Raspberry Pi IoT In C, 2nd Edition

411 Pages·2020·11.942 MB·English

by  Harry Fairhead

Raspberry Pi IoT In C
2nd Edition
Harry Fairhead
I/O Press
I Programmer Library
Copyright © 2020 IO Press
All rights reserved. This book or any portion thereof may not be reproduced
or used in any manner whatsoever without the express written permission of
the publisher except for the use of brief quotations in a book review.
Harry Fairhead, Raspberry Pi IoT In C
ISBN: 9781871962635
Second Edition
First Printing, October 2020
Revision 0
Published by IO Press www.iopress.info
In association with I Programmer www.i-programmer.info
and with I o T Programmer www.iot-programmer.com
The publisher recognizes and respects all marks used by companies and
manufacturers as a means to distinguish their products. All brand names and
product names mentioned in this book are trade marks or service marks of
their respective companies and our omission of trade marks is not an attempt
to infringe on the property of others.
In particular we acknowledge that Raspberry Pi is a registered trademark of
the Raspberry Pi Foundation.
For updates, errata, links to resources and the source code for the programs in
this book, visit its dedicated page on the IO Press website: iopress.info.
2
Preface
This is the second edition of my book about IoT in C featuring the Raspberry
Pi and things have changed a lot since the first edition. The Pi range has
evolved and expanded over the time to provide both a low-cost, almost
disposable, device, the Pi Zero and a desktop replacement, the Pi 4. What has
stayed the same is the way that the Pi has enabled a full Linux system to be
used for hardware-oriented IoT applications. Where in the past you might
have opted for a microcontroller that didn’t run any sort of operating system,
now even the simplest of IoT devices can be a full Linux server if the need
arises.
This is a different way to think about physical computing, but it brings many
challenges and the biggest is speed. You could say that this is always an issue,
but getting a hardware task complete can mean trimming timings to the
microsecond. To do this means working close to the hardware and there is no
language better for this than C. It may be an old language, but C is still at or
close to number one in programming language rankings. It is worth learning
and it isn’t difficult.
The first problem any beginner at almost any programming project has to
solve is what tools to use.. In the case of programming the Pi things are
initially easy in that the GCC C compiler is installed as standard. When it
comes to an editor or IDE to help you write programs, things become more
difficult. Ideally you need a remote development system running on a desktop
machine which will download, build and run the program on a remote Pi.
The only IDE that does the job properly is NetBeans, but this is in a state of
flux having moved from Oracle to Apache. This book still majors on
NetBeans, but there is now an increasingly attractive alternative by way of
Visual Studio Code. VS Code is slightly more difficult to set up, something
that is covered in Appendix II, but works well once you are over that hurdle.
I have also taken the opportunity to reorganize some of the material so that
the more introductory topics come before the more advanced ones. So only
after dealing with simple GPIO output, do we look at simple input. Input is
generally harder to program than output because you aren’t in control of
when things happen. The usual solution to this is to use events or interrupts.
Linux doesn’t support user mode interrupts and to create something that
behaves like an interrupt we need to look at the use of the new, character-
mode, GPIO driver. In a later chapter the idea of using a finite state machine
approach to input is described, which solves nearly all our problems without
having to use interrupts.
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Moving on to other forms of output, we explore Pulse Width Modulation
(PWM) and what it can be used for – servos, music, LED control. The
important topics of I2C, the serial port and the SPI bus are all covered in
detail, including the new ports available on the Pi 4. A custom protocol is
implemented to make use of the DHT22 temperature sensor and the very
useful 1-wire bus is also implemented both as a custom protocol and using
the serial port. As connecting with the web is key to the IoT, we look at how
to use sockets to implement an HTML/2 client and server. Finally we consider
how things work at the very lowest level via memory mapping to access
registers and how to make the best of Linux in a real-time system, including
the relatively new deadline scheduling.
The main idea in this book is to work directly with the hardware. This
approach frees you from dependence on the current Linux configuration and
isolates you from the restrictions of the drivers. Perhaps more importantly, it
ensures you understand what is going on and are in complete control.
