Programming the Raspberry Pi Pico/W in MicroPython, Second Edition

360 Pages·2022·11.509 MB·English

by  Harry Fairhead, Mike James

Programming the Raspberry Pi
Pico/W in MicroPython
Second Edition
Harry Fairhead & Mike James
I/O Press
I Programmer Library
Copyright © 2022 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 & Mike James,
Programming the Raspberry Pi Pico in MicroPython
2nd Edition
ISBN Paperback: 9781871962802
ISBN Hardback: 9781871962062
First Printing, 2022
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 and Pico are registered
trademarks 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
The Raspberry Pi Pico is a remarkable microcontroller. Based on the RP2040
processor chip, designed by Raspberry Pi specially for this small form factor
it was originally launched in 2020. It experienced something of a relaunch
in 2022 with the release of the Pico W, a version of that has Wi-Fi capability,
overcoming the limitation that the original Pico lacked network connectivity.
It was the advent of the Pico W that prompted this Second Edition. You can
think of as the Pico W as a super-set of the original – everything you can do
with an original Pico you can do with a Pico W, but not vice versa. The
differences between the two are set out in Chapter 1 and two new chapters
are dedicated to how to make use of the additional features of the Pico W
and in particular how to create a web client and a web server. In the rest of
the book references to “the Pico” encompass both variants unless there is a
need to make a distinction.
The MicroPython language is a good choice for programming the Pico. It
isn’t the fastest way, but in most cases it is fast enough to interface with the
Pico’s hardware and its big advantage is that it is easy to use. As a high-level
language, MicroPython is based on Python 3 and is fully object-oriented.
This means that you can create classes to encapsulate hardware and make
your code easier to use and understand. It is also easier to implement
complex algorithms and so make your data processing easier. In general, you
can take an existing Python 3 program and simply run it under MicroPython,
usually with no changes. If there are any changes then they are generally
minor.
Another good thing about MicroPython on the Pico is that it is very easy to
get started. After a simple installation procedure you have a working
MicroPython machine which you can program almost at once using the
Thonny IDE. Despite Thonny being a really useful IDE we also show how to
use VS Code because it has more extensive syntax checking and input
prompting. Both IDEs are easy to get started with and easy to use so the
choice between them is up to you.
The Pico has so many resources that a comprehensive account would fill a
book twice this size. In order to make things fit in the space available we
have concentrated on things that are accessible from MicroPython and that
are basic to getting started. We have avoided “advanced” topics which
generally lead the beginner into deep water far too quickly. However, this
edition covers of use of the second core, uasyncio and asynchronous
programming in general.
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Another “advanced” topic that is covered is the use of the PIO
(Programmable I/O) because it is one of the key advantages of using the Pico,
allowing you to delegate interaction with external hardware to a peripheral.
Using this facility you can avoid any problems that you might have with
MicroPython’s lack of speed as you can write a PIO program which runs at
the full speed of the Pico. The PIO isn’t the solution to every problem, but
although challenging it is very useful and a lot of fun. You will find out
about the PIO in a dedicated chapter and then use it to implement a “driver”
for the DHT22 and the 1-Wire bus.
This book doesn’t teach you Python or MicroPython in the sense of basic
programming, but a knowledge of how to program in almost any language is
all you really need. All examples are written in a very simple style that
avoids the use of some features of Python that are very “neat” but tend to
obscure the meaning of the code from a beginner. You can easily refactor
any of the examples into classes that suit your particular purpose and
programming style.
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 our tireless editors Sue Gee and Kay Ewbank. Programming is the
art of great precision, but English doesn’t come with a built-in linter. Errors
that remain, and we hope they are few, are ours.
For the source code for the programs in this book, together with any updates
or errata, links to resources including recommendations for obtaining
electronic components, visit its dedicated page on the IO Press website:
iopress.info.
