Embedded system
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Reverted edits by [[Special:Contributions/OrgasGirl|OrgasGirl]] ([[User talk:OrgasGirl|talk]]) to last version by Jonathan Cardy
{{Unreferenced|date=April 2008}}
[[Image:ADSL modem router internals labeled.jpg|thumb|300px|A [[router]], an example of an embedded system. Labelled parts include a [[microprocessor]] (4), [[RAM]] (6), and [[flash memory]] (7).]]
An '''embedded system''' is a special-purpose [[computer]] system designed to perform one or a few dedicated functions<ref name="barr-glossary">{{cite web|author=[[Michael Barr (software engineer)|Michael Barr]]|title=Embedded Systems Glossary|work=Netrino Technical Library|accessdate=2007-04-21|url=http://www.netrino.com/Embedded-Systems/Glossary}}</ref>, often with [[real-time computing]] constraints. It is usually ''embedded'' as part of a complete device including hardware and mechanical parts. In contrast, a general-purpose computer, such as a [[personal computer]], can do many different tasks depending on programming. Embedded systems control many of the common devices in use today.
Since the embedded system is dedicated to specific tasks, design engineers can optimize it, reducing the size and cost of the product, or increasing the reliability and performance. Some embedded systems are mass-produced, benefiting from [[economies of scale]].
Physically, embedded systems range from portable devices such as digital watches and [[MP3 player]]s, to large stationary installations like [[traffic light]]s, factory controllers, or the systems controlling [[nuclear power plant]]s. Complexity varies from low, with a single [[microcontroller]] chip, to very high with multiple units, peripherals and networks mounted inside a large chassis or enclosure.
In general, "embedded system" is not an exactly defined term, as many systems have some element of programmability. For example, [[Handheld computer]]s share some elements with embedded systems — such as the operating systems and microprocessors which power them — but are not truly embedded systems, because they allow different applications to be loaded and peripherals to be connected.
== Examples of embedded systems ==
[[Image:Alix.1C_board_with_AMD_Geode_LX_800_(PC_Engines).jpg|thumb|250px|PC Engines' ALIX.1C [[Mini-ITX]] embedded board with [[Geode (processor)|AMD Geode]] LX 800 together with [[Compact Flash]], [[miniPCI]] and PCI slots, 44-pin [[Integrated_Drive_Electronics|IDE]] interface and 256MB RAM]]
[[Image:RouterBoard 112 with U.FL-RSMA pigtail and R52 miniPCI Wi-Fi card.jpg|thumb|250px|An [[Embedded system|embedded]] RouterBoard 112 with [[U.FL]]-[[SMA connector|RSMA]] pigtail and R52 [[miniPCI]] [[Wi-Fi]] card widely used by [[wireless]] [[Internet]] service providers ([[Wireless internet service provider|WISP]]s) in the [[Czech Republic]].]]
Embedded systems span all aspects of modern life and there are many examples of their use.
Telecommunications systems employ numerous embedded systems from [[telephone switch]]es for the network to [[mobile phone]]s at the end-user.
Computer networking uses dedicated [[router]]s and [[network bridge]]s to route data.
[[Consumer electronics]] include [[personal digital assistant]]s (PDAs), [[mp3 player]]s, mobile phones, [[videogame console]]s, [[digital camera]]s, [[DVD player]]s, [[Global Positioning System|GPS]] receivers, and [[computer printer|printer]]s.
Many household appliances, such as [[microwave oven]]s, [[washing machine]]s and [[dishwashers]], are including embedded systems to provide flexibility, efficiency and features.
Advanced [[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, 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 [[Global Positioning System|GPS]] receivers that also have considerable safety requirements.
Various electric motors — [[brushless DC motor]]s, [[induction motor]]s and [[DC motor]]s — are using electric/electronic [[motor controller]]s.
[[Automobile]]s, [[electric vehicle]]s. and [[hybrid vehicle]]s are increasingly using embedded systems to maximize efficiency and reduce pollution.
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 stethoscope]]s for amplifying sounds, and various [[medical imaging]] ([[Positron emission tomography|PET]], [[Single photon emission computed tomography|SPECT]], [[Computed tomography|CT]], [[Magnetic resonance imaging|MRI]]) for non-invasive internal inspections.
== History ==
In the earliest years of computers in the 1930-40s, computers were sometimes dedicated 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 concept of [[programmable controllers]] evolved from traditional [[electromechanical]] sequencers, via solid state devices, to the use of computer technology.
<!-- Commented out because image was deleted: [[Image:Apollo-guidance-computer.jpg|thumb|300px|right|The Apollo Guidance Computer, the first recognizably modern embedded system.<br>Image courtesy of Computer History Museum.]] -->
One of the first recognizably modern embedded systems was the [[Apollo Guidance Computer]], developed 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 circuits to reduce the size and weight. An early mass-produced embedded 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 new computer that was the first high-volume use of integrated circuits. This program alone 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 in price and there has been a dramatic rise in processing power and functionality. The first [[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 replace expensive knob-based [[analog electronics|analog]] components such as [[potentiometer]]s and [[variable capacitor]]s with up/down buttons or knobs read out by a microprocessor even in some consumer products. By the mid-1980s, most of the common previously external system components had been integrated into the same chip as the processor and this modern form of the [[microcontroller]] allowed an even more widespread use, which by the end of the decade were the norm rather than the exception for almost all electronics devices.
