip, an embedded [[Ethernet]] chip]]

Embedded systems talk with the outside world via [[peripheral]]s, such as:
* [[Serial communication interface]]s (SCI): [[RS-232]], [[RS-422]], [[RS-485]], etc.
* [[Synchronous Serial Interface]]: [[I2C]], [[Serial Peripheral Interface Bus|SPI]], SSC and ESSI (Enhanced Synchronous Serial Interface)
* [[Universal Serial Bus]] (USB)
* Media cards ([[SD card]]s, [[CompactFlash]], etc.)
* [[Network interface controller]]: [[Ethernet]], [[WiFi]], etc.
* [[Fieldbus]]es: [[CAN bus]], [[Local Interconnect Network|LIN-Bus]], [[PROFIBUS]], etc.
* Timers: [[Phase-locked loop]]s, [[programmable interval timer]]s
* [[General Purpose Input/Output]] (GPIO)
* [[Analog-to-digital converter|Analog-to-digital]] and [[digital-to-analog converter]]s
* Debugging: [[JTAG]], [[In-system programming]], [[background debug mode interface]] port, BITP, and DB9 ports.

===Tools===
As with other software, embedded system designers use [[compiler]]s, [[Assembly language|assemblers]], and [[debugger]]s to develop embedded system software. However, they may also use more specific tools:
* In circuit debuggers or emulators (see [[#Debugging|next section]]).
* Utilities to add a checksum or [[Cyclic redundancy check|CRC]] to a program, so the embedded system can check if the program is valid.
* For systems using [[digital signal processing]], developers may use a [[computational notebook]] to simulate the mathematics.
* System-level modeling and simulation tools help designers to construct simulation models of a system with hardware components such as processors, [[memories]], [[Direct memory access|DMA]], [[Interface (computing)|interface]]s, 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 as [[Forth (programming language)|Forth]] or [[BASIC Stamp|Basic]].
* Another alternative is to add a RTOS or [[embedded operating system]]
* Modeling and code generating [[UML tool|tools]] often based on [[UML state machine|state machines]]

Software tools can come from several sources:
* Software companies that specialize in the embedded market
* Ported from the [[GNU]] software 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

Embedded software often requires a variety of development tools, including programming languages such as [[C++]], [[Rust (programming language)|Rust]], or [[Python (programming language)|Python]], and frameworks like [[Qt (software)|Qt]] for graphical interfaces. These tools enable developers to create efficient, scalable, and feature-rich applications tailored to the specific requirements of embedded systems. The choice of tools is driven by factors such as real-time performance, integration with hardware, or energy efficiency.

As the complexity of embedded systems grows, higher-level tools and operating systems are migrating into machinery where it makes sense. For example, [[cellphone]]s, [[personal digital assistant]]s and 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 as [[Linux]], [[NetBSD]], [[FreeBSD]], [[OSGi]] or [[Embedded Java]] is required so that the third-party software provider can sell to a large market.

==Debugging==
Embedded [[debugging]] may 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 particular [[program counter]] value 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 are 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.<ref>{{Cite book |last1=Tancreti |first1=Matthew |last2=Sundaram |first2=Vinaitheerthan |last3=Bagchi |first3=Saurabh |last4=Eugster |first4=Patrick |title=Proceedings of the 14th International Conference on Information Processing in Sensor Networks |chapter=TARDIS |date=2015 |series=IPSN '15 |location=New York, NY, USA |publisher=ACM |pages=286–297 |doi=10.1145/2737095.2737096 |isbn=9781450334754 |s2cid=10120929}}</ref>
* External debugging using logging or serial port output to trace operation using either a monitor in flash or using a debug server like the [[Remedy Debugger]] that even works for heterogeneous [[multicore]] systems.
* An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via a [[JTAG]] or [[Nexus (standard)|Nexus]] interface.<ref>{{Cite book|last1=Tancreti|first1=Matthew|last2=Hossain|first2=Mohammad Sajjad|last3=Bagchi|first3=Saurabh|last4=Raghunathan|first4=Vijay|title=Proceedings of the 9th ACM Conference on Embedded Networked Sensor Systems |chapter=Aveksha |date=2011|series=SenSys '11|location=New York, NY, USA|publisher=ACM|pages=288–301|doi=10.1145/2070942.2070972|isbn=9781450307185|s2cid=14769602}}</ref> This allows the operation of the microprocessor to be controlled externally, but is typically restricted to specific debugging capabilities in the processor. 
* An [[in-circuit emulator]] (ICE) 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 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 Certus<ref>{{cite web |url=http://www.eejournal.com/archives/articles/20121030-tektronix/ |title=Tektronix Shakes Up Prototyping, Embedded Instrumentation Boosts Boards to Emulator Status |publisher=Electronic Engineering Journal |date= 2012-10-30 |access-date=2012-10-30}}</ref> are 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 as [[high-level programming language]], [[assembly code]] or mixture of both.