You can also contact us at [email protected] or
[email protected]
Harry Fairhead
Mike James
December, 2022
4
Table of Contents
Chapter 1
The Raspberry Pi Pico – Before We Begin 13
The Pico And Its Family..................................................................14
What To Expect................................................................................16
What Do You Need?.........................................................................16
Community......................................................................................18
Summary.........................................................................................19
Chapter 2
Getting Started 21
Installing MicroPython....................................................................21
Hello World Using Thonny.............................................................23
Hello World VS Code......................................................................25
Summary.........................................................................................28
Chapter 3
Getting Started With The GPIO 29
Pico Pins..........................................................................................29
Basic GPIO Functions......................................................................31
Blinky...............................................................................................32
Which Hardware?............................................................................34
Summary.........................................................................................36
Chapter 4
Simple Output 37
Basic GPIO Functions......................................................................37
How Fast?.........................................................................................38
Including Pauses..............................................................................40
Fixed Time Delay.............................................................................41
Phased Pulses..................................................................................42
Setting Multiple GPIO Lines...........................................................43
Summary.........................................................................................45
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Chapter 5
Some Electronics 47
Electrical Drive Characteristics.......................................................47
Driving An LED...............................................................................48
LED BJT Drive..................................................................................50
A BJT Example................................................................................51
MOSFET Driver...............................................................................54
MOSFET LED..................................................................................55
Setting Drive Type...........................................................................56
Setting Output Mode.......................................................................58
Basic Input Circuit - The Switch....................................................58
Debounce.........................................................................................60
The Potential Divider......................................................................60
Summary.........................................................................................62
Chapter 6
Simple Input 63
GPIO Input.......................................................................................63
Basic Input Functions......................................................................64
The Simple Button..........................................................................64
Press Or Hold...................................................................................67
How Fast Can We Measure?............................................................68
The Finite State Machine................................................................70
FSM Button......................................................................................72
FSM Hold Button.............................................................................73
FSM Ring Counter...........................................................................74
Summary.........................................................................................77
Chapter 7
Advanced Input – Events and Interrupts 79
Events...............................................................................................79
Interrupts Considered Harmful?.....................................................80
Hardware Events.............................................................................81
An Edgy Button...............................................................................83
Measuring Pulses With Events........................................................85
Interrupts.........................................................................................86
How Fast Is An Interrupt?...............................................................88
Race Conditions and Starvation.....................................................90
Responding To Input.......................................................................91
Summary.........................................................................................92
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Chapter 8
Pulse Width Modulation 93
Some Basic Pico PWM Facts...........................................................93
Pico PWM........................................................................................95
Changing The Duty Cycle...............................................................98
Duty Cycle Resolution.....................................................................99
Uses Of PWM – Digital To Analog................................................100
Frequency Modulation..................................................................102
Controlling An LED.......................................................................103
What Else Can You Use PWM For?...............................................106
Summary.......................................................................................107
Chapter 9
Controlling Motors And Servos 109
DC Motor.......................................................................................109
Brushed Motors.............................................................................110
Unidirectional Brushed Motor......................................................112
Unidirectional PWM Motor Controller........................................114
Bidirectional Brushed Motor........................................................116
Bidirectional Motor Software........................................................120
Using A Single Full H-Bridge As Two Half H-Bridges.................121
Controlling a Servo........................................................................121
Brushless DC Motors.....................................................................125
Stepper Motors..............................................................................127
Stepper Motor Driver....................................................................130
Stepper Motor Rotation – Using Timers.......................................134
Summary.......................................................................................138
Chapter 10
Getting Started With The SPI Bus 139
SPI Bus Basics...............................................................................139
Pico SPI Interfaces.........................................................................142
The SPI Functions.........................................................................143
Using the Data Transfer Functions...............................................144
A Loopback Example.....................................................................146
Using The CS Line.........................................................................147
The BME280 Humidity, Pressure and Temperature Sensor........149
Problems........................................................................................153
Summary.......................................................................................154
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Chapter 11
A-To-D And The SPI Bus 155
Pico ADC........................................................................................155
The MCP3008 SPI ADC.................................................................157
