Embedded system
Encyclopedia
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
(PC), is designed to be flexible and to meet a wide range of end-user needs. Embedded systems control many devices in common use today.
Embedded systems contain processing cores that are typically either microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s or digital signal processor
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
s (DSP). The key characteristic, however, is being dedicated to handle a particular task. They may require very powerful processors and extensive communication, for example air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
systems may usefully be viewed as embedded, even though they involve mainframe computers and dedicated regional and national networks between airports and radar sites (each radar probably includes one or more embedded systems of its own).
Since the embedded system is dedicated to specific tasks, design engineers can optimize it to reduce the size and cost of the product and increase the reliability and performance. Some embedded systems are mass-produced, benefiting from economies of scale
Economies of scale
Economies of scale, in microeconomics, refers to the cost advantages that an enterprise obtains due to expansion. There are factors that cause a producer’s average cost per unit to fall as the scale of output is increased. "Economies of scale" is a long run concept and refers to reductions in unit...
.
Physically, embedded systems range from portable devices such as digital watches and MP3 players, to large stationary installations like traffic light
Traffic light
Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic...
s, factory controllers
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
, or the systems controlling nuclear power plant
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...
s. Complexity varies from low, with a single microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
chip, to very high with multiple units, peripheral
Peripheral
A peripheral is a device attached to a host computer, but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture....
s and networks mounted inside a large chassis
Chassis
A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...
or enclosure.
In general, "embedded system" is not a strictly definable term, as most systems have some element of extensibility or programmability. For example, handheld computers share some elements with embedded systems such as the operating systems and microprocessors that power them, but they allow different applications to be loaded and peripherals to be connected. Moreover, even systems that do not expose programmability as a primary feature generally need to support software updates. On a continuum from "general purpose" to "embedded", large application systems will have subcomponents at most points even if the system as a whole is "designed to perform one or a few dedicated functions", and is thus appropriate to call "embedded".
Variety of embedded systems
.jpg)
Telecommunications systems employ numerous embedded systems from telephone switches for the network to mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s at the end-user.
Computer networking uses dedicated routers and network bridges to route data.
Consumer electronics
Consumer electronics
Consumer electronics are electronic equipment intended for everyday use, most often in entertainment, communications and office productivity. Radio broadcasting in the early 20th century brought the first major consumer product, the broadcast receiver...
include personal digital assistant
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
s (PDAs), mp3 players, mobile phones, videogame consoles, digital camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
s, DVD player
DVD player
A DVD player is a device that plays discs produced under both the DVD-Video and DVD-Audio technical standards, two different and incompatible standards. These devices were invented in 1997 and continue to thrive...
s, GPS
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
receivers, and printer
Computer printer
In computing, a printer is a peripheral which produces a text or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most new printers, a...
s.
Many household appliances, such as microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
s, washing machine
Washing machine
A washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
s and dishwashers, are including embedded systems to provide flexibility, efficiency and features.
Advanced HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
systems use networked thermostat
Thermostat
A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...
s to more accurately and efficiently control temperature that can change by time of day and season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
.
Home automation
Home automation
Home automation is the residential extension of "building automation". It is automation of the home, housework or household activity. Home automation may include centralized control of lighting, HVAC , appliances, and other systems, to provide improved convenience, comfort, energy efficiency and...
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
Avionics
Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...
such as inertial guidance systems and GPS
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
receivers that also have considerable safety requirements.
Various electric motors — brushless DC motors, induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
s and DC motor
DC motor
A DC motor is an electric motor that runs on direct current electricity.-Brush:The brushed DC electric motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets , and rotating electrical magnets.Like all electric motors or generators,...
s — are using electric/electronic motor controller
Motor controller
A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and...
s.
Automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s, electric vehicle
Electric vehicle
An electric vehicle , also referred to as an electric drive vehicle, uses one or more electric motors or traction motors for propulsion...
s, and hybrid vehicle
Hybrid vehicle
A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles , which combine an internal combustion engine and one or more electric motors.-Power:...
s are increasingly using embedded systems to maximize efficiency and reduce pollution.
