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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 pre-defined tasks, usually with very specific requirements. Since the system is dedicated to a specific task, 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 multipled 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.

Examples of embedded systems

History

The first recognizably modern embedded system was the Apollo Guidance Computer, developed by Charles Stark Draper at the MIT Instrumentation Laboratory. Each flight to the moon had two. They ran the inertial guidance systems of both the command module and LEM.

At the project's inception, the Apollo guidance computer was considered the riskiest item in the Apollo project. The use of the then new monolithic integrated circuits, to reduce the size and weight, increased this risk.

The first mass-produced embedded system was the Autonetics D-17 guidance computer for the Minuteman missile, released in 1961. It was built from discrete transistor logic and had a hard disk for main memory. When the Minuteman II went into production in 1966, the D-17 was replaced with a new computer that was the first high-volume use of integrated circuits. This program alone reduced prices on quad nand gate ICs from $1000/each to $3/each, permitting their use in commercial products.

Since these early applications in the 1960s, where cost was no object, embedded systems have come down in price. There has also been an enormous rise in processing power and functionality. For example the first microprocessor was the Intel 4004, which found its way into calculators and other small systems, but required external memory and support chips. By the mid-1980s, most of the previously external system components had been integrated into the same chip as the processor, resulting in integrated circuits called microcontrollers, and widespread use of embedded systems became feasible.

As the cost of a microcontroller fell below $1, it became feasible to replace expensive analog components such as potentiometers and variable capacitors with digital electronics controlled by a small microcontroller. By the end of the 80s, embedded systems were the norm rather than the exception for almost all electronics devices, a trend which has continued since.

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. Therefore usability considerations vary widely.

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.

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 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, an application-specific integrated circuit, 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, 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:

  • An in-circuit emulator (ICE) is a hardware device that replaces or plugs into the microprocessor, and provides facilities to quickly load and debug experimental code in the system.
  • Utilities to add a checksum or CRC to a program, so the embedded system can check its program is valid
  • For systems using digital signal processing, developers may use a math workbench such as MathCad or Mathematica to simulate the mathematics.
  • Custom compilers and linkers may be used to improve optimisation for the particular hardware.
  • An embedded system may have it's own special language or design tool, or add enhancements to an existing language.

Software tools can come from several sources:

  • Software companies that specialize in the embedded market
  • Ported from the GNU software development tools (see cross compiler)
  • Sometimes, development tools for a personal computer can be used if the embedded processor is a close relative to a common PC processor

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:
  1. Testing the computer: CPU, RAM, and program memory. These often run once at power-up.
  2. Tests of peripherals: These simulate inputs and read-back or measure outputs.
  3. Tests of power supply, including batteries or other backup.
  4. Consumables tests: These measure what a system uses up, and warn when the quantities are low, for example a fuel gauge in a car, or chemical levels in a medical system.
  5. Safety tests: These run within a 'safety interval', and assure that the system is still reliable. The safety interval is usually a time less than the minimum time that can cause harm.

Some tests may require interaction with a technician:

  1. Cabling tests, where a loop is made to allow the unit to receive what it transmits
  2. Rigging tests: allow a system to be adjusted when it is installed.
  3. Operational tests: These measure things that a user would care about to operate the system. Notably, these have to run when the system is operating. This includes navigational instruments on aircraft, a car's speedometer, and disk-drive lights.
After self-test passes, it is common to indicate this by some visible means like LEDs, providing simple diagnostics to technicians and users.

Reliability regimes

Reliability has different definitions depending on why people want it.
  1. The system is too unsafe, or inaccessible to repair. Generally, the embedded system tests subsystems, and switches redundant spares on line, or incorporates "limp modes" that provide partial function. Examples include space systems, undersea cables, navigational beacons, bore-hole systems, and automobiles. Often mass-produced equipment for consumers falls in this category because repairs are expensive and repairmen far away, when compared to the initial cost of the unit.
  2. The system must be kept running for safety reasons. Like the above, but "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) These usually have a few go/no-go tests, with on-line spares or limp-modes using alternative equipment and manual procedures.
  4. The system cannot be operated when it is unsafe. Similarly, perhaps a system cannot be operated when it would lose too much money. (Medical equipment, aircraft equipment with hot spares, such as engines, chemical factory controls, automated stock exchanges, gaming systems) The testing can be quite exotic, but the only action is to shut down the whole unit and indicate a failure.

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 it's 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", or 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. A resource kernel allocates or multiplexes access to CPU time, memory and other resources.

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 embeded 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:

  • 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.
  • 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.
  • Many RTOS systems have a per-unit cost. When used on a product that is or will be come a commodity, that cost is significant.

Exotic custom operating systems

Some systems require safe, timely, reliable or efficient behavior unobtainable with the above architectures. In this case companies will build a system to suit. However, this is increasingly unusual due to the enormous cost of building an operating system from scratch, and it is more common to adapt or add features to one of the above approaches.

See also

External links

Embedded systems

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This article is licensed under the GNU Free Documentation License. It uses material from the "Embedded system".

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