===Tracing===
Real-time operating systems often support [[tracing (software)|tracing]] of 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.<ref>{{Citation |last1=Kraft |first1=Johan |title=Trace Recording for Embedded Systems: Lessons Learned from Five Industrial Projects |date=2010 |url=http://link.springer.com/10.1007/978-3-642-16612-9_24 |work=Runtime Verification |volume=6418 |pages=315–329 |editor-last=Barringer |editor-first=Howard |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-16612-9_24 |isbn=978-3-642-16611-2 |access-date=2022-08-16 |last2=Wall |first2=Anders |last3=Kienle |first3=Holger |editor2-last=Falcone |editor2-first=Ylies |editor3-last=Finkbeiner |editor3-first=Bernd |editor4-last=Havelund |editor4-first=Klaus|url-access=subscription }}</ref> One example of a software-based tracing method used in RTOS environments is the use of empty [[Macro (computer science)|macros]] which are invoked by the operating system at strategic places in the code, and can be implemented to serve as [[Hooking|hooks]].

===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 as [[memory leak]]s, and also [[soft error]]s in the hardware:
* [[watchdog timer]] that resets and restarts the system unless the software periodically notifies the watchdog subsystems
* Designing with a [[trusted computing base]] (TCB) architecture ensures a highly secure and reliable system environment<ref>{{cite journal |url=http://c59951.r51.cf2.rackcdn.com/5557-528-heiser.pdf |archive-url=https://web.archive.org/web/20141129070740/http://c59951.r51.cf2.rackcdn.com/5557-528-heiser.pdf |archive-date=2014-11-29 |url-status=live |title=Your System is secure? Prove it! |first1=Gernot |last1=Heiser |date=December 2007 |volume=2 |issue=6 |pages=35–8 |journal=[[;login:]]}}</ref>
* A [[hypervisor]] designed 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.<ref>{{cite book|last1=Moratelli|first1=C|last2=Johann|first2=S|last3=Neves|first3=M|last4=Hessel|first4=F|title=Proceedings of the 27th International Symposium on Rapid System Prototyping: Shortening the Path from Specification to Prototype|chapter=Embedded virtualization for the design of secure IoT applications|pages=2–6|date=2016|chapter-url=https://ieeexplore.ieee.org/document/7909116|access-date=2 February 2018|doi=10.1145/2990299.2990301|isbn=9781450345354|s2cid=17466572}}</ref> 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.<ref name=":1">{{Cite book|last=Short|first=Michael|title=2008 IEEE/ACS International Conference on Computer Systems and Applications |chapter=Development guidelines for dependable real-time embedded systems |date=March 2008|chapter-url=https://ieeexplore.ieee.org/document/4493674|pages=1032–1039|doi=10.1109/AICCSA.2008.4493674|isbn=978-1-4244-1967-8|s2cid=14163138|url=https://figshare.com/articles/conference_contribution/Development_Guidelines_for_Dependable_Real-Time_Embedded_Systems_/10083272 }}</ref><ref>{{Cite web|last=Motor Industry Software Reliability Association|title=MISRA C:2012 Third Edition, First Revision|url=https://www.misra.org.uk/product/misra-c2012-third-edition-first-revision/|access-date=2022-02-03|language=en-GB}}</ref> Guidelines and coding rules such as [[MISRA C|MISRA 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 code [[Static program analysis|static checkers]] or bounded [[model checking]] for functional verification purposes, and also assist in determination of code [[Worst-case execution time|timing properties]].<ref name=":1"/>

===High vs. low volume===
For high-volume systems such as [[mobile phone]]s, 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|Embedded software}}

In 1978 [[National Electrical Manufacturers Association]] released ICS&nbsp;3-1978, a standard for programmable microcontrollers,<ref>{{cite web |title=FAQs: Programmable Controllers |url=https://www.nema.org/docs/default-source/standards-document-library/faq-programmable-controllers.pdf?sfvrsn=a03312d_2 |access-date=2020-01-10}}</ref> including almost any computer-based controllers, such as [[single-board computer]]s, 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 a [[loop (computing)|loop]] which monitors the input devices. The loop calls [[subroutine]]s, 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 by [[interrupt]]s. 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 multitasking]] is very similar to the simple control loop scheme, except that the loop is hidden in an [[API]].<ref name=":0" /><ref name="Barr-glossary" /> 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 a [[memory management unit]]) programs must be carefully designed and tested, and access to shared data must be controlled by some synchronization strategy such as [[message queue]]s, [[semaphore (programming)|semaphores]] or a [[non-blocking synchronization]] scheme.

