Microcontrollers: The Basics

Originally written on July 16, 2014 by Tom Igoe
Last modified on August 27, 2016 by Tom Igoe

Overview

Different kinds of computers are designed for different purposes. The computer at the heart of your laptop is optimized for different purposes than the one in your phone or the one in your mouse. The simplest computers are those that are designed to take inout from the physical world and control output devices in the physical world. These are called microcontrollers.

Most electronic devices you use today have a microcontroller at their core. Microcontrollers are optimized for control of general input and output. They’re generally less computationally capable than the processors used in multimedia computers or servers, for example. They require less power than a those other processors, and they’re easier to interface with the physical world through input circuits called sensors and output circuits called actuators. They can communicate with other processors through various communication interfaces.

Microcontrollers: Computers for the Physical World

When you’re building something that controls digital media from the physical world, it’s common to use microcontrollers to sense the user’s actions, then pass information about those actions to a multimedia processor like the one in your laptop. Keyboards and computer mice have microcontrollers inside that communicate with personal computers using the USB communications protocol.

Microcontrollers come in many different size packages

Like any other computer, a microcontroller has to have input ports to detect action by a user, and output ports through which it expresses the results of its programs. The metal pins or contact points on a microcontroller are the inputs and outputs. Other devices, like light, heat, or motion sensors, motors, lights, our sound devices, are attached to these pins to allow the microcontroller to be sensitive to the world and to express itself. A microcontroller also requires power connections and communications connections, just like any other computer.

The image below shows an Atmel microcontroller with its pins labelled. You can see which ones are general purpose input and output (GPIO), which ones are for power and communications, and which ones have specialty functions as well, the most common of which is analog input. For more on the typical functions of a microcontroller, see the Microcontroller Pin Functions page.

atmega328_bb

There are several different types of microcontrollers. The simplest have very little program memory, only one or two GPIO pins and no specialty functions. These might only cost a fraction of a dollar apiece, in large quantities. Slightly more capable ones will have more memory, more GPIO pins and will include specialty functions as well. The Atmega328 processor that’s at the heart of the Arduino Uno is one of these processors. You can buy these processors for a few dollars apiece in quantity. More powerful than those are the controllers that have connections to interface to a display screen, like those in your mobile phone. These might cost several dollars, or tens of dollars. The more memory, processing power and input/output ports that a microcontroller has, the more expensive it tends to be.

Complex electronic devices like mobile phones or media players or televisions often contain multiple microcontrollers. A very simple controller might be used to control the power systems for the device, another might control the physical input interface, like the touchscreen of a phone or the remote control of a TV, another might control the display, while a more powerful processor controls the whole system, communicating with each lesser processor as needed. When your device is complex enough to need an operating system, then the control of displays, power, and physical IO is usually farmed out to microcontrollers, while the central processor runs the operating system.

The line between microcontrollers and operating system computers is getting blurry these days, so it helps to understand types of programs that different devices might run in order to understand the difference.

Programs for Microcontrollers and Other Processors

Programs for any processors fall into a few different classes: firmware, bootloaders, basic input-output systems, and operating systems. When you understand how they’re all related, you gain a better picture of how different classes of processors are related as well.

Microcontrollers generally run just one program as long as they are powered. That program is programmed onto the controller from a personal computer using a dedicated hardware programming device. The hardware programmer puts the program on the controller by shifting the instructions onto it one bit at a time, through a set of connections dedicated for this purpose. If you want to change the program, you have to use the programmer again. This is true of any processor, by the way: even the most powerful server or multimedia processor has to have a piece of firmware put on it with a hardware programmer at first.

Microcontrollers generally don’t run operating systems, but they often run bootloaders. A bootloader is a firmware program that lives in a part of the controller’s memory, and can re-program the rest of that memory. If you want to program a microcontroller directly from a personal computer without a separate hardware programmer, or from the internet, then you need a bootloader. Whenever your phone is upgrading its firmware, it’s doing it through a bootloader. Bootloaders allow a processor to accept new programs through more than just the dedicated programming port.

Any processor that will run an operating system will run a Basic Input-Output System, or BIOS as well. A BIOS may be loaded onto a processor using a bootloader. A BIOS runs before, or instead of, the operating system. It can control any display device attached to the processor, and any storage attached (such as a disk drive), and any input device attached as well.

