Embedded system
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Embedded systems)
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
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
-
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
-
engine controllers 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/recorders
-
medical equipment
-
handheld computers
-
videogame consoles
- even computer peripherals themselves such as
routers and
printers have embedded processors
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 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.
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:
- 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
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:
- Testing the computer: CPU, RAM, and program memory. These often run once at
power-up.
- Tests of
peripherals: These simulate inputs and read-back or measure outputs.
- Tests of power supply, including batteries or other backup.
- 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.
- 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:
- Cabling tests, where a loop is made to allow the unit to receive what it
transmits
- Rigging tests: allow a system to be adjusted when it is installed.
- 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.
- 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.
- 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 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.
- 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 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.
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:
- 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.
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
Wikibooks has more about this subject:
Embedded Systems
Wikibooks Wikiversity has more about this subject:
Embedded System Engineering
External links
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