An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions^[1], often with real-time computing constraints. A computer is a Machine that manipulates data according to a list of instructions. In Computer science, real-time computing (RTC is the study of hardware and software systems that are subject to a "real-time constraint"—i 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. A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated 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 players, to large stationary installations like traffic lights, factory controllers, or the systems controlling nuclear power plants. A digital audio player, more commonly referred to as an MP3 player, is a Consumer electronics device that stores organizes and plays audio files Some The traffic light, also known as traffic signal, stop light, traffic lamp, stop-and-go lights, robot or semaphore, Nuclear power is any Nuclear technology designed to extract usable Energy from atomic nuclei via controlled Nuclear reactions 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. A microcontroller (also MCU or µC is a functional Computer system-on-a- chip.

In general, "embedded system" is not an exactly defined term, as many systems have some element of programmability. For example, Handheld computers 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. A mobile device (also known as cellphone device, handheld device, handheld computer, "Palmtop" or simply handheld) is a pocket-sized

Contents 1 Examples of embedded systems 2 History 3 Characteristics 3.1 User interfaces 3.2 Simple systems 3.3 In more complex systems 3.4 CPU platforms 3.4.1 Ready made computer boards 3.4.2 ASIC and FPGA solutions 3.5 Peripherals 3.6 Tools 3.7 Debugging 3.8 Reliability 3.9 High vs Low Volume 4 Embedded software architectures 4.1 Simple control loop 4.2 Interrupt controlled system 4.3 Cooperative multitasking 4.4 Preemptive multitasking or multi-threading 4.5 Microkernels and exokernels 4.6 Monolithic kernels 4.7 Exotic custom operating systems 4.8 Additional software components 5 See also 6 References 7 Further reading 8 External links

Examples of embedded systems

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

Telecommunications systems employ numerous embedded systems from telephone switches for the network to mobile phones at the end-user. In the field of Telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls Computer networking uses dedicated routers and network bridges to route data. A router ('rautər in the USA 'rutər in the UK and Ireland, or either pronunciation in Australia and Canada is a Computer whose software and hardware are usually A network bridge connects multiple Network segments at the Data link layer (layer 2 of the OSI model, and the term layer 2 switch is often

Consumer electronics include personal digital assistants (PDAs), mp3 players, mobile phones, videogame consoles, digital cameras, DVD players, GPS receivers, and printers. Consumer electronics include electronic equipment intended for everyday use A digital audio player, more commonly referred to as an MP3 player, is a Consumer electronics device that stores organizes and plays audio files Some Many compact digital still cameras can record Sound and moving Video as well as still Photograph. This page relates to a Hardware device used to play DVDs For the Apple Software program, see DVD Player (Apple. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth Many household appliances, such as microwave ovens, washing machines and dishwashers, are including embedded systems to provide flexibility, efficiency and features. A microwave oven, or a microwave, is a Kitchen appliance that cooks or heats Food by Dielectric heating. A washing machine, or washer, is a machine designed to clean Laundry, such as Clothing, Towels and sheets The term is mostly applied A dishwasher is a mechanical device for cleaning dishes and eating utensils They can be found in Restaurants and private homes Advanced HVAC systems use networked thermostats to more accurately and efficiently control temperature that can change by time of day and season. HVAC (pronounced either "H-V-A-C" or occasionally " H-vak " is an Initialism or Acronym that stands for " Heating A thermostat is a device for regulating the Temperature of a System so that the system's temperature is maintained near a desired setpoint A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. Home automation uses wired- and wireless-networking that can be used to control lights, climate, security, audio/visual, etc. Home automation (also called smart homes or domotics) is a field within Building automation, specializing in the specific Automation requirements , 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 systems and GPS receivers that also have considerable safety requirements. Avionics means "aviation electronics" It comprises electronic systems for use on aircraft artificial satellites and spacecraft comprising Communications An Inertial Navigation System (INS is a Navigation aid that uses a Computer and motion sensors to continuously track the position orientation and Velocity Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth Various electric motors — brushless DC motors, induction motors and DC motors — are using electric/electronic motor controllers. The statement Brushless DC electric motor may be misleading and controversial among professionals because it's actually an 3 phase AC motor driven by a DC source An induction motor (IM is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction. A DC motor is an Electric motor that runs on Direct current (DC electricity A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an Electric motor. Automobiles, electric vehicles. The Electric Vehicle was an American Automobile manufactured only in 1899 and hybrid vehicles are increasingly using embedded systems to maximize efficiency and reduce pollution. A hybrid vehicle is a vehicle that uses two or more distinct power sources to propel the vehicle Other automotive safety systems such as anti-lock braking system (ABS), Electronic Stability Control (ESC/ESP), and automatic four-wheel drive. An anti-lock braking system, or ABS (from the German, A nti' b' lockier' s' ystem) is a safety system on Motor vehicles Electronic stability control ( ESC) is a computerized technology that improves the safety of a vehicle's handling by detecting and preventing skids Four-wheel drive, 4WD, or 4x4 ("four by four" is a four-wheeled Vehicle with a drivetrain that allows all four Wheels to