Bootloaders and BIOSes are often called firmware because they’re loaded into the flash memory of the processor itself. Other programs live on external storage devices like disk drives, and are loaded by the BIOS. When you change a processor’s firmware, you need to stop the firmware from running, upload the new firmware, and reset the processor for the changes to take effect.

An operating system is a program that manages other programs. The operating system schedules access to the processor to do the tasks that other programs need done, manages network and other forms of communications, communicates with a display processor, and much more.

Programs are compiled into the binary instructions that a processor can read using programs called compilers. A compiler is just one of the many applications that an operating system might run, however. The applications that an operating system runs also live on external storage devices like disk drives.

Firmware Stored on Details
Single program Processor's program memory Is the only program running; must be loaded by hardware programmer
Bootloader Processor's program memory Must be loaded by hardware programmer; Takes small amount of program memory; can load another program into the rest of program memory
BIOS Processor's program memory Usually loaded by bootloader; can load operating system into RAM memory
Software Stored on Details
Operating System External mass storage Runs other programs; loaded into RAM by BIOS; unloaded from RAM on reset
Applications External mass storage Loaded into RAM by operating system and unloaded as needed

Generally, the term microcontroller refers to firmware-only processor, and a processor that runs an operating system from external storage is called an embedded processor, or a central processor if it’s in a device with lots of other processors. For example, the Arduino is a microcontroller. The Raspberry Pi and BeagleBone Black are embedded processors, and phones, tablets, and laptops are multi-processor devices running a central processor.

Microcontroller Development Boards and Activity Boards

A processor, whether microcontroller or multimedia processor, can’t operate alone. It needs support components. For a microcontoller, you need at least a voltage regulator and usually an external clock called a crystal. You might also add circuitry to protect it in case it’s powered wrong, or in case the wrong voltage and current are plugged into the IO pins. You might include communications interfaces as well. This extra circuitry determines the base cost of a development board like the Arduino or the Raspberry Pi.

Development boards usually include:

  • The processor itself
  • Power regulation circuitry
  • Hardware programmer connector
  • Communications interface circuitry
  • Basic indicator LEDs
Development boards. Top: Arduino Uno. Bottom: Raspberry Pi

More advanced development boards might also include multiple communications interface circuits, including wireless interfaces; sensors already attached to some of the GPIO pins; a mass storage connector like an SD card; and video or audio circuitry, if the processor supports that. The more features a board offers, the more it costs.

A development board allows you to program the controller’s firmware and software, but an activity board may not. Activity boards contain a pre-programmed microcontroller and some sensors and actuators along with a communications interface and a communications protocol so that you can interface the board and its sensors and actuators with software running on your personal computer. Boards like the MaKey MaKey or the PicoBoard are activity boards. Activity boards generally can’t operate on their own without being connected to a personal computer, while development boards can.

Activity boards: Top: PicoBoard. bottom: MaKey MaKey

Do I Really Need A Development Board?

You can buy and program microcontrollers without a development board or activity board, but you’ll need some extras to do so. First, you’ll need to design your own support circuitry, at least a programmer interface and a power supply. You’ll need a hardware programmer as well, in most cases. And you’ll need to buy breakout boards or learn to make very small surface-mount circuits, since fewer and fewer microcontrollers come in the large dual inline package (DIP) that can plug into a solderless breadboard anymore. Generally, until you are very comfortable with electronics and circuit fabrication, it’s best to start with an activity board or a development board.

Toolchains and Development Environments

The two most common languages for microcontrollers are the assembly language of each particular processor, or the C programming language.  A toolchain is the combination of compilers and  linkers needed to convert the instructions you write into a binary file that the microcontroller can interpret as its instructions and the programmer software needed to upload that into the processor.  Every manufacturer and processor family has its own assembly language (the beginning of the toolchain), but there’s a C compiler for almost every microcontroller on the market. Beyond that, a toolchain might include a compiler or firmware to convert a higher level language into the controller’s assembly language.