Other automotive safety systems include anti-lock braking system
Anti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...
(ABS), Electronic Stability Control
Electronic Stability Control
Electronic stability control is a computerized technology that may potentially improve the safety of a vehicle's stability by detecting and minimizing skids. When ESC detects loss of steering control, it automatically applies the brakes to help "steer" the vehicle where the driver intends to go...
(ESC/ESP), traction control
Traction control system
A traction control system , also known as anti-slip regulation , is typically a secondary function of the anti-lock braking system on production motor vehicles, designed to prevent loss of traction of driven road wheels...
(TCS) and automatic four-wheel drive
Four-wheel drive
Four-wheel drive, 4WD, or 4×4 is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously...
.
Medical equipment
Medical equipment
Medical equipment is designed to aid in the diagnosis, monitoring or treatment of medical conditions.-Types:There are several basic types:* Diagnostic equipment includes medical imaging machines, used to aid in diagnosis...
is continuing to advance with more embedded systems for vital signs
Vital signs
Vital signs are measures of various physiological statistics, often taken by health professionals, in order to assess the most basic body functions. Vital signs are an essential part of a case presentation. The act of taking vital signs normally entails recording body temperature, pulse rate ,...
monitoring, electronic stethoscopes for amplifying sounds, and various medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
(PET
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
, SPECT
Single photon emission computed tomography
Single-photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information...
, CT
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
, MRI
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
) for non-invasive internal inspections.
Embedded systems are especially suited for use in transportation, fire safety, safety and security, medical applications and life critical systems as these systems can be isolated from hacking and thus be more reliable. For fire safety, the systems can be designed to have 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. http://www.embeddedsystem.com
In addition to commonly described embedded systems based on small computers, a new class of miniature wireless devices called motes are quickly gaining popularity as the field of wireless sensor networking rises. Wireless sensor networking, WSN, makes use of miniaturization made possible by advanced 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 IT monitoring and control systems. These motes are completely self contained, and will typically run off a battery source for many years before the batteries need to be changed or charged.
History
One of the first recognizably modern embedded systems was the Apollo Guidance ComputerApollo Guidance Computer
The Apollo Guidance Computer provided onboard computation and control for guidance, navigation, and control of the Command Module and Lunar Module spacecraft of the Apollo program...
, developed by Charles Stark Draper
Charles Stark Draper
Charles Stark Draper was an American scientist and engineer, often referred to as "the father of inertial navigation." He was the founder and director of the MIT Instrumentation Laboratory, later renamed the Charles Stark Draper Laboratory, which under his direction designed and built the Apollo...
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, released in 1961. It was built from transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
logic
Digital circuit
Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...
and had a hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
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 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
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
for example, the Intel 4004
Intel 4004
The Intel 4004 was a 4-bit central processing unit released by Intel Corporation in 1971. It was the first complete CPU on one chip, and also the first commercially available microprocessor...
, was designed for calculator
Calculator
An electronic calculator is a small, portable, usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.The first solid-state electronic...
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 components such as potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
s and variable capacitor
Variable capacitor
A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio , or as a variable reactance, e.g...
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
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
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.
The integration of microcontrollers has further increased the applications for which embedded systems are used into areas where traditionally a computer would not have been considered. A general purpose and comparatively low-cost microcontroller may often be programmed to fulfill the same role as a large number of separate components. 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. The intangible nature of software makes it much easier to prototype and test new revisions compared with the design and construction of a new circuit not using an embedded processor.
Characteristics
1. Embedded systems are designed to do some specific task, rather than be a general-purpose computer for multiple tasks. Some also have real-timeReal-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
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.
2. 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
Gibson Robot Guitar
The Gibson Robot Guitar typically refers to a sub-class of Les Paul style guitars from Gibson. This is because the first run of limited edition Robot Guitars was exclusively made up of Les Paul bodies...
features an embedded system for tuning the strings, but the overall purpose of the Robot Guitar is, of course, to play music. Similarly, an embedded system in an automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
provides a specific function as a subsystem of the car itself.
