EEmmbbeeddddeedd CCoonnttrroolllleerrss UUssiinngg CC aanndd AArrdduuiinnoo // 22EE JJaammeess MM.. FFiioorree 2 Embedded Controllers Embedded Controllers Using C and Arduino by James M. Fiore Version 2.0.2, 31 August 2016 Embedded Controllers 3 This Embedded Controllers Using C and Arduino, by James M. Fiore is copyrighted under the terms of a Creative Commons license: This work is freely redistributable for non-commercial use, share-alike with attribution Published by James M. Fiore via dissidents For more information or feedback, contact: James Fiore, Professor Electrical Engineering Technology Mohawk Valley Community College 1101 Sherman Drive Utica, NY 13501 [email protected] www.mvcc.edu/jfiore Cover art by the author 4 Embedded Controllers Introduction This text is designed to introduce and expand upon material related to the C programming language and embedded controllers, and specifically, the Arduino development system and associated Atmel ATmega microcontrollers. It is intended to fit the time constraints of a typical 3 to 4 credit hour course for electrical engineering technology and computer engineering technology programs, although it could also fit the needs of a hardware-oriented course in computer science. As such, the text does not attempt to cover every aspect of the C language, the Arduino system or Atmel AVR microcontrollers. The first section deals with the C language itself. It is assumed that the student is a relative newcomer to the C language but has some experience with another high level language, for example, Python. This means concepts such as conditionals and iteration are already familiar and the student can get up and running fairly quickly. From there, the Arduino development environment is examined. Unlike the myriad Arduino books now available, this text does not simply rely on the Arduino libraries. As convenient as the libraries may be, there are other, sometimes far more efficient, ways of programming the boards. Many of the chapters examine library source code to see “what’s under the hood”. This more generic approach means it will be easier for the student to use other processors and development systems instead of being tightly tied to one platform. All Atmel schematics and data tables are derived from the latest version (October, 2014) of the Atmel 328P documentation which may be found at http://www.atmel.com/devices/ATMEGA328P.aspx This serves as the final word on the operation and performance of the 328P and all interested parties should become familiar with it. There is a companion lab manual to accompany this text. Other OER (Open Educational Resource) lab manuals in this series include DC and AC Electrical Circuits, Computer Programming with Python and Semiconductor Devices. An OER is available for Operational Amplifiers and Linear Integrated Circuits, and a Semiconductor Devices text is due in early 2017. Please check my web sites for the latest versions. A Note from the Author This text is used at Mohawk Valley Community College in Utica, NY, for our ABET accredited AAS program in Electrical Engineering Technology. Specifically, it is used in our second year embedded controllers course. I am indebted to my students, co-workers and the MVCC family for their support and encouragement of this project. While it would have been possible to seek a traditional publisher for this work, as a long-time supporter and contributor to freeware and shareware computer software, I have decided instead to release this using a Creative Commons non-commercial, share-alike license. I encourage others to make use of this manual for their own work and to build upon it. If you do add to this effort, I would appreciate a notification. “When things get so big, I don’t trust them at all You want some control-you gotta keep it small” - Peter Gabriel Embedded Controllers 5 6 Embedded Controllers Table of Contents 1. Course Introduction . . . . 8 2. C Memory Organization . . . . 10 3. C Language Basics . . . . . 14 4. C Language Basics II . . . . 24 5. C Storage Types and Scope . . . 32 6. C Arrays and Strings . . . . . 36 7. C Conditionals and Looping . . . 40 8. C Pointers . . . . . . 48 9. C Look-Up Tables . . . . . 52 10. C Structures . . . . . . 56 11. C Linked Lists* . . . . . 60 12. C Memory* . . . . . 64 13. C File I/O* . . . . . . 68 14. C Command Line Arguments* . . . 72 15. Embedded Programming . . . . 74 16. Hardware Architecture . . . . 78 17. AVR ATmega 328P Overview** . . . 84 18. Bits & Pieces: includes and defines . . 90 19. Bits & Pieces: Digital Output Circuitry . . 98 20. Bits & Pieces: Digital Input Circuitry . . 102 21. Bits & Pieces: pinMode . . . . 106 22. Bits & Pieces: digitalWrite . . . . 112 23. Bits & Pieces: delay . . . . . 116 24. Bits & Pieces: digitalRead . . . . 124 25. Bits & Pieces: Analog Input Circuitry . . 132 26. Bits & Pieces: analogRead . . . . 136 27. Bits & Pieces: analogWrite . . . . 142 28. Bits & Pieces: Timer/Counters . . . 146 29. Bits & Pieces: Interrupts . . . . 154 Appendices . . . . . . 160 Index . . . . . . . 165 * Included for more complete language coverage but seldom used for small to medium scale embedded work. ** Including modest comic relief for film noir buffs. Embedded Controllers 7 1. Course Introduction 1.1 Overview This course introduces the C programming language and specifically addresses the issue of embedded programming. It is assumed that you have worked with some other high level language before, such as Python, BASIC, FORTRAN or Pascal. Due to the complexities of embedded systems, we begin with a typical desktop system and examine the structure of the language along with basic examples. Once we have a decent grounding in syntax, structure, and the development cycle, we switch over to an embedded system, namely an Arduino based development system. This course is designed so that you can do considerable work at home with minimal cost, if you choose (entirely optional, but programming these little beasties can be addicting so be fore warned). Along with this course text you will need an Arduino Uno board (about $25) and a USB host cable. A small “wall wart” power adapter for it may also be useful. There’s a lot of free C programming info on the ‘net but if you prefer print books and want more detail, you may also wish to purchase one of the many C programming texts available. Two good titles are Kochan’s book Programming in C and the one by Deitel & Deitel C-How to Program. Whichever book you choose, make sure that its focus is C, not C++. You will also need a desktop C compiler. Just about any will do, including Visual C/C++, Borland, Code Warrior, or even GCC. A couple of decent freeware compilers available on the ‘net include Pelles C and Miracle C. 1.2 Frequently Asked Questions Why learn C language programming? C is perhaps the most widely used development language today. That alone is a good reason to consider it, but there’s more:  It is a modern structured language that has been standardized (ANSI).  