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Fast and Effective Embedded
Systems Design
Applying the ARM mbed
Second Edition
Rob Toulson
Tim Wilmshurst
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Introduction
It’s now just 4years since the first edition of this book was published. Microprocessors are
of course still everywhere, providing “intelligence” in cars, mobile phones, household and
office equipment, TVs and entertainment systems, medical products, aircraftdthe list
seems endless. Those everyday products, where a microprocessor is hidden inside to add
intelligence, are called embedded systems. It is still not so long ago when designers of
embedded systems had to be electronics experts, or software experts, or both. Nowadays,
with user-friendly and sophisticated building blocks available for our use, both the
specialist and the beginner can quickly engage in successful embedded system design. One
such building block is the mbed, launched in 2009 by the renowned computer giant ARM.
The mbed is the central theme of this book; through it, all the main topics of embedded
system design are introduced.
Technology has continued its onward gallop since the first edition of this book was
published in 2012, and there is already much in that edition which needs updating,
rewriting, or replacing. The single mbed device that we based the first book around has
spawned an extended family of “mbed-enabled” devices, an “ecosystem” of products,
development tools, community support has emerged, and the phrase “Internet of Things”
(IoT) is on everyone’s lips. The company who created the mbed, ARM, has meanwhile
repositioned the mbed concept, notably by placing it at the heart of their IoT
developments. The original mbed device is, however, still there, well-used and well-
established, in industry, among hobbyists, and in colleges and universities worldwide. Its
concept and ecosystem are, however, changing and developing very fast.
In light of this exciting but rapidly changing field, we as authors made a number of
decisions regarding this second edition. We decided that it should still be based around the
original mbed, now more commonly called the LPC1768 mbed. This clever device is after
all well-established and has much life left in it. We further recognized that the first edition
may have tried to cover too many things, with too little depth. Therefore in this edition,
we remove a few topics, but aim to ensure that each remaining one gets fuller treatment.
Overall it has grown by one chapter, and is a bigger book; we hope that the increased
depth will be appreciated by the reader. In retaining the focus just described, we are
xv
Introduction
deliberately choosing to exclude some important mbed-related topics, notably around IoT
developments. We anticipate making these the subject of further writing projects.
Broadly, the book divides into two parts, as it did with the first edition. Chapters 1 to 10
provide a wide-ranging introduction to embedded systems, using the mbed and
demonstrating how it can be applied to rapidly produce successful embedded designs.
These chapters aim to give full support to a reader, moving you through a carefully
constructed series of concepts and exercises. These start from basic principles and simple
projects, to more advanced system design. The final six chapters cover more advanced or
specialist topics, building on the foundations laid in the early chapters. The pace here may
be a little faster, and you may need to contribute more background research.
All this book asks of you at the very beginning is a basic grasp of electrical/electronic
theory. The book adopts a “learning through doing” approach. To make best use of it, you
will need an mbed, an internet-connected computer, and the various additional electronic
components identified through the book. You won’t need every single one of these if you
choose not to do a certain experiment or book section. You’ll also need a digital voltmeter,
and ideally access to an oscilloscope.
Each chapter is based around a major topic in embedded systems. Each has some
theoretical introduction and may have more theory within the chapter. The chapter then
proceeds as a series of practical experiments. Have your mbed ready to hook up the next
circuit, and download and compile the next example program. Run the program, and aim
to understand what’s going on. As your mbed confidence grows, so will your creativity
and originality; start to turn your own ideas into working projects.
You will find that this book rapidly helps you to:
(cid:129) understand and apply the key aspects of embedded systems,
(cid:129) understand and apply the key aspects of the ARM mbed,
(cid:129) learn from scratch, or develop your skills in embedded C/C++ programming,
(cid:129) develop your understanding of electronic components and configurations,
(cid:129) understand how the mbed can be applied in some of the most exciting and innovative
intelligent products emerging today,
(cid:129) produce designs and innovations you never thought you were capable of!
If you get stuck, or have questions, support is available for all readers through the book
website, the mbed website, or by email discussion with the authors.
If you are a university or college instructor, then this book offers a “complete solution” to
your embedded systems course. Both authors are experienced university lecturers, and had
your students in mind when writing this book. The book contains a structured sequence of
practical and theoretical learning activity. Ideally you should equip every student or
xvi
Introduction
student pair with an mbed, prototyping breadboard and component kit. These are highly
portable; so development work is not confined to the college or university lab. Later in the
course, students will start networking their mbeds together. Complete Microsoft
PowerPoint presentations for each chapter are available to instructors via the book website,
as well as answers to Quiz questions, and example solution code for the exercises and
mini projects.