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.<ref>
{{cite web
|url=http://www.clarinox.com/docs/whitepapers/Whitepaper_06_CrossPlatformDiscussion.pdf |archive-url=https://web.archive.org/web/20110219200027/http://www.clarinox.com/docs/whitepapers/Whitepaper_06_CrossPlatformDiscussion.pdf |archive-date=2011-02-19 |url-status=live
|title= Working across Multiple Embedded Platforms
|publisher=clarinox
|access-date=2010-08-17
}}</ref> 

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 of [[embedded middleware]] in addition to an RTOS.

===Microkernels and exokernels===
A [[microkernel]] allocates memory and switches the CPU to different threads of execution. User-mode processes implement major functions such as file systems, network interfaces, etc.

[[Exokernel]]s communicate efficiently by normal subroutine calls. The hardware and all the software in the system are available to and extensible by application programmers.

===Monolithic kernels===
A [[monolithic kernel]] is a relatively large kernel with sophisticated capabilities adapted to suit an embedded environment. This gives programmers an environment similar to a desktop operating system like [[Linux]] or [[Microsoft 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 are [[embedded Linux]], [[VXWorks]] and [[Windows CE]].

Despite the increased cost in hardware, this type of embedded system is increasing in popularity, especially on the more powerful embedded devices such as [[wireless router]]s and [[Automotive navigation system|GPS 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 like [[Controller–area network|CAN]], [[TCP/IP]], [[FTP]], [[HTTP]], and [[HTTPS]], and storage capabilities like [[File Allocation Table|FAT]] and 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, [[AUTOSAR]] is a standard architecture for embedded software.<!--[[User:Kvng/RTH]]-->

==See also==
{{Portal|Electronics}}
{{Div col|colwidth=22em}}
* [[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]]

{{Div col end}}

==Notes==
<references group="nb" />

==References==
{{Reflist}}

==Further reading==
* {{Cite book |title=Designing Embedded Hardware, 2nd Edition |author=John Catsoulis |date=May 2005 |publisher=O'Reilly |isbn=0-596-00755-8}}
* {{Cite book |title=Embedded Systems, An Introduction Using the Renesas RX62N Microcontroller |author1=James M. Conrad |author2=Alexander G. Dean |date=September 2011 |publisher=Micrium |isbn=978-1935-7729-96}}
* {{Cite book |title=Embedded Software Development for the Internet Of Things, The Basics, The Technologies and Best Practices |author=Klaus Elk |date=August 2016 |publisher=CreateSpace Independent Publishing Platform |isbn=978-1534602533}}

==External links==
{{Commons category|Embedded systems}}
{{Wikibooks|Embedded Systems}}
{{Wikiversity|Embedded System Engineering}}
* [https://www.youtube.com/watch?v=H-OKGOMoCSI&list=PLo7bVbJhQ6qwlDa-R6pz7tA7kPzn1s5Ae Embedded Systems course with mbed] YouTube, ongoing from 2015
* [http://geer.tinho.net/geer.nro.6xi13.txt Trends in Cyber Security and Embedded Systems] Dan Geer, November 2013
* [https://www.youtube.com/playlist?list=PLPW8O6W-1chwyTzI3BHwBLbGQoPFxPAPM Modern Embedded Systems Programming Video Course] YouTube, ongoing from 2013
* [http://www.esweek.org/ Embedded Systems Week (ESWEEK)] yearly event with conferences, workshops and tutorials covering all aspects of embedded systems and software
* {{Webarchive|url=https://web.archive.org/web/20180211173413/http://www.emsig.net/conf/2015/wese/ |date=2018-02-11 |title=Workshop on Embedded and Cyber-Physical Systems Education}}, workshop covering educational aspects of embedded systems
* [https://microcontrollershop.com/An%20Embedded%20Tools%20Introduction.php Developing Embedded Systems - A Tools Introduction]

{{Computer sizes}}
{{Computer science}}
{{Embedded systems}}
{{Authority control}}

[[Category:Embedded systems| ]]