It is modular, allowing reuse of code.  It is widely supported, allowing source code to be used for several different platforms by just recompiling for the new target.  Its popularity means that several third-party add-ons (libraries and modules) are available to “stretch” the language.  It has type checking which helps catch errors.  It is very powerful, allowing you to get “close to the metal”.  Generally, it creates very efficient code (small space and fast execution). What’s the difference between C and C++? C++ is a superset of C. First came C, then came C++. In fact, the name C++ is a programmer’s joke because ++ is the increment operator in C. Thus, C++ literally means “increment C”, or perhaps “give me the next C”. C++ does everything C does plus a whole lot more. These extra features don’t come free and embedded applications usually cannot afford the overhead. Consequently, although much 8 Embedded Controllers desktop work is done in C++ as well as C, most embedded work is done in C. Desktop development systems are usually referred to as C/C++ systems meaning that they’ll do both. Embedded development systems may be strictly C (as is ours). Where can I buy an Arduino development board? The Arduino Uno board is available from a variety of sources including Digi-Key, Mouser, Parts Express and others. Shop around! What’s the difference between desktop PC development and embedded programming? Desktop development focuses on applications for desktop computers. These include things like word processors, graphing utilities, games, CAD programs, etc. These are the things most people think of when they hear the word “computer”. Embedded programming focuses on the myriad nearly invisible applications that surround us every day. Examples include the code that runs your microwave oven, automobile engine management system, cell phone, and many others. In terms of total units, embedded applications far outnumber desktop applications. You may have one or even a few PCs in your house, but you probably use dozens of embedded applications every day. Embedded microcontrollers tend to be much less powerful but also much less expensive than their PC counterparts. The differing programming techniques are an integral part of this course and we shall spend considerable time examining them. How does C compare with Python? If, like many students taking this course, your background is with the Python language, you may find certain aspects of C a little odd at first. Some of it may seem overly complicated. Do not be alarmed though. The core of the language is actually simple. Python tends to hide things from the programmer while C doesn’t. Initially, this seems to make things more complicated, and it does for the most simple of programs. For more complicated tasks C tends to cut to the heart of the matter. Many kinds of data manipulation are much easier and more efficient in C than in other languages. One practical consideration is that C is a compiled language while most versions of Python are essentially interpreted. This means that there is an extra step in the development cycle, but the resulting compiled program is much more efficient. We will examine why this is so a little later. How does C compare with assembly language? Assembly has traditionally been used when code space and speed are of utmost importance. Years ago, virtually all embedded work was done in assembly. As microcontrollers have increased in power and the C compilers have improved, the tables have turned. The downside of assembly now weighs against it. Assembly is processor-specific, unstructured, not standardized, nor particularly easy to read or write. C now offers similar performance characteristics to assembly but with all the advantages of a modern structured language. Embedded Controllers 9 2. C Memory Organization 2.1 Introduction When programming in C, it helps if you know at least a little about the internal workings of simple computer systems. As C tends to be “close to the metal”, the way in which certain things are performed as well preferred coding techniques will be more apparent. First off, let’s narrow the field a bit by declaring that we will only investigate a fairly simple system, the sort of thing one might see in an embedded application. That means a basic processor and solid state memory. We won’t worry about disk drives, monitors, and so forth. Specific details concerning controller architecture, memory hardware and internal IO circuitry are covered in later chapters. 2.2 Guts 101 A basic system consists of a control device called a CPU (central processing unit), microprocessor, or microcontroller. There are subtle distinctions between these, but we have little need to go very deep at this point. Microcontrollers tend not to be as powerful as standard microprocessors in terms of processing speed, but they usually have an array of input/output ports and hardware functions (such as analog to digital or digital to analog converters) on chip that typical microprocessors do not. To keep things simple we shall use the term “processor” as a generic. Processors are often connected to external memory (RAM chips). Microcontrollers generally contain sufficient on-board memory to alleviate this requirement, but it is worthwhile to note that we are not talking about large (megabyte) quantities. A microcontroller may only contain a few hundred bytes of memory, but in simple applications that may be sufficient. Remember, a byte of memory consists of 8 bits, each bit being thought of as a 1/0, high/low, yes/no, or true/false pair. In order for a processor to operate on data held in memory, the data must first be copied into a processor’s register (it may have dozens of registers). Only in a register can mathematical or logical operations be carried out. For example, if you desire to add one to variable, the value of the variable must first be copied into a register. The addition is then performed on the register contents yielding the answer. This answer is then copied back to the original memory location of the variable. It seems a little roundabout at first, but don’t worry, the C language compiler will take care of most of those details for you. 2.3 Memory Maps Every byte of memory in a computer system has an address associated with it. This is a requirement. Without an address, the processor has no way of identifying a specific location in memory. Generally, memory addressing starts at 0 and works its way up, although some addresses may be special or “reserved” in some systems. That is, a specific address might not refer to normal memory, but might refer to a certain input/output port for external communication. Very often it is useful to draw a “memory map”. This is nothing more than a huge array of memory slots. Some people draw them with the lowest (starting) address at the top and other people draw them with the lowest address at the bottom. 10 Embedded Controllers