Embedded Computer

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, 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 computer system designed to perform one or a few dedicated functions. 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.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. 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.

In general, "embedded system" is not an exactly defined term, as many systems have some element of programmability. For example, handheld computer 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.

Examples of embedded systems

Audio like mp3 players and telephone switches for interactive voice response systems
Avionics, such as inertial guidance systems, flight control hardware/software and other integrated systems in aircraft and missiles
Cellular telephones and telephone switches
Electric or Electronic Motor controller for Brushless DC motors, Induction motors and DC Motors
Electronic control units and antilock brake controllers for automobiles
Home automation products, such as thermostats, air conditioners, sprinklers, and security monitoring systems
Handheld calculators
Household appliances, including microwave ovens, washing machines, television sets, DVD players and DVD recorder
Medical equipment
Personal digital assistant
Videogame consoles
Computer peripherals such as routers and computer printer
Industrial controllers for remote machine operation
Electronic musical instrument (digital synthesizers and digital pianos).
Security applications such as Digital video recorder and video server.

History In the earliest years of computers in the 1940s, computers were sometimes dedicated to a single task, but were too large to be considered "embedded". Over time however, the concept of programmable controllers developed from a mix of computer technology, solid state devices, and traditional electromechanical sequences.

The first recognizably modern embedded system was the Apollo Guidance Computer, developed by Charles Stark Draper at the MIT Instrumentation Laboratory. 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 transistor digital circuit 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 Sheffer stroke#NAND gate from $1000/each to $3/each, permitting their use in commercial products.

Since these early applications in the 1960s, 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.

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 perform event-based instructions.

By the mid-1980s, many 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 electronics 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.

Characteristics net4801, an embedded system targeted at network applications.

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 computing 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.

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. 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 (computing).

In more complex systems A full graphical screen, with touch screen 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.

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. 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. In particular, routers take advantage of this ability.

CPU platform Embedded processors can be broken into two distinct categories: microprocessors (μP) and microcontrollers (μC). Microcontrollers have built-in peripherals on the chip, reducing size of the system.

There are many different CPU architectures used in embedded designs such as ARM architecture, MIPS architecture, Coldfire/68k, PowerPC, x86 architecture, PIC microcontroller, 8051, Atmel AVR, Renesas H8, SuperH, V850, FR-V, M32R, Z80, Zilog_Z8, etc. This in contrast to the desktop computer market, which is currently limited to just a few competing architectures.

PC/104 and PC/104+ are 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 MicroC/OS-II, QNX or VxWorks.

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. A related scheme is to use a field-programmable gate array (FPGA), and program it with all the logic, including the CPU.

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, Serial Peripheral Interface Bus, 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, Assembly language#Assemblers, and debuggers to develop embedded system software. However, they may also use some more specific tools:

In circuit debuggers or emulators (see next section).
Utilities to add a checksum or Cyclic redundancy check 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 MATLAB, Simulink, MathCad, or Mathematica to simulate the mathematics. 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.
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 (programming language) or BASIC Stamp.
Another alternative is to add a Real-time operating system or Embedded operating system, which may have DSP capabilities like DSPnano RTOS.

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. 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.

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.

An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via a JTAG or NEXUS interface. 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.

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.

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.

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.

Recovery from errors may be achieved with techniques such as a watchdog timer that resets the computer unless the software periodically notifies the watchdog.

Specific reliability issues may include:

The system cannot safely be shut down for repair, or it is too inaccessible to repair. Solutions may involve subsystems with redundant spares that can be switched over to, or software "limp modes" that provide partial function. Examples include space systems, undersea cables, navigational beacons, bore-hole systems, and automobiles.

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.

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.

High vs Low Volume For high volume systems such as Digital audio player 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 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.

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

Sometimes 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.

Cooperative multitasking A nonpreemptive multitasking system is very similar to the simple control loop scheme, except that the loop is hidden in an Application programming interface. 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. 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 or threads running seemingly at the same time.

Any piece of task or thread code can damage the data of another task or thread; they must be precisely separated. Access to shared data must be controlled by some synchronization strategy, such as message queues, semaphore (programming) 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 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.

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 Routers and Automotive navigation system. Here are some of the reasons:

Ports to common embedded chip sets are available.
They permit re-use of publicly available code for Device driver, Web servers, Firewall (networking), 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 Programmable logic device.
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.

See also

Communications server
DSP
Electronic Control Unit
Embedded operating systems
System on module
System on chip
Firmware
Information appliance
Microprocessor
Programming languages
Real-time operating system
Software engineering
Ubiquitous computing

References

External links

Free Embedded Systems Glossary
The Embedded Systems Institute
How a real time operating system works

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