Embedded Systems Design

433 Pages·2002·7.899 MB·English

by  Steve Heath (Auth.)

Preface
The term embedded systems design covers a very wide
range of microprocessor designs and does not simply start and
end with a simple microcontroller. It can be a PC running software
other than Windows and word processing software. It can be a
sophisticated multiprocessor design using the fastest processors
on the market today.
The common thread to embedded systems design is an
understanding of the interaction that the various components
within the system have with each other. It is important to under-
stand how the hardware works and the restraints that using a
certain peripheral may have on the rest of the system. It is essential
to know how to develop the software for such systems and the
effect that different hardware designs can have on the software
and vice versa. It is this system design knowledge that has been
captured in this book as a series of tutorials on the various aspects
of embedded systems design.
Chapter I defines what is meant by the term and in essence
defines the scope of the rest of the book. The second chapter
provides a set of tutorials on processor architectures explaining
the different philosophies that were used in their design and
creation. It covers many of the common processor architectures
ranging from 8 bit microcontrollers through CISC and RISC
processors and finally ending with digital signal processors and
includes information on the ARM processor family.
The third chapter discusses different memory types and
their uses. This has been expanded in this edition to cover caches
in more detail and the challenges associated with them for embed-
ded design. The next chapter goes through basic peripherals such
as parallel and serial ports along with timers and DMA control-
lers. This theme is continued in the following chapter which
covers analogue to digital conversion and basic power control.
Interrupts are covered in great detail in the sixth chapter
because they are so essential to any embedded design. The differ-
ent types that are available and their associated software routines
are described with several examples of how to use them and,
perhaps more importantly, how not to use them.
The theme of software is continued in the next two chapters
which cover real-time operating systems and software develop-
ment. Again, these have a tremendous effect on embedded de-
signs but whose design imp li cati ons are often not well und erstood
or explained. Chapter 9 discusses debugging and emulation tech-
niques.
xviii ecaferP
The remaining five chapters are dedicated to design exam-
ples covering buffer and data structures, memory and processor
performance trade-offs and techniques, software design examples
including using a real-time operating system to create state ma-
chines and finally a couple of design examples. In this edition, an
example real-time system design is described that uses a non-real-
time system to create an embedded system. The C source code is
provided so that it can be run and experimented with on a PC
running MS-DOS.
stnemegdelwonkcA
By the nature of this book, many hardware and software
products are identified by their tradenames. In these cases, these
designations are claimed as legally protected trademarks by the
companies that make these products. It is not the author's nor the
publisher's intention to use these names generically, and the
reader is cautioned to investigate a trademark before using it as a
generic term, rather than a reference to a specific product to which
it is attached.
Many of the techniques within this book can destroy data
and such techniques must be used with extreme caution. Again,
neither author nor publisher assume any responsibility or liability
for their use or any results.
While the information contained in this book has been
carefully checked for accuracy, the author assumes no responsibil-
ity or liability for its use, or any infringement of patents or other
rights of third parties which would result.
As technical characteristics are subject to rapid change, the
data contained are presented for guidance and education only. For
exact detail, consult the relevant standard or manufacturers' data
and specification.
1 What is an embedded
system?
Whenever the word microprocessor is mentioned, it con-
jures up a picture of a desktop or laptop PC running an application
such as a word processor or a spreadsheet. While this is a popular
application for microprocessors, it is not the only one and the fact
is most people use them indirectly in common objects and appli-
ances without realising it. Without the microprocessor, these
products would not be as sophisticated or cheap as they are today.
The embedding of microprocessors into equipment and
consumer appliances started before the appearance of the PC and
consumes the majority of microprocessors that are made today. In
this way, embedded microprocessors are more deeply ingrained
into everyday life than any other electronic circuit that is made. A
large car may have over 50 microprocessors controlling functions
such as the engine through engine management systems, brakes
with electronic anti-lock brakes, transmission with traction con-
trol and electronically controlled gearboxes, safety with airbag
systems, electric windows, air-conditioning and so on. With a
well-equipped car, nearly every aspect has some form of elec-
tronic control associated with it and thus a need for a microproc-
essor within an embedded system.
A washing machine may have a microcontroller that con-
tains the different washing programs, provides the power control
for the various motors and pumps and even controls the display
that tells you how the wash cycles are proceeding.
Mobile phones contain more processing power than a desk-
top processor of a few years ago. Many toys contain microproces-
sors and there are even kitchen appliances such as bread machines
that use microprocessor-based control systems. The word control
is very apt for embedded systems because in virtually every
embedded system application, the goal is to control an aspect of a
physical system such as temperature, motion, and so on using a
variety of inputs. With the recent advent of the digital age replac-
ing many of the analogue technologies in the consumer world, the
dominance of the embedded system is ever greater. Each digital
consumer device such as a digital camera, DVD or MP3 player all
depend on an embedded system to realise the system. As a result,
the skills behind embedded systems design are as diverse as the
systems that have been built although they share a common
heritage.
deddebmE systems ngised
What is an embedded system?
