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An embedded system is a special-purpose system in which the computer is completely encapsulated by the device it controls. Unlike a general-purpose computer, such as a personal computer, an embedded system performs one or a few pre-defined tasks, usually with very specific requirements. Since the system is dedicated to specific tasks, design engineers can optimize it, reducing the size and cost of the product. Embedded systems are often mass-produced, so the cost savings may be multiplied by millions of items. Handheld computers or PDAs are generally considered embedded devices because of the nature of their hardware design, even though they are more expandable in software terms. This line of definition continues to blur as devices expand. Physically, embedded systems range from portable devices such as MP3 players, to large stationary installations like traffic lights or factory controllers. Examples of embedded systems History
Characteristics 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 reason such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs. For high volume systems such as portable music players or mobile phones, minimizing cost is usually the primary design consideration. Engineers typically select hardware that is just “good enough” to implement the necessary functions. For example, a digital set-top box for satellite television has to process large amounts of data every second, but most of the processing is done by custom integrated circuits. The embedded CPU "sets up" this process, and displays menu graphics, etc. for the set-top's look and feel. For low-volume or prototype embedded systems, personal computer hardware can be used, by limiting the programs or by replacing the operating system with a real-time operating system. The software written for embedded systems is often called firmware, and is stored in ROM or Flash memory chips rather than a disk drive. It often runs with limited hardware resources: small or no keyboard, screen, and little RAM memory. Embedded systems 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. Recovery from errors may be achieved with techniques such as a watchdog timer that resets the computer unless the software periodically notifies the watchdog. 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. In between are devices with small character- or digit-only displays and a few buttons. One approach widely used in embedded systems without sophisticated displays, uses a few buttons to control a menu system, with some for movement and some for adjustments. On such devices simple, obvious, and low-cost approaches like red-yellow-green lights (mirroring traffic lights) are common. On larger screens, a touch-screen or screen-edge soft buttons also provides good flexibility while minimising space used. The advantage of this system is that the meaning of the buttons can change with the screen, and selection can be very close to the natural behavior of pointing at what's desired. Handheld systems often have a screen with a "joystick button" for a pointing device. The rise of the World Wide Web has given embedded designers another quite different option, by providing a web page interface over a network connection. This is successful for remote, permanently installed equipment. This avoids the cost of a sophisticated display, yet provides complex input and display capabilities when needed, on another computer. CPU Platform There are many different CPU architectures used in embedded designs such as ARM, MIPS, Coldfire/68k, PowerPC, X86, PIC, 8051, Atmel AVR, Renesas H8, SH, V850, FR-V, M32R etc. This in contrast to the desktop computer market, which is currently limited to just a few competing architectures. PC/104 is a typical base for small, low-volume embedded and ruggedized system design. These often use DOS, Linux, NetBSD, or an embedded real-time operating system such as QNX or Inferno. 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 was purchased as intellectual property to add to the IC's design. A related scheme is to use a field-programmable gate array (FPGA), and program it with all the logic, including the CPU. Tools As for other software, embedded system designers use compilers, assemblers, and debuggers to develop embedded system software. However, they may also use some more specific tools: Software tools can come from several sources: Debugging Embedded Debugging may be performed at different levels, depending on the facilities available, ranging from assembly- or source-level debugging with an in-circuit emulator, to output from serial debug ports, to an emulated environment running on a personal computer. 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 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. Most such open environments have a reference design that runs on a PC. Much of the software for such systems can be developed on a conventional PC. However, the porting of the open environment to the specialized electronics, and the development of the device drivers for the electronics are usually still the responsibility of a classic embedded software engineer. In some cases, the engineer works for the integrated circuit manufacturer, but there is still such a person somewhere. Start-up All embedded systems have start-up code. Usually it sets up the electronics, runs a self-test, and then starts the application code. The startup process is commonly designed to be short, such as less than a tenth of a second, though this may depend on the application. Self-Test Most embedded systems have some degree or amount of built-in self-test. In safety-critical systems, they are also run periodically or continuously. There are several basic types: Some tests may require interaction with a technician: Reliability regimes Reliability has different definitions depending on why people want it. 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. A common model for this kind of design is a state machine, which identifies a set of states that the system can be in and how it changes between them, with the goal of providing tightly defined system behaviour. This system's strength is its simplicity, and on small pieces of software the loop is usually so fast that nobody cares that its timing is not predictable. It is common on small devices with a stand-alone microcontroller dedicated to a simple task. Weaknesses of a simple control loop are that it does not guarantee a time to respond to any particular hardware event (although careful design may work around this), and that it can become difficult to maintain or add new features. Nonpreemptive multitasking A nonpreemptive multitasking system is very similar to the above, except that the loop is hidden in an API. The programmer defines a series of tasks, and each task gets its own environment to "run" in. Then, when a task is idle, it calls an idle routine (usually called "pause", "wait", "yield", etc.). An architecture with similar properties is to have an event queue, and have a loop that processes the events one at a time. 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 In this type of system, a low-level piece of code switches between tasks based on a timer. This is the level at which the system is generally considered to have an "operating system", and introduces all the complexities of managing multiple tasks running seemingly at the same time. Any piece of task code can damage the data of another task; they must be precisely separated. Access to shared data must be controlled by some synchronization strategy, such as message queues, semaphores or a non-blocking synchronization scheme. Because of these complexities, it is common for organizations to buy a real-time operating system, allowing the application programmers to concentrate on device functionality rather than operating system services. Microkernels and Exokernels A microkernel is a logical step up from a real-time OS. The usual arrangement is that the operating system kernel allocates memory and switches the CPU to different threads of execution. User mode processes implement major functions such as file systems, network interfaces, etc. In general, microkernels succeed when the task switching and intertask communication is fast, and fail when they are slow. Exokernels 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 full kernel with sophisticated capabilities is adapted to suit an embedded environment. This gives the programmers a full environment similar to a desktop operating system like Linux or Microsoft Windows, and is therefore very productive for development; on the downside, it requires considerably more hardware resources, is often more expensive, and because of the complexity of these kernels can be less predictable and reliable. Common examples of embedded monolithic kernels are Embedded Linux and Windows CE. Despite the increased cost in hardware, this type of embedded system is increasing in popularity, especially on the more powerful embedded devices such as Wireless Routers and GPS Navigation Systems. Here are some of the reasons: Exotic custom operating systems About 20% 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. Since these systems are often developed by programmers without real-time expertise, horror stories are common. However, some techniques are widely known and used by experienced implementors, but rarely taught in universities. For example, many operating systems use queues to serialize and prioritize events. At high event rates, this can exhaust memory reserves or slow responses. In these cases, "private drivers" run directly from interrupts may summarize a sequence for an operating system. To prevent starvation, the run time of each task may be controlled, and tasks may be run at multiples of a heartbeat timer, a simple technique called "harmonic tasking" widely used in safety-critical multitasking systems. To prevent deadlock, a system may be limited to exactly two priorities, usually "running" and "interrupts disabled," in order to prevent priority inversion. A design with an RTOS may use Rate-monotonic scheduling to assure responsiveness. See also | |||||||||
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