==Characteristics==
[[Image:Soekris net4801 board.jpg|thumb|[[Soekris]] net4801, an embedded system targeted at network applications.]]
# 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 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 consist of small, computerized parts 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, of course, to play music <ref>http://www.embedded.com/underthehood/207401418</ref>. Similarly, an embedded system in an [[automobile]] provides a specific function as a subsystem of the car itself.
# The software written for embedded systems is often called [[firmware]], and is usually stored 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, and little memory.
=== User interfaces ===
Embedded systems range from no user interface at all — dedicated only to one task — to complex [[Desktop_operating_system#Graphical_user_interfaces|graphical user interfaces]] that resemble modern computer desktop operating systems.
===Simple systems===
Simple embedded devices use buttons, [[LED]]s, and small character- or digit-only displays, often with a simple [[Menu (computing)|menu system]].
===In more complex systems===
A full graphical screen, with [[touch screen|touch]] sensing or screen-edge buttons provides flexibility while minimising space used: the meaning of the buttons can change with the screen, and selection involves the natural behavior of pointing at what'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 quite different option: providing a web page interface over a network connection. This avoids the cost of a sophisticated display, yet provides complex input and display capabilities when needed, on another computer. This is successful for remote, permanently installed equipment such as Pan-Tilt-Zoom cameras and network routers.
===CPU platforms===
Embedded processors can be broken into two broad categories: ordinary microprocessors (μP) and microcontrollers (μC), which have many more peripherals on chip, reducing cost and size. Contrasting to the personal computer and server markets, a fairly large number of basic [[CPU architecture]]s are used; there are [[Von Neumann architecture|Von Neumann]] as well as various degrees of [[Harvard architecture]]s, [[RISC]] as well as non-RISC and [[VLIW]]; word lengths vary from 4-bit to 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 several different 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]], [[x86 architecture|x86]], [[XE8000]], [[Z80]], etc.
====Ready made computer boards====
[[PC/104]] and PC/104+ are examples of available ''ready made'' computer boards intended 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]].
====ASIC and FPGA solutions====
A common configuration for very-high-volume embedded systems is the [[system on a chip]] (SoC), an [[application-specific integrated circuit]] (ASIC), for which the CPU core was purchased and added as part of the chip design. A related scheme is to use a [[field-programmable gate array]] (FPGA), and program it with all the logic, including the CPU.
===Peripherals===
Embedded Systems talk with the outside world via [[peripheral]]s, such as:
*Serial Communication Interfaces (SCI): [[RS-232]], [[RS-422]], [[RS-485]] etc
*Synchronous Serial Communication Interface: [[I2C]], [[JTAG]], [[Serial Peripheral Interface Bus|SPI]], SSC and ESSI
*[[Universal Serial Bus]] (USB)
*Networks: [[Ethernet]], [[Controller Area Network]], [[LonWorks]], etc
*Timers: [[PLL]](s), Capture/Compare and [[Time Processing Unit]]s
*Discrete IO: aka [[General Purpose Input/Output]] (GPIO)
*Analog to Digital/Digital to Analog (ADC/DAC)
===Tools===
As for other software, embedded system designers use [[compiler]]s, [[Assembly language#Assembler|assembler]]s, and [[debugger]]s to develop embedded system software. However, they may also use some more specific tools:
* In circuit debuggers or emulators (see 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 math workbench such as [[MATLAB]], [[Simulink]], [[MathCad]], or [[Mathematica]] to simulate the mathematics. They might also use libraries for both the host and target which eliminates developing DSP routines as done in [[DSPnano RTOS]] and [[Unison Operating System]].
* Custom compilers and linkers may be used to improve optimisation for the particular 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 [[Real-time operating system]] or [[Embedded operating system]], which may have DSP capabilities like [[DSPnano RTOS]].
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
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]], [[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. From simplest to most sophisticated they can 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)
* 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]] which even works for heterogeneous [[multicore]] systems.
* An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via a [[JTAG]] or NEXUS interface. 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]] 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.
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 [[assembly code]] or [[source-code]].
Because an embedded system is often composed of a wide variety of elements, the debugging strategy may vary. For instance, debugging a software- (and microprocessor-) centric embedded system is different from debugging an embedded system where most of the processing is performed by peripherals (DSP, FPGA, co-processor).
An increasing number of embedded systems today use more than one single processor core. A common problem with multi-core development is the proper synchronization of software execution. In such a case, the embedded system design may wish to check the data traffic on the busses between the processor cores, which requires very low-level debugging, at signal/bus level, with a logic analyzer, for instance.