There are many definitions for this but the best way to
define it is to describe it in terms of what it is not and with examples
of how it is used.
An embedded system is a microprocessor-based system
that is built to control a function or range of functions and is not
designed to be programmed by the end user in the same way that
a PC is. Yes, a user can make choices concerning functionality but
cannot change the functionality of the system by adding/replac-
ing software. With a PC, this is exactly what a user can do: one
minute the PC is a word processor and the next it's a games
machine simply by changing the software. An embedded system
is designed to perform one particular task albeit with choices and
different options. The last point is important because it differenti-
ates itself from the world of the PC where the end user does
reprogram it whenever a different software package is bought and
run. However, PCs have provided an easily accessible source of
hardware and software for embedded systems and it should be no
surprise that they form the basis of many embedded systems. To
reflect this, a very detailed design example is included at the end
of this book that uses a PC in this way to build a sophisticated data
logging system for a race car.
If this need to control the physical world is so great, what is
so special about embedded systems that has led to the widespread
use of microprocessors? There are several major reasons and these
have increased over the years as the technology has progressed
and developed.
tnemecalpeR for discrete logic-based circuits
The microprocessor came about almost by accident as a
programmable replacement for calculator chips in the 1970s. Up to
this point, most control systems using digital logic were imple-
mented using individual logic integrated circuits to create the
design and as more functionality became available, the number of
chips was reduced.
This was the original reason for a replacement for digital
systems constructed from logic circuits. The microprocessor was
originally developed to replace a mass of logic that was used to
create the first electronic calculators in the early 1970s. For exam-
ple, the early calculators were made from discrete logic chips and
many hundreds were needed just to create a simple four function
calculator. As the integrated circuit developed, the individual
logic functions were integrated to create higher level functions.
Instead of creating an adder from individual logic gates, a com-
plete adder could be bought in one package. It was not long before
complete calculators were integrated onto a single chip. This
enabled them to be built at a very low cost compared to the original
machines but any changes or improvements required that a new
What is an embedded system ?
chip be developed. The answer was to build a chip that had some
form of programmable capability within it. Why not build a chip
that took data in, processed it and sent it out again? In this way,
instead of creating new functions by analysing the gate level logic
and modifying it ~ a very time-consuming process ~ new
products could be created by changing the program code that
processed the information. Thus the microprocessor was born.
Provide functional sedargpu
In the same way that the need to develop new calculator
chips faster and with less cost prompted the development of the
first microprocessors, the need to add or remove functionality
from embedded system designs is even more important. With
much of the system's functionality encapsulated in the software
that runs in the system, it is possible to change and upgrade
systems by changing the software while keeping the hardware the
same. This reduces the cost of production even lower because
many different systems can share the same hardware base.
In some cases, this process is not possible or worthwhile but
allows the manufacturer to develop new products far quicker and
faster. Examples of this include timers and control panels for
domestic appliances such as VCRs and televisions.
In other cases, the system can be upgraded to improve
functionality. This is frequently done with machine tools, tel-
ephone switchboards and so on. The key here is that the ability to
add functionality now no longer depends on changing the hard-
ware but can be done by simply changing the software. If the
system is connected to a communications link such as a telephone
or PC network, then the upgrade can be done remotely without
having to physically send out an engineer or technician.
Provide easy maintenance sedargpu
The same mechanism that allows new functionality to be
added through reprogramming is also beneficial in allowing bugs
to be solved through changing software. Again it can reduce the
need for expensive repairs and modifications to the hardware.
Improves mechanical ecnamrofrep
For any electromechanical system, the ability to offer a finer
degree of control is important. It can prevent excessive mechanical
wear, better control and diagnostics and, in some cases, actually
compensate for mechanical wear and tear. A good example of this
is the engine management system. Here, an embedded microproc-
essor controls the fuel mixture and ignition for the engine and will
alter the parameters and timing depending on inputs from the
engine such as temperature, the accelerator position and so on. In
this way, the engine is controlled far more efficiently and can be
configured for different environments like power, torque, fuel
efficiency and so on. As the engine components wear, it can even
deddebmE systems ngised
adjust the parameters to compensate accordingly or if they are
dramatically out of spec, flag up the error to the driver or indicate
that servicing is needed.
This level of control is demonstrated by the market in
'chipped' engine management units where third party companies
modify the software within the control unit to provide more
power or torque. The differences can range from %01 to nearly
%05 for some turbo charged engines! All this from simply chang-
ing a few bytes. Needless to say, this practice may invalidate any
guarantee from the manufacturer and may unduly stress and limit
the engine's mechanical life. In some cases, it may even infringe
the original manufacturer's intellectual property rights.