Connecting To The Pico................................................................159
Basic Configuration.......................................................................160
The Protocol..................................................................................160
SPI ADC Class...............................................................................163
How Fast?.......................................................................................164
Summary.......................................................................................165
Chapter 12
Using The I2C Bus 167
I2C Hardware Basics.....................................................................167
The Pico I2C..................................................................................169
The I2C Functions.........................................................................170
Slow Read Protocols......................................................................175
A Real Device.................................................................................175
A First Program..............................................................................176
The I2C Protocol In Action...........................................................178
Reading The Raw Temperature Data............................................179
Processing The Data......................................................................180
Reading Humidity.........................................................................181
Checksum Calculation..................................................................182
Complete Listing...........................................................................183
Summary.......................................................................................185
Chapter 13
Using The PIO 187
PIO Basic Concepts.......................................................................188
State Machines And GPIO............................................................189
PIO Blinky......................................................................................190
Clock Division And Timing..........................................................192
Writing Loops................................................................................194
Data To The PIO............................................................................195
Output To GPIO.............................................................................197
Side Effects....................................................................................200
Input...............................................................................................203
Edges..............................................................................................205
Advanced PIO................................................................................207
Summary.......................................................................................209
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Chapter 14
The DHT22 Sensor Implementing A Custom Protocol 211
The DHT22....................................................................................211
The Electronics..............................................................................214
The DHT Driver.............................................................................214
The Protocol..................................................................................215
DHT22 Using the PIO – Counting................................................219
DHT22 Using the PIO – Sampling................................................224
Complete Listing...........................................................................226
Summary.......................................................................................228
Chapter 15
The 1-Wire Bus And The DS1820 229
The Hardware................................................................................229
Initialization..................................................................................230
Using The Driver...........................................................................232
Writing Bits....................................................................................232
Writing With The Driver...............................................................234
A First Command - Writing Bytes.................................................235
Writing Bytes With the Driver.......................................................236
Reading Bits...................................................................................237
Reading Using The Driver.............................................................238
Working With 1-Wire Devices.......................................................239
Computing The CRC.....................................................................239
The DS18B20 Hardware................................................................241
The DS18B20 Driver.....................................................................243
Initialization..................................................................................244
Initiating Temperature Conversion...............................................245
Reading the Scratchpad................................................................246
Getting The Temperature..............................................................246
A Temperature Class.....................................................................247
The Complete Program..................................................................248
Driver Version................................................................................250
A PIO DS18B20 Program...............................................................251
Complete Program.........................................................................257
Other Commands..........................................................................259
Summary.......................................................................................262
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Chapter 16
The Serial Port 263
Serial Protocol...............................................................................263
UART Hardware............................................................................265
Setting Up the UART.....................................................................266
Data Transfer.................................................................................267
Serial Polling..................................................................................269
Timeouts........................................................................................270
Polling On Write............................................................................271
Reading and Writing Characters and Strings...............................273
Summary.......................................................................................274
Chapter 17
Using The Pico W - WiFi 275
The Pico W...................................................................................275
The WLAN Class...........................................................................276
A Practical Connect.......................................................................277
WiFi Scan.......................................................................................280
A Simple HTTP Client..................................................................280
Request Methods...........................................................................281
A Custom Server...........................................................................282
The urequests Module..................................................................284
A Temperature Sensor Client........................................................286
Sockets...........................................................................................289
A Socket Web Client......................................................................293
SSL Socket Based HTTPS Client..................................................294
Socket Server.................................................................................296
A Socket Temperature Server.......................................................297
An SSL HTTPS Server..................................................................301
Non-Blocking Sockets...................................................................306
The Connection Queue.................................................................307
Summary.......................................................................................309
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