3. The program instructions written for embedded systems are referred to as firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
, and are stored in read-only memory or Flash memory
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
chips. They run with limited computer hardware resources: little memory, small or non-existent keyboard and/or screen.
User interface
Simple embedded devices use buttons, LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
s, graphic or character LCDs (for example popular HD44780 LCD) with a simple menu system
Menu (computing)
In computing and telecommunications, a menu is a list of commands presented to an operator by a computer or communications system. A menu is used in contrast to a command-line interface, where instructions to the computer are given in the form of commands .Choices given from a menu may be selected...
.
More sophisticated devices which use a graphical screen with touch sensing or screen-edge buttons 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's desired. Handheld systems often have a screen with a "joystick button" for a pointing device.
Some systems provide user interface remotely with the help of a serial (e.g. RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, USB, I²C
I²C
I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; Inter-Integrated Circuit;...
, etc.) or network (e.g. Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
) connection. In spite of the potentially necessary proprietary client software and/or specialist cables that are needed, this approach usually gives a lot of advantages: extends the capabilities of embedded system, avoids the cost of a display, simplifies BSP
Board support package
In embedded systems, a board support package is implementation specific support code for a given board that conforms to a given operating system...
, allows to build rich user interface on the PC. A good example of this is the combination of an embedded web server
Embedded HTTP server
An embedded HTTP server is a component of a software system that implements the HTTP protocol. Examples of usage within an application might be:* To provide a thin-client interface for a traditional application.....
running on an embedded device (such as an IP camera
IP camera
An Internet protocol camera, or IP camera, is a type of digital video camera commonly employed for surveillance, and which unlike analog closed circuit television cameras can send and receive data via a computer network and the Internet...
) or a network routers. The user interface is displayed in a web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
on a PC connected to the device, therefore needing no bespoke software to be installed.
Processors in embedded systems
Secondly, 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 architectures are used; there are Von NeumannVon Neumann architecture
The term Von Neumann architecture, aka the Von Neumann model, derives from a computer architecture proposal by the mathematician and early computer scientist John von Neumann and others, dated June 30, 1945, entitled First Draft of a Report on the EDVAC...
as well as various degrees of Harvard architecture
Harvard architecture
The Harvard architecture is a computer architecture with physically separate storage and signal pathways for instructions and data. The term originated from the Harvard Mark I relay-based computer, which stored instructions on punched tape and data in electro-mechanical counters...
s, RISC as well as non-RISC and VLIW; word lengths vary from 4-bit to 64-bits and beyond (mainly in DSP
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
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
68k
The Motorola 680x0/m68000/68000 is a family of 32-bit CISC microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and were the primary competitors of Intel's x86 microprocessors...
, 78K0R/78K0, 8051
Intel 8051
The Intel MCS-51 is a Harvard architecture, single chip microcontroller series which was developed by Intel in 1980 for use in embedded systems. Intel's original versions were popular in the 1980s and early 1990s. While Intel no longer manufactures the MCS-51, binary compatible derivatives remain...
, ARM
ARM architecture
ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...
, AVR
Atmel AVR
The AVR is a modified Harvard architecture 8-bit RISC single chip microcontroller which was developed by Atmel in 1996. The AVR was one of the first microcontroller families to use on-chip flash memory for program storage, as opposed to one-time programmable ROM, EPROM, or EEPROM used by other...
, AVR32
AVR32
The AVR32 is a 32-bit RISC microprocessor architecture designed by Atmel. The microprocessor architecture was designed by a handful of people educated at the Norwegian University of Science and Technology, including lead designer Øyvind Strøm, PhD and CPU architect Erik Renno, M.Sc in Atmel's...
, Blackfin
Blackfin
The Blackfin is a family of 16- or 32-bit microprocessors developed, manufactured and marketed by Analog Devices. The family is characterized by their built-in, fixed-point digital signal processor functionality supplied by 16-bit Multiply–accumulates , accompanied on-chip by a small and...