Protection of intellectual property
To retain a competitive edge, it is important to keep the
design knowledge within the company and prevent others from
understanding exactly what makes a product function. This knowl-
edge, often referred to as IPR (intellectual property rights), be-
comes all important as markets become more competitive. With a
design that is completely hardware based and built from off-the-
shelf components, it can be difficult to protect the IPR that was
used in its design. All that is needed to do is to take the product,
identify the chips and how they are connected by tracing the tracks
on the circuit board. Some companies actually grind the part
numbers off the integrated circuits to make it harder to reverse
engineer in this way.
With an embedded system, the hardware can be identified
but the software that really supplies the system's functionality can
be hidden and more difficult to analyse. With self-contained
microcontrollers, all that is visible is a plastic package with a few
connections to the outside world. The software is already burnt
into the on-chip memory and is effectively impossible to access. As
a result, the IPR is much more secure and protected.
tnemecalpeR for eugolana stiucric
The movement away from the analogue domain towards
digital processing has gathered pace recently with the advent of
high performance and low cost processing.
To understand the advantages behind digital signal process-
ing, consider a simple analogue filter. The analogue implementa-
tion is extremely simple compared to its digital equivalent. The
analogue filter works by varying the gain of the operational
amplifier which is determined by the relationship between r i and
rf.
In a system with no frequency component, the capacitor ic
plays no part as its impedance is far greater than that of .fr As the
frequency component increases, the capacitor impedance de-
creases until it is about equal with rf where the effect will be to
reduce the gain of the system. As a result, the amplifier acts as a
What is an embedded system ?
low pass filter where high frequencies will be filtered out. The
equation shows the relationship where o~j is the frequency compo-
nent. These filters are easy to design and are cheap to build. By
making the CR (capacitor-resistor) network more complex, differ-
ent filters can be designed.
The mathematical function
Analogue signal gnissecorp
signal Digital gnissecorp )PSD(
The digital equivalent is more complex requiring several
electronic stages to convert the data, process it and reconstitute the
data. The equation appears to be more involved, comprising of a
summation of a range of calculations using sample data multi-
plied by a constant term. These constants take the place of the CR
deddebmE systems ngised
components in the analogue system and will define the filter's
transfer function. With digital designs, it is the tables of coeffi-
cients that are dynamically modified to create the different filter
characteristics.
Given the complexity of digital processing, why then use it?
The advantages are many. Digital processing does not suffer from
component ageing, drift or any adjustments which can plague an
analogue design. They have high noise immunity and power
supply rejection and due to the embedded processor can easily
provide self-test features. The ability to dynamically modify the
coefficients and therefore the filter characteristics allows complex
filters and other functions to be easily implemented. However, the
processing power needed to complete the 'multiply-accumulate'
processing of the data does pose some interesting processing
requirements.
Data sampling A/D
at frequency sF conversion
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PSD gnissecorp stnemeriuqer
The diagram shows the problem. An analogue signal is
sampled at a frequency sf and is converted by the A/D converter.
This frequency will be first determined by the speed of this
conversion. At every period, ,st there will be a new sample to
process using N instructions. The table shows the relationship
between sampling speed, the number of instructions and the
What is an embedded system ?
instruction execution time. It shows that the faster the sampling
frequency, the more processing power is needed. To achieve the 1
MHz frequency, a 01 MIPS processor is needed whose instruction
set is powerful enough to complete the processing in under 01
instructions. This analysis does not take into account A/D conver-
sion delays. For DSP algorithms, the sampling speed is usually
twice the frequency of the highest frequency signal being proc-
essed: in this case the I MHz sample rate would be adequate for
signals up to 005 kHz.
One major difference between analogue and digital filters is
the accuracy and resolution that they offer. Analogue signals may
have definite limits in their range, but have infinite values be-
tween that range. Digital signal processors are forced to represent
these infinite variations within a finite number of steps deter-
mined by the number of bits in the word. With an 8 bit word, the
increases are in steps of I / 652 of the range. With a 61 bit word, such
steps are in 1 / 65536 and so on. Depicted graphically as shown, a
61 bit word would enable a low pass filter with a roll-off of about
09 .Bd A 42 bit word would allow about 021 dB roll-off to be
achieved.
BdO
L
dB
24 bit
ycneuqerF
Word size and cutoff frequencies
DSP can be performed by ordinary microprocessors, al-
though their more general-purpose nature often limits perform-
ance and the frequency response. However, with responses of
only a few hundred Hertz, even simple microcontrollers can
perform such tasks. As silicon technology improved, special build-
ing blocks appeared allowing digital signal processors to be
developed, but their implementation was often geared to a hard-
ware approach rather than designing a specific processor architec-
ture for the job. It is now common for processors to claim DSP