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An embedded system is an electronic computing system which is embedded
within a larger system through a "dense" interface. An interface is
considered to be dense if it hides the complexity of the underlying
embedded system from the environment or user. By hiding computational
complexity behind a simple interface, embedded systems technology
enables complex computing power to be used by average people. One
example of a device which leverages embedded technology is a digital
camera which performs sophisticated computations with a very simple
control interface. Embedded systems are primarily defined by their
interface with the user and the environment.
To consider the design, validation, and test issues related to
embedded systems, it is necessary to understand constraints imposed a
dense interface, and by the applications in which embedded systems are
used.
User/Environmental Interface
The user interface must be simple, relying on standard
mechanisms for human interface including buttons, keypads, video
screens, speakers, etc. Human interface devices must be suitable to
the human sensory system; for example a speaker should not generate
frequencies outside the human auditory range and a video screen should
not produce frequencies outside of the human visual range. An embedded
system can also interact directly with its environment through a set
of sensors and actuators . An automatic braking
system (ABS) in a car must contain sensors which detect the rate of
wheel rotation, and actuators to apply pressure to the brake pads.
Applications of Embedded Systems
The simple user interface of a embedded systems makes their use
appropriate in applications whose complexity is in their interaction
with the environment rather than the user. Consumer electronics is a
common application area because complicated video and audio processing
is performed with little user interaction. A cellular phone is a good
example of a device with a simple interface which performs complex
audio (and recently video) processing tasks using sensor data
(i.e. microphone and lens). Feedback control is another area in which
embedded systems are widely used because the data used by the system
is primarily environmental data rather than user data. Vehicle control
systems are embedded systems which rely primarily on data from
internal sensors (i.e. RPM, valve pressure) to perform their tasks.
Important Design/Test Issues
Based on the interface properties of embedded systems and the
application space in which they are used, we identify the following
broad research areas which are important to embedded system design and
test.
- Hardware-Software Codesign/Covalidation - Satisfying the
varied design requirements imposed by the range of embedded
applications generally necessitates the use of both hardware and
software components. Hardware components are required to perform the
time-sensitive tasks (i.e. video processing) while software is most
appropriate to implement complex control tasks. For this reason,
almost all embedded systems are implemented as hardware/software
systems.
- Real-Time System Design - Many embedded system
applications must satisfy real-time deadlines to ensure correct
operation. Real-time constraints are imposed by the human interface
which requires a given video frame rate or sampling
frequency. Real-time constraints are also imposed by the nature of the
feedback control applications. Feedback control systems require a
guanranteed response rate to ensure stability, and instability in
life-critical control systems (i.e. braking control) cannot be
tolerated.
Since many embedded control systems are implemented using software
components, the software side of the real-time systems problem has
garnered significant attention. Satisfying timing deadlines in
software requires complex interaction between the application code and
the operating system. Real-time software research can be partitioned
along these lines: Real-Time Operating Systems (RTOS) and
Embedded Software Development
- Hybrid Systems - Embedded systems combine discrete
components used for control with components whose behavior is best
modeled continuously, including analog electronic components and
mechanical components. The tasks of characterizing, designing, and
testing a hybrid system are not yet well formalized.
- Mobile Computing - Aside from laptop computers, all
sophisticated mobile devices are embedded systems because a simple
user interface is required to enable widespread acceptance of a
product. The functionality of a mobile device is strictly limited by
battery lifetime, so research in Low Power Design and
Power-Aware Design are important. Mobile embedded computing
systems present new challenges in Wireless Networking and
Distributed Control .
Further Information
An NSF/ACM workshop on Embedded Systems was held in 2001 and the workshop
report contains a general description of the different facets of
the Embedded System design problem.
Many books are available on the various aspects of Embedded Systems
design individually, but a book which gives a broad perspective on the
field is
Computers as Components by Wayne Wolf.
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