This book is appropriate for any course or module which wants to introduce the concepts
of embedded systems, in a hands-on and interesting way. Because the need for electronic
theory is limited, the book is accessible to those disciplines which would not normally aim
to take embedded systems. The book is meant to be accessible to Year 1 undergraduates,
though we expect it will more often be used in the years which follow. Students are likely
to be studying one of the branches of engineering, physics, or computer science. The book
will also be of interest to the practicing professional and the hobbyist.
This book is written by Rob Toulson, Professor of Creative Industries at the University of
Westminster, and Tim Wilmshurst, Head of Electronics at the University of Derby. After
completing his PhD, Rob spent a number of years in industry, where he worked on digital
signal processing and control systems engineering projects, predominantly in audio and
automotive fields. He then moved to an academic career, where his main focus is now in
developing collaborative research between the technical and creative industries. Tim led
the Electronics Development Group in the Engineering Department of Cambridge
University for a number of years, before moving to Derby. His design career has spanned
much of the history of microcontrollers and embedded systems. Aside from our shared
interest in embedded systems, we both share an interest in music and music technology.
The book brings together our wide range of experiences. Having planned the general
layout of the book, and after some initial work, we divided the chapters between us. Tim
took responsibility for most of the early ones and issues relating to electronic and
computing hardware. Rob worked on programming aspects, and Internet and Audio
applications. This division of labor was mainly for convenience, and at publication we
both take responsibility for all the chapters! Because Tim has written several books on
embedded systems in the past, a few background sections and diagrams have been taken
from these and adapted, for inclusion in this book. There seemed no point in “reinventing
the wheel” where background explanations were needed.
xvii
CHAPTER 1
Embedded Systems, Microcontrollers,
and ARM
1.1 Introducing Embedded Systems
1.1.1 What Is an Embedded System?
We’re all familiar with the idea of a desktop or laptop computer, and the amazing
processing that they can do. These computers are general purpose; we can get them to do
different things at different times, depending on the application or program we run on
them. At the very heart of such computers we would find a microprocessor, a tiny and
fantastically complicated electronic circuit which contains the core features of a computer.
All of this is fabricated on a single slice of silicon, called an integrated circuit (IC). Some
people, particularly those who are not engineers themselves, call these circuits microchips,
or just chips.
What is less familiar to many people is the idea that instead of putting a microprocessor
in a general-purpose computer, it can also be placed inside a product which has nothing
to do with computing, like a washing machine, toaster, or camera. The microprocessor is
then customized to control that product. The computer is there, inside the product; but it
can’t be seen, and the user probably doesn’t even know it’s there. Moreover, those add-
ons which we normally associate with a computer, like a keyboard, screen, or mouse, are
also nowhere to be seen. We call such products embedded systems, because the
microprocessor that controls them is embedded right inside. Because such a
microprocessor is developed to control the device, in many cases, those used in
embedded systems have different characteristics from the ones used in more general-
purpose computing machines. We end up calling these embedded ones microcontrollers.
Though much less visible than their microprocessor cousins, microcontrollers sell in far
greater volume, and their impact has been enormous. To the electronic and system
designer they offer huge opportunities.
Embedded systems come in many forms and guises. They are extremely common in the
home, the motor vehicle, and the workplace. Most modern domestic appliances, like a
washing machine, dishwasher, oven, central heating, and burglar alarm, are embedded
systems. The motor car is full of them, in engine management, security (for example,
locking and antitheft devices), air-conditioning, brakes, radio, and so on. They are found
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Copyright©2017RTSixtyLtd.andTimWilmshurst.PublishedbyElsevierLtd.Allrightsreserved.
3
4 Chapter 1
across industry and commerce, in machine control, factory automation, robotics, electronic
commerce, and office equipment. The list has almost no end, and it continues to grow.
Fig. 1.1 expresses the embedded system as a simple block diagram. There is a set of
inputs from the controlled system. The embedded computer, usually a microcontroller,
runs a program dedicated to this application, permanently stored in its memory. Unlike the
general-purpose desktop computer, which runs many programs, this is the only program it
ever runs. Based on information supplied from the inputs, the microcontroller computes
certain outputs, which are connected to actuators and other devices within the system. The
actual electronic circuit, along with any electromechanical components, is often called the
hardware; the program running on it is often called the software. Aside from all of this,
there may also be interaction with a user, for example via a keypad and display, and there
may be interaction with other subsystems elsewhere, though neither of these is essential to
the general concept. One other variable will affect all that we do in embedded systems,
and this is time, represented therefore as a dominating arrow which cuts across the figure.