, C167
C167 family
The C167 family is a 16 bit microcontroller architecture from Infineon. It is a further development of the C166 family, which has been brought by Siemens 1993 on the market as controller for measurement and control tasks....
, Coldfire, COP8
COP8
The COP8 microcontroller from National Semiconductor is an 8 bit CISC core microcontroller, whose main features are:* Large amount of I/O pins.* Plenty of Flash memory/ROM for code and data .* Very low EMI. No known bugs....
, Cortus APS3, eZ8
Zilog Z8
The Zilog Z8 is a microcontroller architecture, originally introduced in 1979, which today also includes the eZ8 Encore!, eZ8 Encore! XP, and eZ8 Encore! MC families....
, eZ80, FR-V
FR-V
The Fujitsu FR-V is a VLIW-based RISC microprocessor, including FR-400 and FR-450 which runs Linux, and are also supported by the GNU Compiler Collection. Some processors include support with an MMU while others do not....
, H8, HT48, M16C, M32C, MIPS
MIPS architecture
MIPS is a reduced instruction set computer instruction set architecture developed by MIPS Technologies . The early MIPS architectures were 32-bit, and later versions were 64-bit...
, MSP430, PIC
PIC microcontroller
PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division...
, PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...
, R8C
R8C
The Renesas R8C is a 16-bit microcontroller that was developed as a smaller and cheaper version of the Renesas M16C . It retains the M16C's 16-bit CISC architecture and instruction set, but trades size for speed by cutting the internal data bus from 16 bits to 8 bits...
, RL78, SHARC
Super Harvard Architecture Single-Chip Computer
The Super Harvard Architecture Single-Chip Computer is a high performance floating-point and fixed-point DSP from Analog Devices,...
, SPARC
SPARC
SPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
, ST6
ST6/ST7
ST6/ST7 is an 8-bit microcontroller product line from STMicroelectronics that is based on the ST6 instruction set architecture.Common usage is main controller in appliances like washing machines....
, SuperH
SuperH
SuperH is a 32-bit reduced instruction set computer instruction set architecture developed by Hitachi. It is implemented by microcontrollers and microprocessors for embedded systems....
, TLCS-47, TLCS-870, TLCS-900, TriCore
Tricore
TriCore™ is a 32-bit microcontroller architecture from Infineon. It unites the elements of a RISC processor core, a microcontroller and a DSP in one chip package.- History and background :...
, V850
V850
The Renesas Electronics V850 is a 32-bit RISC CPU core architecture for embedded microcontrollers originally developed and manufactured by NEC, succeeded by V850 variants named V850ES, V850E, and V850E2 which run uClinux. Compilers available for it include the GNU Compiler Collection, IAR Systems...
, x86
X86 architecture
The term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...
, XE8000
XE8000
The XE8000 series is a low-power microcontroller family from XEMICS . Advanced analog features are combined with a proprietary RISC CPU named CoolRISC on all XE8000 devices. The CPU has 8-bits data bus and 22 bits instruction bus. All instructions are executed in 1 clock cycle...
, Z80, AsAP
Asynchronous Array of Simple Processors
The asynchronous array of simple processors architecture comprises a 2-D array of reduced complexity programmable processors with small memories interconnected by a reconfigurable mesh network...
etc.
Ready made computer boards
PC/104PC/104
PC/104 is an embedded computer standard controlled by the which defines both a form factor and computer bus. PC/104 is intended for specialized embedded computing environments where applications depend on reliable data acquisition despite an often extreme environment...
and PC/104+ are examples of standards for ready made computer boards intended for small, low-volume embedded and ruggedized systems, mostly x86-based. These are often physically small compared to a standard PC, although still quite large compared to most simple (8/16-bit) embedded systems. They often use MSDOS, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
, or an embedded real-time operating system
Real-time operating system
A real-time operating system is an operating system intended to serve real-time application requests.A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter...
such as MicroC/OS-II
MicroC/OS-II
MicroC/OS-II , is a low-cost priority-based pre-emptive real-time multitasking operating system kernel for microprocessors, written mainly in the C programming language...