===Reliability===
Embedded systems often reside in machines that are expected to run continuously for years 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 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. "Limp modes" are less tolerable. Often backups are selected by an operator. Examples include aircraft navigation, reactor control systems, safety-critical chemical factory controls, train signals, 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 notifies the watchdog
* subsystems with redundant spares that can be switched over to
* software "limp modes" that provide partial function
* [[Immunity Aware Programming]]
===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 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 a [[real-time operating system]].
== Embedded software architectures ==
There are several different types of software architecture in common use.
=== Simple control loop ===
In this design, the software simply has a loop. The loop calls subroutines, each of which manages a part of the hardware or software.
=== Interrupt controlled system ===
Some embedded systems are predominantly interrupt controlled. 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 in a predefined frequency, or by a serial port controller receiving a byte.
These kinds of systems are used if event handlers need low latency and the event handlers are short and simple.
Usually these kinds of 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 ===
A [[nonpreemptive multitasking]] system is very similar to the simple control loop scheme, except that the loop is hidden in an [[Application programming interface|API]]. The programmer defines a series of tasks, and each task gets its own environment to "run" in. Then, when a task is idle, it calls an idle routine (usually called "pause", "wait", "yield", "nop" (Stands for no operation), etc.).
The advantages and disadvantages are very similar to the control loop, except that adding new software is easier, by simply writing a new task, or adding to the queue-interpreter.
=== 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 (connected to 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 larger systems using an MMU) 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 buy a [[real-time operating system]], allowing the application programmers to concentrate on device functionality rather than operating system services, at least for large systems; smaller systems often cannot afford the overhead associated with a ''generic'' real time system, due to limitations regarding memory size, performance, and/or battery life.
=== Microkernels and exokernels ===
A [[microkernel]] is a logical step up from a real-time OS. The usual arrangement is that the operating system kernel allocates memory and switches the CPU to different threads of execution. User mode processes implement major functions such as file systems, network interfaces, etc.
In general, microkernels succeed when the task switching and intertask communication is fast, and fail when they are slow.
[[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 ===
In this case, a relatively large kernel with sophisticated capabilities is 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]] 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 [[Routers|Wireless Routers]] and [[Automotive navigation system|GPS Navigation Systems]]. Here are some of the reasons:
* Ports to common embedded chip sets are available.
* They permit re-use of publicly available code for [[Device driver|Device Drivers]], [[Web servers|Web Servers]], [[Firewall (networking)|Firewalls]], and other code.
* Development systems can start out with broad feature-sets, and then the distribution can be configured to exclude unneeded functionality, and save the expense of the memory that it would consume.
* Many engineers believe that running application code in user mode is more reliable, easier to debug and that therefore the development process is easier and the code more portable.
* Many embedded systems lack the tight real time requirements of a control system. A system such as Embedded Linux has fast enough response for many applications.
* Features requiring faster response than can be guaranteed can often be placed in [[Programmable logic device|hardware]].
* Many RTOS systems have a per-unit cost. When used on a product that is or will become a commodity, that cost is significant.
=== Exotic custom operating systems ===
A small fraction of embedded systems require safe, timely, reliable or efficient behavior unobtainable with the one of the above architectures. In this case an organization builds a system to suit.
In some cases, the system may be partitioned into a "mechanism controller" using special techniques, and a "display controller" with a conventional operating system. A communication system passes data between the two.
=== Additional software components ===
In addition to the core operating system, many embedded systems have additional upper-layer software components. These components consists of networking protocol stacks like TCP/IP, FTP, HTTP, and HTTPS, and also included storage capabilities like FAT and Flash memory management systems. If the embedded devices 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 are included. In the RTOS category, the availability of the additional software components depends upon the commercial offering.
== See also ==
<div style="-moz-column-count:2; column-count:2;">
* [[Communications server]]
* [[DSP]]
* [[Electronic Control Unit]]
* [[Embedded operating system]]s
* [[System on module]]
* [[System on a chip]]
* [[Firmware]]
* [[Information appliance]]
* [[Microprocessor]]
* [[Programming language]]s
* [[Real-time operating system]]
* [[Software engineering]]
* [[Ubiquitous computing]]
</div>
==References==
{{reflist}}
== Further reading ==
*John Catsoulis, [http://www.oreilly.com/catalog/dbhardware2/ Designing Embedded Hardware], O'Reilly, May 2005, ISBN: 0-596-00755-8.
* Anoop MS, [http://www.infosecwriters.com/text_resources/pdf/Anoopms_Embedded_Systems.pdf Security needs in embedded systems], Tata Elxsi, India, May 2008.
==External links==
{{Wikibooks|Embedded Systems}}
{{Wikiversity|Embedded System Engineering}}
*[http://www.esi.nl/ The Embedded Systems Institute]
*[http://www.FreeRTOS.org/implementation/ How a real time operating system works]
*[http://electronicdesign.com/subject/embedded Embedded in Electronic Design]
*[http://www.embedded.com Home of the ''Embedded Systems Programming'' magazine]
*[http://embeddedsystemnews.com Embedded System News]
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