We will need to be able to measure time, make things happen at precisely predetermined
times, generate data streams or other signals with a strong time dependence, and respond
to unexpected things in a timely fashion.
This chapter introduces and reviews many concepts relating to computers,
microprocessors, microcontrollers, and embedded systems. It does this in overview form,
to give a platform for further learning. We return to most concepts in later chapters,
building on them and adding detail. More details can also be found in Ref. [1].
(User Interface)
TIME
EEmmbbeeddddeedd CCoommpuptuetrer
Input Output
SSooffttwwaarree
Variables Variables
HHaarrddwwaarree
(Link to other Systems)
Figure 1.1
The embedded system.
Embedded Systems, Microcontrollers, and ARM 5
1.1.2 An Example Embedded System
A snack vending machine is a good example of an embedded system. One is represented
in Fig. 1.2, in block diagram form. At its heart is a single microcontroller. As the diagram
shows, this accepts a number of input signals, from the user keypad, the coin counting
module, and from the dispensing mechanism itself. It generates output signals dependent
on those inputs.
A hungry customer may approach the machine and start jabbing at the buttons, or feeding
in coins. In the first case, the keypad sends signals back to the microcontroller, so that it
can recognize individual keys pressed and then hook these key values together to decipher
a more complex message. The coin counting module will also send information telling the
amount of money paid. The microcontroller will attempt to make deductions from the
information it’s receiving, and will output status information on the display. Has a valid
product been selected? Has enough money been paid? If yes, then it will energize an
actuator to dispense the product. If no, it will display a message asking for more money,
or a reentry of the product code. A good machine will be ready to give change. If it does
dispense, there will be sensors on the mechanism to ensure that the chosen product is
available, and finally that the action is complete. These are shown in the diagram as gate
position sensors. A bad machine (and haven’t we all met these?) will give annoying or
useless messages on its display, demand more money when you know you’ve given the
Inputs Outputs
gate
actuators
keypad
gate
position microcontroller liquid
sensors crystal
display
mobile
coin comms.
counter module
module
Figure 1.2
Vending machine embedded system.
6 Chapter 1
right amount, or leave that chocolate bar you desperately want teetering on a knife-edge,
not dropping down into the dispensing tray.
What’s been described so far is a fairly conventional machine. We can, however, take it
further. In modernvending systems, there might be a mobile communications feature
which allows the vending machine to report directly to the maintenance team if a fault is
diagnosed; they will then come and fix the fault. Similarly, the machine might report stock
levels through mobile or internet communications to allow the service team to visit and
replenish whenever supplies run low.
This simple example reflects exactly the diagram of Fig. 1.1. The microcontroller accepts
input variables, makes calculations and decisions with this, and generates outputs in
response. It does this in a timely manner, and correct use of time is implicit in these
actions. There is in this case a user interface, and in a modern machine there is a network
interface. While the above paragraphs seem to be describing the physical hardware of the
system, in fact it’s all controlled by the software that the designer has written. This runs in
the microcontroller and determines what the system actually does.
1.1.3 A Second Example System: A Word on Control Systems and the Segway
In this book we will see a number of control systems. Such systems are very common and
range from home to office equipment, industrial machinery to cars, autonomous vehicles,
robotics, toys, flying drones, and aerospace. Think of how much control there must be in a
humble inkjet printer: drawing in and guiding the paper, shooting the print cartridges
backward and forward with extraordinary precision, and squirting just the right amount of
ink, of the right color, in just the right place. The accuracy of a control system is a very
important aspect as errors and inaccuracy can have huge impact on performance, wear,
and safety.
Closed loop control systems employ internal error analysis to achieve accurate control of
actuators, given continuous sensor input readings. Usually, this means that the system
measures the position of an actuator, compares it with a signal representing the position
it’s meant to be at (called the setpoint), and from this information tries to move the
actuator to the correct position. The fundamental components of a closed loop system are
therefore an actuator which causes some form of action, a sensor which measures the
effect of the action, and a controller to compute the difference between the setpoint and
the actual position. Although we say position, any physical variable can be controlled. For
example, a heater element might be used to heat an oven to a specified temperature, and a
temperature sensor used to measure the actual heat of the oven. If the closed loop
(cid:1)
controller tells the oven to be at 200 C, then the heater will provide heat until the sensor
(cid:1)
signals to the controller that 200 C has been achieved. If the sensor measurement shows
