Anembedded system is acomputer that has been built to solve only a few very specific problems and is not easily changed.[1] The wordembedded means it is built into the system. It is a permanent part in a bigger system. It usually does not look like a computer, often no keyboard or monitor or mouse. But like any computer it has aprocessor,software, input and output.
For example, the controller embedded in anelevator tells the motor to move the elevator to different floors, based on buttons that are pushed. A decoder is embedded in asatellite televisionset-top box to read a signal from the dish and send something that a TV understands. Often this type of system must do its work in a specific amount of time. This is calledreal-time computing. If a set-top box got interrupted to do another task, you would see a bad picture on the TV, for example. A general purpose computer will often have short pauses while it does something else, it is not real-time.
Embedded systems control many of the commondevices such as card readers in hotel door locks or several things in a car. They can control small things like anMP3 player or adigital camera, and large things liketraffic light systems,airplanes, orassembly lines in a factory.
Embedded systems are used in many kinds of electrical devices, including:
Embedded systems are not always standalone devices. Sometimes they are built as a set, like the various parts of a car - the radio, the throttle control, thepollution control, etc.[2] Sometimes they can communicate to the internet or a cell-phone network and they may have aUSB reader or other connections.
Embedded systems range from nouser interface at all — only sending and receiving electric signals — to a fullgraphical user interface like on a modern computer. Quite often they will have a fewpush-buttons and a small display and someLEDs. A more complex system may have atouch screen, allowing the meaning of the buttons to change with each screen as insmart phones.
Hardware includes the chips, wires,circuit boards, buttons and displays.
The most important chip is thecentral processing unit or CPU. This runs the software instructions. It can be a standardmicroprocessor or amicrocontroller. Microcontrollers include the microprocessor as well as simpleperipheral equipment so the system can be smaller and cheaper. They have less flexibility because those parts cannot be changed. Usually these parts includeFlash memory and support forserial ports,USB, etc.
Unlike a microprocessor for a general-purpose computer, bigger and faster is not always better. Many embedded processors are very small. Sometimes this is to use less space or less power, sometimes it is to be cheaper. General-purpose computers will use microprocessors that read 32-bit or 64-bit words and run at speeds measured in GHz, but embedded processors are usually 4 to 32 bits and run at speeds usually measured in tens of MHz (a hundred times slower). (But the programs are also smaller and do not check for things that are not used).
There are"ready-made" computer boards that can be used in some embedded systems. These often useWindows CE,Linux,NetBSD, or an embeddedreal-time operating system.
Sometimes it may be easier to use acircuit board that is already made. These usually share manycomponents with general purpose computers, but are smaller than one in a general purpose computer. Boards such as the VIA EPIA can runMicrosoft Windows. The advantage is that it saves someelectrical engineering time and can use the samesoftware development tools used for PC-type software development. Examples of such embedded devices are theATMs or displays incasinos. This works well if the real-time requirements are not strict (it doesn't matter much if a job takes eight seconds instead of five, for example).
If the device needs to be very small or will be sold in very big numbers ("high-volume"), it makes sense to make a custom or specialized chip that does exactly what is needed. This is asystem on a chip (SoC) which holds a complete system -processor,floating point unit, memorycache andinterfaces on a singleintegrated circuit. SoCs can be made as a special-orderapplication-specific integrated circuit or by using afield-programmable gate array which is programmed by the people building the embedded system.
Embedded systems talk with the outside world or other components usingperipherals such as:
Embedded systems often do not need a fulloperating system. Some use specially-built small and simple operating systems that start very quickly, others do not need one at all. Embedded systems are not adapted as easily, but they are built to perform their tasks much more reliably. Because the hardware is simpler, it is also often cheaper to build and runs faster. In contrast to this, a general-purpose computer needs to be ready for newdevice drivers and software to runhardware it doesn't know about yet, like new printers orhard drives. It needs to run different application programs.
As embedded systems get bigger, things that used to be only on general-purpose computers or evenmainframes are now becoming common on embedded systems. This includes protected memory space, andopen programming environment includingLinux,NetBSD, etc.
Some examples of operating systems, from simple to complex:
Real-time operating systems include products likeMicroC/OS-II, Green Hills INTEGRITY,QNX orVxWorks. UnlikeMacOS orWindows 7, these operating systems are not known very well by most people. But they are used in many places where time and safety is very important. People use them every day and do not realise it.
Common examples of larger kernels areEmbedded Linux andWindows CE. Although these do not have the tight time limits needed for a strict real-time system, they are becoming more common, especially for more powerful devices such asWireless Routers andGPS. They allow re-use of code in thepublic domain forDevice Drivers,Web Servers,Firewalls, and other code. Software developers that are more comfortable writing applications for PCs will find this more familiar as well. If needed, an FPGA or other special hardware can be used for things that do need tight time limits.
Like othersoftware, embedded system designers usecompilers,assemblers, anddebuggers to develop embeddedsystem software. However, they may also use some more specific tools:
Debugging tools:
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 asassembly code orsource-code. Some integrated systems (likeVxWorks orGreen Hills) have special features, like keeping track of how much space the software takes as it runs, what tasks are running, and when things happen.
Depending on what kind of embedded system is being made will affect how it can be debugged. For instance, debugging a single microprocessor system is different from debugging a system where processing is also done on a peripheral (DSP, FPGA, co-processor).
Embedded systems are often in machines that are expected to run for years without errors, and in some cases recover by themselves if an error occurs. This means the software is usually developed and tested more carefully than that for personal computers, and unreliable mechanical moving parts such asdisk drives and fans are avoided.
Places where safety and reliability are important:
Ways to recover from errors — both software bugs such asmemory leaks, and alsosoft errors in the hardware:
