EMBEDDED SYSTEMS by liuhongmei


                             EMBEDDED SYSTEMS

                                 An Embedded system is a special-purpose system in
       which the computer is completely encapsulated by the device it controls, such as
       a personal computer, an embedded system performs pre-defined tasks, usually
       with very specific requirements.

Examples of Embedded systems:

       Automatic teller machines (ATMs)
       Avionics, such as inertial guidance systems, flight control hardware/software
        and other integrated systems in aircraft and missiles
       Cellular telephones and telephone switches
       Computer equipment such as routers and printers
       Engine controllers and antilock brake controllers for automobiles

                                                 The Apollo Guidance Computer, the
first recognizably modern embedded system.

        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.

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. 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.

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.


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.

Users 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.

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.
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 are common.

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

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.

PC/104 is a typical base for small, low-volume embedded and ruggedized system
design. These often use DOS, Linux, or an embedded real-time operating system such
as QNX or Inferno.


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.

   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


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, cell phones,
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
electronicsMost 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.


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.


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

   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.

Reliability regimes:

        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.
        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.
      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,

Embedded software architectures:

      Simple control loop

      Nonpreemptive multitasking

      Preemptive multitasking

Simple control loop:

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.

Preemptive multitasking:

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.

Non preemptive multitasking:

A non preemptive 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-

Exotic custom operating systems:
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.


An embedded system is a special type of           system with   some device controls,
and it is integrated with in a single chip and acts as a microcontroller.
So, in future it should spread to all the technologies.

     1. "http://en.wikipedia.org/wiki/Embedded_system
     2. Embedded system engineering


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