Medical equipment is continuing to advance with more embedded systems for vital signs monitoring, electronic stethoscopes for amplifying sounds, and various medical imaging (PET, SPECT, CT, MRI) for non-invasive internal inspections. Medical equipment is designed to aid in the diagnosis monitoring or treatment of medical conditions Vital signs are measures of various physiological statistics often taken by Health professionals in order to assess the most basic body functions The stethoscope (from Greek στηθοσκόπιο, of στήθος stéthos - chest and σκοπή skopé - examination) is an acoustic Medical imaging refers to the techniques and processes used to create Images of the human body (or parts thereof for clinical purposes ( Medical procedures seeking to Positron emission tomography ( PET) is a Nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the Single photon emission computed tomography (SPECT or less commonly SPET is a Nuclear medicine tomographic imaging technique using Gamma rays. Computed tomography (CT is a Medical imaging method employing Tomography.

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. A programmable logic controller ( PLC) or programmable controller is a Digital computer used for Automation of industrial processes such as In Engineering, electromechanics combines the Sciences of Electromagnetism of Electrical engineering and mechanics.

One of the first recognizably modern embedded systems was the Apollo Guidance Computer, developed by Charles Stark Draper at the MIT Instrumentation Laboratory. Charles Stark Draper ( October 2, 1901 &ndash July 25, 1987) was an American Scientist and Engineer, often referred 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. The LGM-30 Minuteman is a United States nuclear Missile, a land-based Intercontinental ballistic missile (ICBM It was built from transistor logic and had a hard disk for main memory. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device 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. Definition The NAND operation is a Logical operation on two Logical values typically the values of two Propositions that produces a value

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 calculators and other small systems but still required many external memory and support chips. A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated The Intel 4004 is a 4-bit Central processing unit (CPU released by Intel Corporation in 1971 A calculator is device for performing mathematical calculations distinguished from a Computer by having a limited problem solving ability and an interface optimized for interactive 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 potentiometers and variable capacitors with up/down buttons or knobs read out by a microprocessor even in some consumer products. Analogue electronics (or analog in American English) are those electronic systems with a continuously Variable signal A potentiometer is a three-terminal Resistor with a sliding contact that forms an adjustable Voltage divider. A variable capacitor is a Capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically 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. A microcontroller (also MCU or µC is a functional Computer system-on-a- chip.

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-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. In Computer science, real-time computing (RTC is the study of hardware and software systems that are subject to a "real-time constraint"—i
2. Embedded systems are not always separate devices. Most often they are physically built-in to the devices they control. .
3. The software written for embedded systems is often called firmware, and is stored in read-only memory or Flash memory chips rather than a disk drive. In Computing, firmware is a computer program that is Embedded in a hardware device for example a Microcontroller. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed 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 full user interfaces similar to desktop operating systems in devices such as PDAs.

Simple systems

Simple embedded devices use buttons, LEDs, and small character- or digit-only displays, often with a simple menu system. In Computing and Telecommunications a menu is a list of commands presented to an operator by a Computer or Communications system.

In more complex systems

A full graphical screen, with 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. A Touch Screen is a display which can detect the presence and location of a touch within the display area