A toolchain does the work of converting your code, but an integrated development environment (IDE) is needed to connect you, the programmer, to the toolchain. An IDE usually contains a text editor with user interface elements to send your text to the toolchain and upload the result to the processor. IDEs will also include a display to give you error messages about your code, and a monitor of some sort so that you can see what your code does when it’s running on the processor.

Things to consider when picking a microcontroller:

Here’s a guide to picking a microcontroller for this class. What follows are the economic considerations for picking a microcontroller more generally.

Costs

How much do I want to spend? The more features and flexibility, the higher the cost. But if it reduces the time taken between setting up and expressing yourself, it may be worth spending the extra money.

Time

How much work do I want to do? An activity board or higher level development board will generally minimize the amount of work you do to build your interface to the world. Lower level dev boards or building your own boards will take more work before you have things working. Don’t go build your own boards unless you have to. Many good projects never get completed on time because the maker wanted to use their project as a way to learn how to make a circuit.

What programming languages/communications protocols/electronics do I already know? All other things being equal, pick a system whose components you know something about.

What’s the knowledge base like? Most microcontrollers have several websites and listserves dedicated to their use and programming. Quite often, the best ones are linked right off the manufacturer’s or distributor’s website. Check them out, look at the code samples and application notes. Read a few of the discussion threads. Do a few web searches for the microcontroller environment you’re considering. Is there a lot of collected knowledge available in a form you understand? This is a big factor to consider. Sometimes a particular processor may seem like the greatest thing in the world, but if nobody besides you is using it, you’ll find it much harder to learn.

Expandability/Compatibility

What other components is the microcontroller compatible with? Can you add on modules to your microcontroller? For example, are their motor controllers compatible with it? Display controllers? Sensors or sensor modules? Often these modules are expensive but they just snap into place without you making any special circuitry. If your time is worth a great deal, then these modules are a good buy. Sometimes even if you know how to build it with a lower level controller, a higher level system is worth the cost because it saves building and maintenance time.

What do I have to connect to? Are you connecting to a MIDI synthesizer? A DMX-512 lighting board? A desktop computer? The phone system? The Internet? Different microcontrollers will have different interface capabilities. Make sure you can connect everything together. Sometimes this requires some creative combinations of controllers if no one controller can speak to all the devices you want it to speak to.

Physical and Electrical Characteristics

How many inputs/outputs do I need? Every system has a certain number of ins and outs. If you can, decide how many things you want to sense or control before you pick your controller.

What kinds of inputs and outputs do I need? Do you need analog inputs and outputs, for sensing changing values, or do you only need digital ins and outs, for sensing whether something is on or off? Most of the embedded Linux boards (for example, the Raspberry Pi) do not have analog inputs, so be careful of that.

What kind of power is available to me? Does it need to be battery powered? Does it need to match the voltage of another device? Does it need to draw very little amperage?

How fast do I need to process data? Lower level processors will generally give you more speed.

How much memory do I need? If you’re planning some complex data processing or logging, you may need a microprocessor with lots memory, or the ability to interface with external memory.

How small does it need to be? A lower level controller generally lets you build your own circuitry, allowing you to reduce the size of the hardware you need.

The Economics of Microcontroller Development

So where does this leave you, the hobbyist or beginner with microcontrollers? What should you choose?

Using mid-level microcontrollers will cost you relatively little up front, in terms of peripherals. The various components you’ll need to make a typical project will run you about $75 – $100, including the controller. Starter kits are a good investment if you’ve never done it before, as they get you familiar with the basics. If you know your way around a circuit, you can start with just a development board. You can always keep the project intact and re-use the microcontroller for other projects. You’ll save yourself time not having to learn how a hardware programmer works, or which compiler to choose, or how to configure it. For the beginner seeking immediate gratification, mid-level is the way to go. The only downside is that if you want to build many more projects, you’ve got to buy another development board.

Using the controllers by themselves, on the other hand, is more of a hassle up front. You’ve got to know how to build the basic support and communications circuits, how to use a hardware programmer, and how to set up a toolchain. You’ll spend a lot of time early on cursing and wishing you’d bought a development board. The advantage comes a bit later on, once everything’s set up. You’ll eventually save money on development boards, and can make them in any shape you want. It gets better the longer you continue making microcontroller projects. So start with development or activity boards, and move up as your needs demand and knowledge can accommodate.