, QNX
QNX
QNX is a commercial Unix-like real-time operating system, aimed primarily at the embedded systems market. The product was originally developed by Canadian company, QNX Software Systems, which was later acquired by Canadian BlackBerry-producer Research In Motion.-Description:As a microkernel-based...
or VxWorks
VxWorks
VxWorks is a real-time operating system developed as proprietary software by Wind River Systems of Alameda, California, USA. First released in 1987, VxWorks is designed for use in embedded systems.- History :...
. Sometimes these boards use non-x86 processors.
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
EPIA
VIA EPIA is a series of mini-ITX, nano-ITX and pico-ITX motherboards with integrated VIA processors...
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 ATMs 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 ISA or PCI busses. When a System-on-a-chip
System-on-a-chip
A system on a chip or system on chip is an integrated circuit that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions—all on a single chip substrate...
processor is involved, there may be little benefit to having a standarized 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 BGA
Ball grid array
A ball grid array is a type of surface-mount packaging used for integrated circuits.- Description :The BGA is descended from the pin grid array , which is a package with one face covered with pins in a grid pattern. These pins conduct electrical signals from the integrated circuit to the printed...
chips such as an ARM
ARM architecture
ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...
-based System-on-a-chip
System-on-a-chip
A system on a chip or system on chip is an integrated circuit that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions—all on a single chip substrate...
processor and peripherals, external flash memory
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
for storage, and DRAM
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
for runtime memory. The module vendor will usually provide boot software and make sure there is a selection of operating systems, usually including Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
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. Gumstix
Gumstix
Gumstix is a company founded in 2003 by Gordon Kruberg that produces small single-board computers. The name Gumstix refers to the size of the first such computer being the approximate size of a stick of gum....
product lines are a Linux-centric example of this model.
ASIC and FPGA solutions
A common array of n configuration for very-high-volume embedded systems is the system on a chip (SoC) which contains a complete system consisting of multiple processors, multipliers, caches and interfaces on a single chip. SoCs can be implemented as an application-specific integrated circuitApplication-specific integrated circuit
An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...
(ASIC) or using a field-programmable gate array
Field-programmable gate array
A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...
(FPGA).
Peripherals
Embedded Systems talk with the outside world via peripheralPeripheral
A peripheral is a device attached to a host computer, but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture....
s, such as:
- Serial Communication Interfaces (SCI): RS-232RS-232In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, RS-422, RS-485 etc. - Synchronous Serial Communication Interface: I2C, SPISerial Peripheral Interface BusThe Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard named by Motorola that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select ...
, SSC and ESSI (Enhanced Synchronous Serial Interface) - Universal Serial BusUniversal Serial BusUSB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
(USB) - Multi Media Cards (SD Cards, Compact Flash etc.)
- Networks: EthernetEthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, LonWorksLonWorksLonWorks is a networking platform specifically created to address the needs of control applications. The platform is built on a protocol created by Echelon Corporation for networking devices over media such as twisted pair, powerlines, fiber optics, and RF...
, etc. - FieldbusFieldbusFieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158....
es: CAN-BusController Area NetworkController–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer....
, LIN-BusLocal Interconnect NetworkThe LIN-Bus is a vehicle bus standard or computer networking bus-system used within current automotive network architectures....
, PROFIBUSProfibusPROFIBUS is a standard for field bus communication in automation technology and was first promoted in 1989 by BMBF...
, etc. - Timers: PLL(s), Capture/Compare and Time Processing Units
- Discrete IO: aka General Purpose Input/Output (GPIO)
- Analog to Digital/Digital to Analog (ADCAnalog-to-digital converterAn analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
/DACDigital-to-analog converterIn electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
) - Debugging: JTAGJTAGJoint Test Action Group is the common name for what was later standardized as the IEEE 1149.1 Standard Test Access Port and Boundary-Scan Architecture. It was initially devised for testing printed circuit boards using boundary scan and is still widely used for this application.Today JTAG is also...