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. The World Wide Web (commonly shortened to the Web) is a system of interlinked Hypertext documents accessed via the Internet. 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 architectures are used; there are Von Neumann as well as various degrees of Harvard architectures, RISC as well as non-RISC and VLIW; word lengths vary from 4-bit to 64-bits and beyond (mainly in DSP processors) although the most typical remain 8/16-bit. CPU design is the Design engineering task of creating a Central processing unit (CPU a component of Computer hardware. The von Neumann architecture is a design model for a stored-program Digital computer that uses a processing unit and a single separate storage structure The Harvard architecture is a Computer architecture with physically separate storage and signal pathways for instructions and data Very Long Instruction Word or VLIW refers to a CPU architecture designed to take advantage of Instruction level parallelism (ILP A digital signal processor ( DSP or DSP micro) is a specialized Microprocessor designed specifically for Digital signal processing, generally 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, AVR, Blackfin, C167, Coldfire, COP8, eZ8, eZ80, FR-V, H8, HT48, M16C, M32C, MIPS, MSP430, PIC, PowerPC, R8C, SHARC, ST6, SuperH, TLCS-47, TLCS-870, TLCS-900, Tricore, V850, x86, XE8000, Z80, etc. The WDC 65816 (also 65C816) a 16-bit Microprocessor CPU developed by the Western Design Center (WDC, is an expanded and compatible The Western Design Center WDC 65C02 Microprocessor is an upgraded CMOS version of the popular NMOS -based MOS Technology 6502 The 68HC08 ( HC08 in short is a broad family of 8-bit Microcontrollers from Freescale Semiconductor (formerly Motorola Semiconductor The 68HC11 ( 6811 or HC11 for short is a 8-bit Microcontroller (µC family originally from Motorola now produced by Freescale Semiconductor The Motorola 680x0 / m68k / 68k / 68K is a family of 32-bit CISC Microprocessor CPU chips and was the primary The Intel 8051 is a Harvard architecture, single chip Microcontroller (µC which was developed by Intel in 1980 for use in Embedded systems The ARM architecture (previously the Advanced RISC Machine, and prior to that Acorn RISC Machine) is a 32-bit RISC processor architecture The AVR is a Modified Harvard architecture 8-bit RISC single chip Microcontroller (µC which was developed by Atmel in 1996 Blackfin refers to a family of 16/32-bit Microprocessors with built-in Digital Signal Processor ( DSP) functionality which is traditionally only accompanied by The C167 family is a 16 bit microcontroller architecture from Infineon. The Freescale ColdFire is a 68k architecture Microprocessor manufactured for Embedded systems development by Freescale Semiconductor (formerly The COP8 microcontroller from National Semiconductor is an 8 bit CISC core Microcontroller, whose main features are Large amount The Zilog Z8 is a Microcontroller architecture originally introduced in 1979 which today also includes the eZ8 Encore!, eZ8 Encore! XP, and The ZiLOG eZ80 is an 8-bit Microprocessor which is essentially an updated version of the company's earlier Z80 8-bit microprocessor The Fujitsu FR-V is a VLIW -based RISC Microprocessor, including FR-400 and FR-450 which runs Linux, and are also supported by H8 is the name of a large family of 8-bit and 16-bit Microcontrollers made by Renesas Technology, originating in the early 1990s within Hitachi The Holtek HT48FXX I/O Series is a series of Holtek devices optimised for multiple I/O control Digital applications MIPS (originally an acronym for Microprocessor without Interlocked Pipeline Stages) is a RISC microprocessor architecture developed by MIPS Technologies The MSP430 is a Microcontroller family from Texas Instruments. PIC is a family of Harvard architecture Microcontrollers made by Microchip Technology, derived from the PIC1640 originally developed by General Instrument PowerPC is a RISC Instruction set architecture created by the 1991 Apple – IBM – Motorola alliance known as AIM The Super Harvard Architecture Single-Chip Computer ( SHARC) is a high performance Floating-point and Fixed-point DSP from Analog Devices The SuperH (or SH) is brandname of a certain Microcontroller and Microprocessor architecture The Toshiba TLCS series are a family of CISC and RISC Microcontrollers from Toshiba. The Toshiba TLCS series are a family of CISC and RISC Microcontrollers from Toshiba. The Toshiba TLCS series are a family of CISC and RISC Microcontrollers from Toshiba. TriCore is the name of a multicore-processor design by Infineon Technologies. The NEC Electronics Corporation V850 is a 32-bit embedded RISC Microcontroller originally developed and manufactured by NEC, succeeded by V850 See also X86 assembly language The generic term x86 refers to the most commercially successful Instruction set architecture in the history of Personal The XE8000 series is a low-power Microcontroller family from XEMICS (now a business unit of Semtech) The Zilog Z80 is an 8-bit Microprocessor designed and sold by Zilog from July 1976 onwards