Similarly, on the hardware side, no HATs or other off-the-shelf expansion
boards are used. Simple and direct electronics are often all that is needed and
finding out how to do it means that you can understand how the expansion
boards do their job.
This is not a projects book, although there isn’t much left for you to do to
round out the embryonic projects that are used as examples. Instead it is
about understanding concepts and the acquisition of skills. The hope is that
by the end of the book you will know how to tackle your own projects and get
them safely to completion without wasting time in trial and error.
Thanks to my tireless editors Sue Gee and Kay Ewbank. I may know what a
comma is for in C, but my understanding of its role in natural language is
severely limited. Errors that remain, and I hope they are few, are mine.
For updates, errata, links to resources and the source code for the programs in
this book, visit its dedicated page on the IO Press website: iopress.info. You
can also contact me at [email protected].
Harry Fairhead
September, 2020
4
Table of Contents
Preface 3
Chapter 1
Why Pi For IoT? 13
The Pi Versus The Microcontroller..................................................13
Which Pi?..........................................................................................14
No HATS Needed..............................................................................16
What To Expect.................................................................................17
User Mode – No Drivers....................................................................17
What Do You Need?..........................................................................18
Safety In Numbers.............................................................................20
Summary...........................................................................................21
Chapter 2 23
Getting Started
Getting Started In C...........................................................................24
Local v Remote..................................................................................25
Installing NetBeans...........................................................................25
Your First C Pi Program.....................................................................28
Troubleshooting................................................................................33
Summary...........................................................................................34
Chapter 3
Getting Started With The GPIO 35
Setting Up the BCM 2835 Library With NetBeans...........................35
A First IoT Program...........................................................................38
Running Programs On NetBeans......................................................40
Pin Numbering..................................................................................41
GPIO Modes.......................................................................................43
Summary...........................................................................................45
Chapter 4
Simple Output 47
Output Mode.....................................................................................47
Multi Functions.................................................................................48
How Fast?...........................................................................................50
The Problem Of Clocks.....................................................................51
Including Pauses................................................................................53
Busy Wait...........................................................................................55
Automatic Busy Wait Calibration.....................................................57
Phased Pulses....................................................................................59
Can We Do Better?.............................................................................61
Summary...........................................................................................62
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Chapter 5
Some Electronics 63
Electrical Drive Characteristics........................................................63
Driving An LED.................................................................................64
LED BJT Drive....................................................................................66
A BJT Example..................................................................................67
MOSFET Driver.................................................................................70
MOSFET LED....................................................................................71
Setting Drive Type.............................................................................72
Setting Output Mode.........................................................................74
Basic Input Circuit - The Switch......................................................74
Debounce...........................................................................................76
The Potential Divider........................................................................76
Summary...........................................................................................78
Chapter 6
Simple Input 79
GPIO Input.........................................................................................79
Basic Input Functions.......................................................................80
The Simple Button............................................................................80
Press Or Hold.....................................................................................83
How Fast Can We Measure?..............................................................84
The Finite State Machine..................................................................87
FSM Button........................................................................................89
FSM Hold Button...............................................................................91
FSM Ring Counter.............................................................................92
Summary...........................................................................................95
Chapter 7
GPIO The Linux Way 97
Why Integrate GPIO With Linux?.....................................................97
GPIO Character Device.....................................................................99
Raw GPIO Character Device In C - ioctl.........................................102
Getting Chip Info.............................................................................103
GPIO Output....................................................................................104
GPIO Input.......................................................................................107
Summary.........................................................................................109
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Chapter 8
Advanced Input – Events, Threads, Interrupts 111
Events..............................................................................................111
Interrupts Considered Harmful?.....................................................112
Events And The BCM 2835 Library................................................113
Measuring Pulses With Events........................................................117
How Fast Are Events.......................................................................119
An Edgy Button...............................................................................120
Interrupts And The GPIO Character Driver...................................121
An Interrupt Function Using A Thread..........................................125
How Fast Is An Interrupt?...............................................................130
The Complete Program...................................................................131
Responding To Input.......................................................................134
Summary.........................................................................................135
GPIO Chapter 9
Pulse Width Modulation - Servos And More 137
Some Basic Pi PWM Facts...............................................................137
PWM Modes....................................................................................139
PWM Functions...............................................................................141
Selecting Clock Rate........................................................................142
Using PWM......................................................................................143
How Fast Can You Modulate?.........................................................145
Uses Of PWM – Digital To Analog..................................................146
Music...............................................................................................148
Controlling An LED.........................................................................149