, ISPIn-System ProgrammingIn-System Programming is the ability of some programmable logic devices, microcontrollers, and other programmable electronic chips to be programmed while installed in a complete system, rather than requiring the chip to be programmed prior to installing it into the system.The primary advantage of...
, ICSPIn Circuit Serial Programming (ICSP)In Circuit Serial Programming is a method of directly programming AVRs, the Parallax Propeller, and PIC microcontrollers.-See also:*In-system programming*Device Programmers...
, BDMBackground Debug Mode interfaceBackground Debug Mode interface is an electronic interface that allows debugging of embedded systems. Specifically, it provides in-circuit debugging functionality in microcontrollers...
Port, BITP, and DP9 ports.
Tools
As with other software, embedded system designers use compilerCompiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
s, assemblers, and debugger
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...
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 CRCCyclic redundancy checkA cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
to a program, so the embedded system can check if the program is valid. - For systems using digital signal processingDigital signal processingDigital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
, developers may use a math workbench such as ScilabScilabScilab is an open source, cross-platform numerical computational package and a high-level, numerically oriented programming language. Itcan be used for signal processing, statistical analysis, image enhancement, fluid dynamics simulations, numerical optimization, and modeling and simulation of...
/ ScicosScicosScicos is a graphical dynamical system modeler and simulator. User can create block diagrams to model and simulate the dynamics of hybrid dynamical systems and compile such models into executable code...
, MATLABMATLABMATLAB is a numerical computing environment and fourth-generation programming language. Developed by MathWorks, MATLAB allows matrix manipulations, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs written in other languages,...
/ SimulinkSimulinkSimulink, developed by MathWorks, is a commercial tool for modeling, simulating and analyzing multidomain dynamic systems. Its primary interface is a graphical block diagramming tool and a customizable set of block libraries. It offers tight integration with the rest of the MATLAB environment and...
, EICASLABEICASLABEICASLAB is a software suite providing a laboratory for automatic control design and time-series forecasting developed as final output of the European ACODUASIS Project IPS-2001-42068 funded by the European Community within the Innovation Programme...
, MathCadMathCadMathcad is computer software primarily intended for the verification, validation, documentation and re-use of engineering calculations. First introduced in 1986 on DOS, it was the first to introduce live editing of typeset mathematical notation, combined with its automatic computations...
, MathematicaMathematicaMathematica is a computational software program used in scientific, engineering, and mathematical fields and other areas of technical computing...
,or FlowStone DSP to simulate the mathematics. They might also use libraries for both the host and target which eliminates developing DSP routines as done in DSPnano RTOSDSPnano RTOSDSPnano is an embedded real-time operating system which is 100% compatible with POSIX and offers a tiny embedded Linux compatible solution. It was first created in 1996 and was one of the first pthread based real-time kernels...
and Unison Operating SystemUnison Operating SystemThe Unison Operating System is a real-time operating system optimized for system-on-a-chip , multi-core and digital signal processor systems. It is open source. It offers Linux compatibility while being much smaller than Linux...
. - 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 or BasicBASIC StampThe BASIC Stamp is a microcontroller with a small, specialized BASIC interpreter built into ROM. It is made by Parallax, Inc. and has been popular with electronics hobbyists since the early 1990s because of its low threshold of learning and ease of use due to its simple to understand BASIC...
. - Another alternative is to add a real-time operating systemReal-time operating systemA real-time operating system is an operating system intended to serve real-time application requests.A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter...
or embedded operating systemEmbedded operating systemAn embedded operating system is an operating system for embedded computer systems. These operating systems are designed to be compact, efficient, and reliable, forsaking many functions that non-embedded computer operating systems provide, and which may not be used by the specialized applications...
, which may have DSP capabilities like DSPnano RTOSDSPnano RTOSDSPnano is an embedded real-time operating system which is 100% compatible with POSIX and offers a tiny embedded Linux compatible solution. It was first created in 1996 and was one of the first pthread based real-time kernels...