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. PC/104 (or PC104) is an Embedded computer standard controlled by the PC/104 Consortium which defines both a Form factor These often use DOS, Linux, NetBSD, or an embedded real-time operating system such as MicroC/OS-II, QNX or VxWorks. DOS, short for "Disk Operating System" is a shorthand term for several closely related Operating systems that dominated the IBM PC compatible market Linux (commonly pronounced ˈlɪnəks NetBSD is a freely redistributable Open source version of the Unix -derivative Berkeley Software Distribution (BSD Computer Operating A real-time operating system ( RTOS; generally pronounced as "are-toss" is a multitasking Operating system intended for real-time UCOS redirects here For the fictional police organisation see New Tricks (TV series MicroC/OS-II (commonly termed µC/OS-II or QNX (pronounced either Q-N-X or Q-nix is a commercial Unix-like Real-time operating system, aimed primarily at the Embedded systems market VxWorks is a real-time Operating system made and sold by Wind River Systems of Alameda, California, USA

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. System-on-a-chip or system on chip ( SoC or SOC) refers to integrating all components of a Computer or other electronic System A related scheme is to use a field-programmable gate array (FPGA), and program it with all the logic, including the CPU. FPGAs should not be confused with the Flip-chip pin grid array, a form of integrated circuit packaging

Peripherals

Embedded Systems talk with the outside world via peripherals, such as:

• Serial Communication Interfaces (SCI): RS-232, RS-422, RS-485 etc
• Synchronous Serial Communication Interface: I2C, JTAG, SPI, SSC and ESSI
• Universal Serial Bus (USB)
• Networks: Ethernet, Controller Area Network, LonWorks, etc
• Timers: PLL(s), Capture/Compare and Time Processing Units
• 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 compilers, assemblers, and debuggers to develop embedded system software. For an account of the words periphery and peripheral as they are used in biology sociology politics computer hardware and other fields see the In Telecommunications, RS-232 (Recommended Standard 232 is a standard for serial binary data signals connecting between a DTE ( Data Terminal Equipment American national standard ANSI/TIA/EIA-422-B (formerly RS-422) and its international equivalent ITU-T Recommendation V In Telecommunications, EIA-485 (formerly RS-485 or RS485) is an Electrical Specification of a two- Wire, half-duplex I²C ( Inter-Integrated Circuit) is a multi-master serial Computer bus invented by Philips that is used to attach low-speed peripherals Joint Test Action Group ( JTAG) is the usual name used for the IEEE 1149 Interface The SPI bus specifies four logic signals SCLK &mdash Serial Clock (output from master MOSI/SIMO &mdash Master Output Slave Input (output Ethernet is a family of frame -based Computer networking technologies for Local area networks (LANs LonWorks is a networking platform specifically created to address the unique performance reliability installation and maintenance needs of Control applications The platform A phase-locked loop or phase lock loop (PLL is a Control system that generates a signal that has a fixed relation to the phase of a "reference" Time Processor Unit The time processor unit has some of the characteristics of a processor General (GPIO is a acronym of General Purpose Input/Output A Microprocessor, Microcontroller or interface device may have one or more GPIO connections A compiler is a Computer program (or set of programs that translates text written in a computer language (the source language) into another See the terminology section below for information regarding inconsistent use of the terms assembly and assembler A debugger is a Computer program that is used to test and Debug other programs However, they may also use some more specific tools:

• In circuit debuggers or emulators (see next section).
• Utilities to add a checksum or CRC to a program, so the embedded system can check if the program is valid. A cyclic redundancy check (CRC is a type of function that takes as input a data stream of any length and produces as output a value of a certain space commonly a 32-bit integer
• For systems using digital signal processing, developers may use a math workbench such as MATLAB, Simulink, MathCad, or Mathematica to simulate the mathematics. Digital signal processing ( DSP) is concerned with the representation of the signals by a sequence of numbers or symbols and the processing of these signals MATLAB is a numerical computing environment and Programming language. Simulink, developed by The MathWorks, is a commercial tool for modeling simulating and analyzing multidomain dynamic systems Mathcad is desktop software for performing and documenting engineering and scientific calculations Mathematica is a computer program used widely in scientific engineering and mathematical fields 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. DSPnano is an embedded Real-time operating system (RTOS which is 100% compatible with POSIX and offers a tiny tiny Embedded Linux compatible solution The Unison Operating System (RTOS is a SoC, Multi-core and DSP optimized Open source RTOS which offers a tiny tiny Linux compatible
• 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 Basic. Forth is a structured, imperative, stack-based, computer Programming language and programming environment The BASIC Stamp is a Microcontroller with a small specialized BASIC interpreter ( PBASIC) built into ROM.
• Another alternative is to add a Real-time operating system or Embedded operating system, which may have DSP capabilities like DSPnano RTOS. A real-time operating system ( RTOS; generally pronounced as "are-toss" is a multitasking Operating system intended for real-time An embedded operating system is an Operating system for embedded computer systems. DSPnano is an embedded Real-time operating system (RTOS which is 100% compatible with POSIX and offers a tiny tiny Embedded Linux compatible solution