Changing LED Brightness Linearly.................................................151
Controlling A Servo.........................................................................153
What Else Can You Use PWM For?.................................................157
Some Hardware Details...................................................................157
Summary.........................................................................................161
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Chapter 10
Using The I2C Bus 163
I2C Hardware Basics.......................................................................164
The Pi I2C........................................................................................166
The I2C Functions...........................................................................168
Slow Read Protocols........................................................................173
A Real Device..................................................................................174
A First Program................................................................................175
The I2C Protocol In Action.............................................................177
Checksum Calculation....................................................................183
Complete Listing.............................................................................184
Summary.........................................................................................186
Chapter 11
The DHT22 Sensor Implementing A Custom Protocol 187
The DHT22......................................................................................187
The Electronics................................................................................190
The Software...................................................................................190
Complete Listing.............................................................................195
Summary.........................................................................................197
Chapter 12
Exploring 1-Wire Bus Basics 199
The Hardware..................................................................................199
Initialization....................................................................................200
Writing Bits......................................................................................203
A First Command - Writing Bytes...................................................204
Reading Bits.....................................................................................205
Summary.........................................................................................208
Chapter 13
Using iButtons 209
The Hardware..................................................................................210
Is There a Button?...........................................................................211
Read The Serial Number.................................................................211
Computing The CRC.......................................................................212
iButton Function..............................................................................215
Complete Program Listing...............................................................216
Summary.........................................................................................219
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Chapter 14
DS18B20 Temperature Sensor 221
The Hardware..................................................................................221
Initialization....................................................................................223
Initiating Temperature Conversion.................................................224
Reading The Scratchpad.................................................................225
Getting The Temperature................................................................225
A Temperature Function.................................................................226
The Complete Program...................................................................227
Other Commands............................................................................230
Summary.........................................................................................233
Chapter 15
The Multidrop 1-Wire Bus 235
The Hardware..................................................................................235
The 1-Wire Search Algorithm.........................................................236
A Recursive Scan.............................................................................238
oneWireScan....................................................................................243
Scanning the Bus.............................................................................245
Error Checking.................................................................................247
How Many Devices?........................................................................248
Reading Specific Devices................................................................248
Alarm Scan......................................................................................252
Final Thoughts................................................................................252
Summary.........................................................................................253
Chapter 16
The Serial Port 255
Serial Protocol.................................................................................255
The UART Hardware.......................................................................259
Linux Device Names.......................................................................261
Getting Rid Of the Console.............................................................262
Opening the Serial Port...................................................................264
Raw Mode........................................................................................269
Sending Data...................................................................................271
Reading Data....................................................................................272
Polling For Data With Blocking.......................................................276
Polling For Data With Non-blocking...............................................278
Receiving A Big Block.....................................................................279
Blocking Versus Non-blocking........................................................280
Using A UART.................................................................................280
Summary.........................................................................................288
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Chapter 17
Getting Started With The SPI Bus 289
SPI Bus Basics.................................................................................289
Pi SPI Interfaces..............................................................................290
How SPI0 Works..............................................................................292
The SPI Functions...........................................................................295
A Loopback Example......................................................................298
Problems..........................................................................................300
Summary.........................................................................................302
Chapter 18
A to D With The SPI Bus 303
The MCP3008..................................................................................303
Connecting To The Pi......................................................................305
Basic Configuration.........................................................................305
The Protocol....................................................................................306
Some Packaged Functions...............................................................309
How Fast..........................................................................................309
Summary.........................................................................................311
Chapter 19
Connecting With The Web - Sockets 313
Socket Basics...................................................................................313
Socket Functions.............................................................................314
A Web Client....................................................................................317
Connecting Using a URL.................................................................320
A Server...........................................................................................323
A Web Server?.................................................................................328
What About Security?.....................................................................330
Summary.........................................................................................331
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