. - Modeling and code generating toolsUML toolA UML tool or UML modeling tool is a software application that supports some or all of the notation and semantics associated with the Unified Modeling Language , which is the industry standard general purpose modeling language for software engineering.UML tool is used broadly here to include...
often based on state machinesUML state machineUML state machine is a significantly enhanced realization of the mathematical concept of a finite automaton in Computer Science applications as expressed in the Unified Modeling Language notation....
Software tools can come from several sources:
- Software companies that specialize in the embedded market
- Ported from the GNUGNUGNU is a Unix-like computer operating system developed by the GNU project, ultimately aiming to be a "complete Unix-compatible software system"...
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, cellphones, personal digital assistant
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
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
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
, OSGi
OSGi
The Open Services Gateway initiative framework is a module system and service platform for the Java programming language that implements a complete and dynamic component model, something that does not exist in standalone Java/VM environments...
or Embedded Java
Embedded Java
[This section needs to be updated, as Embedded Java covers much more than Java ME and Sun SPOT alone -- it also covers embedded use of Java on top of Java platforms such as JavaCard and Java SE ]...
is required so that the third-party software provider can sell to a large market.
Debugging
Embedded debuggingDebugging
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...
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 DebuggerRemedy DebuggerThe Remedy debugger was the first embedded system level debugger in the world. It offered many features that users take for granted today in the days when having a source level debugger was a luxury...
which even works for heterogeneous multicore systems. - An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via a JTAGJTAGJoint Test Action Group is the common name for what was later standardized as the IEEE 1149.1 Standard Test Access Port and Boundary-Scan Architecture. It was initially devised for testing printed circuit boards using boundary scan and is still widely used for this application.Today JTAG is also...
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 emulatorIn-circuit emulatorAn in-circuit emulator is a hardware device used to debug the software of an embedded system. It was historically in the form of bond-out processor which has many internal signals brought out for the purpose of debugging...
(ICE) replaces the microprocessor with a simulated equivalent, providing full control over all aspects of the microprocessor. - A complete emulatorEmulatorIn computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...
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 HLL
High-level programming language
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...
source-code, assembly code or mixture of both.
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
Logic analyzer
A logic analyzer is an electronic instrument which displays signals in a digital circuit. A logic analyzer may convert the captured data into timing diagrams, protocol decodes, state machine traces, assembly language, or correlate assembly with source-level software.Presently, there are three...
, for instance.
Tracing
Real-time operating systems (RTOS) often supports tracing
Tracing (software)
In software engineering, tracing is a specialized use of logging to record information about a program's execution. This information is typically used by programmers for debugging purposes, and additionally, depending on the type and detail of information contained in a trace log, by experienced...
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 behavior. A good example is RTXCview, for RTXC Quadros
RTXC Quadros
RTXC Quadros is a real time operating system written mainly in the C programming language. It is mainly intended for use in embedded systems.The RTXC RTOS was originally developed by AT Barrett and Associates in the 1970s. It is currently maintained by Quadros Systems, Inc...
by Quadros Systems, Inc..
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.
- 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
Soft error
In electronics and computing, a soft error is an error in a signal or datum which is wrong. Errors may be caused by a defect, usually understood either to be a mistake in design or construction, or a broken component. A soft error is also a signal or datum which is wrong, but is not assumed to...
s in the hardware:
- watchdog timerWatchdog timerA watchdog timer is a computer hardware or software timer that triggers a system reset or other corrective action if the main program, due to some fault condition, such as a hang, neglects to regularly service the watchdog A watchdog timer (or computer operating properly (COP) timer) is a computer...
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
- Designing with a Trusted Computing BaseTrusted computing baseThe trusted computing base of a computer system is the set of all hardware, firmware, and/or software components that are critical to its security, in the sense that bugs or vulnerabilities occurring inside the TCB might jeopardize the security properties of the entire system...
(TCB) architecture ensures a highly secure & reliable system environment - An Embedded HypervisorEmbedded HypervisorSoftware virtualization has been a major topic in the enterprise space for quite some time, but has become an important technology for embedded systems only in the last few years. The application of the technology to the enterprise space has been well described elsewhere by companies such as...
