Embedded system

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A router, an example of an embedded system. Labelled parts include a microprocessor (4), RAM (6), and flash memory (7).
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A router, an example of an embedded system. Labelled parts include a microprocessor (4), RAM (6), and flash memory (7).

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.

Contents

[edit] Examples of embedded systems

[edit] History

In the 1960ies, computers possessed an ability to acquire, analyze, and process data, make decisions at very high speeds. However there were some disadvantages with the computer controls. They were: high cost, program complexity, and hesitancy of personnel to learn. However the new concept of electronic devices were evolved. They were called Programmable controllers which later became a part of embedded systems. This concept developed from a mix of computer technology, solid state devices, and traditional electromechanical sequences. The first programmable controller came into world in the year of 1969.

  • In 1978 National Engineering Manufacturers Association released the standard for a programmable microcontroller. The definition was an almost any computer-based controller. They included single board computers, numerical controllers, and sequential controllers in order to perfom even-based instructions.
The Apollo Guidance Computer, the first recognizably modern embedded system.source: The Computer History Museum
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The Apollo Guidance Computer, the first recognizably modern embedded system.
source: The Computer History Museum

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 knob-based analog components such as potentiometers and variable capacitors with digital electronics controlled by a small microcontroller with up/down buttons or knobs. 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.

[edit] 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.

Their functionality examines the interface input status, and in response to it, it controls the the output interface. In order to carry out the control plan, several logic combinations are need. They are called as program and are stored in memory of the device. The period of time during the program execution is called a scan. During the scan all inputs are tested, the control plan is evaluated and the outputs are updated.

  • The input and output devices such as switches, motors, lights, which are attached to the interface, are called field devices.

Programmable controllers do not always come is separate blocks. Very often they are build in, in some devices which can be classified as field devices. They can be integraded into the field devices "housing", and even to be a part of a circuit board of the field device. That is why such systems are called embeded device.

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.

[edit] User interfaces

The physical nature of the user interface depends on the type of programmable controller or an embedded system. It can be,

  • in form of some terminal contacts on device's body (if in a separate housing),
  • A terminal contact circuit board (if on a separate circuit board), or just
  • some intermediate point on a cirucit board (if P.C. is on a single circuit board with a field device)

In reality, more sophisticated devices such as computer ports, dedicated ports, are in use. Very often user interface comes on basis of some standard like RSR232, USB, Bluetooth, GPIB, etc.

[edit] Simple Interface

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.

[edit] In more complex systems

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.

[edit] 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.

[edit] 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 if the 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 its 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

[edit] 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.

[edit] 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.

[edit] 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.

[edit] Reliability regimes

Reliability has different definitions depending on why people want it.

  1. The system cannot safely be shut down for repair, or it is too inaccessible to repair. Generally, the embedded system tests subsystems, and switches redundant spares on line. Instead of hardware substitution, it may use software "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 repairmen are far away and repairs are expensive, 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 without 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.

[edit] Embedded software architectures

There are several different types of software architecture in common use.

[edit] 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.

[edit] 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. The tasks performed in the interrupt handlers should be kept short to keep the interrupt latency to a minimum.

Some times longer tasks are added to a queue structure in the interrupt handler to be processed in the main loop later. This method brings the system close to a multitasking kernel with discrete processes.

[edit] Nonpreemptive multitasking

A nonpreemptive multitasking system is very similar to the simple control loop scheme, 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.

[edit] 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.

[edit] 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.

[edit] 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:

  • 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 become a commodity, that cost is significant.

[edit] 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.

[edit] See also

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