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. GNU ( pronounced) is a computer Operating system composed entirely of Free software. For example, cellphones, personal digital assistants 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. Linux (commonly pronounced ˈlɪnəks NetBSD is a freely redistributable Open source version of the Unix -derivative Berkeley Software Distribution (BSD Computer Operating The OSGi Alliance (formerly known as the Open Services Gateway initiative now an obsolete name is an open Standards organization founded in March 1999 In discussions of J2ME Java specifications such as the CDC / CLDC device capability sets and the MIDP profile definition attention needs to be given to

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. The Remedy debugger was the first embedded system level Debugger in the world
• An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via a JTAG or NEXUS interface. Joint Test Action Group ( JTAG) is the usual name used for the IEEE 1149 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. An in-circuit emulator (ICE is a hardware device used to debug the Software of an Embedded system.
• 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. An emulator duplicates (provides an emulation of the functions of one System using a different system so that the second system behaves like (and appears to

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. See the terminology section below for information regarding inconsistent use of the terms assembly and assembler In Computer science, source code (commonly just source or code) is any sequence of statements or declarations written in some Human-readable

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:

1. 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.
2. 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.
3. 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 errors 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 portable music players or mobile phones, minimizing cost is usually the primary design consideration. In Electronics and Computing, an Error is a signal or datum which is wrong When writing Firmware for an Embedded system, immunity aware programming is a set of programming techniques used in an attempt to tolerate transient errors in A digital audio player, more commonly referred to as an MP3 player, is a Consumer electronics device that stores organizes and plays audio files Some 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. A real-time operating system ( RTOS; generally pronounced as "are-toss" is a multitasking Operating system intended for real-time

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 API. Nonpreemptive multitasking is a style of computer multitasking in which the operating system never initiates a context switch from a running process to 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 queues, semaphores or a non-blocking synchronization scheme. In Computer science, a message queue is a software-engineering component used for Interprocess communication or inter- thread communication For other uses see Semaphore. A semaphore, in computer science is a protected Variable (an entity storing a value or Abstract In Computer science, non-blocking synchronization ensures that threads competing for a shared resource do not have their execution indefinitely

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. A real-time operating system ( RTOS; generally pronounced as "are-toss" is a multitasking Operating system intended for real-time

Microkernels and exokernels

A microkernel is a logical step up from a real-time OS. A microkernel is a minimal Computer Operating system kernel which in its purest form provides no operating-system services at all only the 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.

Exokernels communicate efficiently by normal subroutine calls. Exokernel is an Operating system kernel developed by the MIT Parallel and Distributed Operating Systems group and also a class of similar operating systems 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. Linux (commonly pronounced ˈlɪnəks Microsoft Windows is a series of Software Operating systems and Graphical user interfaces produced by Microsoft.

Common examples of embedded monolithic kernels are Embedded Linux and Windows CE. Embedded Linux is the use of a Linux Operating system in Embedded computer systems such as Mobile phones Personal digital assistants Windows CE (also known officially as Windows Embedded Compact post version 6

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. A router ('rautər in the USA 'rutər in the UK and Ireland, or either pronunciation in Australia and Canada is a Computer whose software and hardware are usually An automotive navigation system is a satellite navigation system designed for use in Automobiles. Here are some of the reasons:

• Ports to common embedded chip sets are available.
• They permit re-use of publicly available code for Device Drivers, Web Servers, Firewalls, and other code. In computing a device driver or software driver is a Computer program allowing higher-level computer programs to interact with a Hardware device The term web server can mean one of two things A Computer program that is responsible for accepting HTTP requests from web clients which are A firewall is an integrated collection of security measures designed to prevent unauthorized electronic access to a networked computer system
• 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 hardware. A programmable logic device or PLD is an electronic component used to build reconfigurable Digital circuits Unlike a Logic gate, which has a
• 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

References

Further reading

• John Catsoulis, Designing Embedded Hardware, O'Reilly, May 2005, ISBN: 0-596-00755-8.
• Anoop MS, Security needs in embedded systems, Tata Elxsi, India, May 2008.

External links

• The Embedded Systems Institute
• How a real time operating system works
• Embedded in Electronic Design
• Home of the Embedded Systems Programming magazine

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