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, improving reliability. This may also allow a subsystem to be automatically shut down and restarted on fault detection. - Immunity Aware ProgrammingImmunity Aware ProgrammingWhen writing firmware for an embedded system, immunity-aware programming refers to programming techniques which improve the tolerance of transient errors in the program counter or other modules of a program that would otherwise lead to failure...
High vs low volume
For high volume systems such as portable music players or mobile phoneMobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
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
Real-time operating system
A real-time operating system is an operating system intended to serve real-time application requests.A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter...
.
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 multitaskingNonpreemptive multitasking
Nonpreemptive multitasking is a style of computer multitasking in which the operating system never initiates a context switch from a running process to another process...
system is very similar to the simple control loop scheme, except that the loop is hidden in an API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
. 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, usually called “pause”, “wait”, “yield”, “nop” (stands for no operation), etc.
The advantages and disadvantages are 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
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
) programs must be carefully designed and tested, and access to shared data must be controlled by some synchronization strategy, such as message queue
Message queue
In computer science, message queues and mailboxes are software-engineering components used for interprocess communication, or for inter-thread communication within the same process. They use a queue for messaging – the passing of control or of content...
s, semaphores
Semaphore (programming)
In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment....
or a non-blocking synchronization
Non-blocking synchronization
In computer science, a non-blocking algorithm ensures that threads competing for a shared resource do not have their execution indefinitely postponed by mutual exclusion...
scheme.
Because of these complexities, it is common for organizations to use a real-time operating system
Real-time operating system
A real-time operating system is an operating system intended to serve real-time application requests.A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter...
(RTOS), 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. The choice that an RTOS is required brings in its own issues however as the selection must be done prior to starting to the application development process. This timing forces developers to choose the embedded operating system for their device based upon current requirements and so restricts future options to a large extent. The restriction of future options becomes more of an issue as product life decreases. Additionally the level of complexity is continuously growing as devices are required to manage many variables 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
Embedded middleware
Analogous to middleware , embedded middleware is a software layer that sits "in the middle" between the embedded application and the real time operating system...
in addition to a real time operating system.
Microkernels and exokernels
A microkernelMicrokernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
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
Exokernel
Exokernel is an operating system kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems....
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 LinuxLinux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
or Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, 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
Embedded Linux
Embedded Linux is the use of Linux in embedded computer systems such as mobile phones, personal digital assistants, media players, set-top boxes, and other consumer electronics devices, networking equipment, machine control, industrial automation, navigation equipment and medical instruments...
and Windows CE
Windows CE
Microsoft Windows CE is an operating system developed by Microsoft for embedded systems. Windows CE is a distinct operating system and kernel, rather than a trimmed-down version of desktop Windows...
.
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 Routers and GPS Navigation Systems
Automotive navigation system
An automotive navigation system is a satellite navigation system designed for use in automobiles. It typically uses a GPS navigation device to acquire position data to locate the user on a road in the unit's map database. Using the road database, the unit can give directions to other locations...
. Here are some of the reasons:
- Ports to common embedded chip sets are available.
- They permit re-use of publicly available code for Device DriversDevice driverIn computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
, Web Servers, 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. Although a system such as Embedded Linux may be fast enough in order to respond to many other applications.
- Features requiring faster response than can be guaranteed can often be placed in hardwareProgrammable logic deviceA programmable logic device or PLD is an electronic component used to build reconfigurable digital circuits. Unlike a logic gate, which has a fixed function, a PLD has an undefined function at the time of manufacture...
. - 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 any 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 consist of networking protocol stacks like CAN, TCP/IP, FTP, HTTP, and HTTPSHttps
Hypertext Transfer Protocol Secure is a combination of the Hypertext Transfer Protocol with SSL/TLS protocol to provide encrypted communication and secure identification of a network web server...
, and also included storage capabilities like FAT
File Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
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.
External links
- Designing Embedded Hardware John Catsoulis, O'Reilly, May 2005, ISBN 0-596-00755-8.