Real-Time Plots with kst and a Microcontroller

Follow along as we build a real-time data graphing system using kst, an Arduino microcontroller and a Linux notebook.
Arduino Open Hardware Primer

A simplified description of a microcontroller is that it's a small-footprint computer that can read and interpret input pin values, make a few calculations or decisions, and then control output pin signals based on the embedded program.

Arduino microcontrollers are known as open hardware. In the spirit of the open-source software tradition, the board designs, schematics and code are freely available to download, modify and enhance. The Arduino home page has active forums for information exchange, and there is a comprehensive set of reference documents.

Figure 2. Arduino Board

Naturally, the boards and raw components cost money. A number of Arduino clones are available, offering board configurations for a variety of specialized applications. Prices for solder-it-yourself boards start at around $20. Complete ready-to-run boards with built-in USB interfaces cost about $30. You'll also need a breadboard, a few resistors, some jumper wires, a wall wart or battery and some sensors (suppliers are listed in the Resources for this article).

Inputs are either analog or digital. An analog device might be a potentiometer or photocell, while a digital device might be a magnetic reed switch or a push button.

Outputs control things. You could turn on a light with a digital output operating through a relay or transistor. In this article, we won't control anything with output pins. Instead, the Arduino will communicate sensor data to a Linux notebook, over the USB line.

Modern microcontroller modules, like the Arduino, take advantage of what's called in-circuit programming. The processor chip uses Flash memory for program storage and is accessed via the USB connection. Flash is a type of Electrically Erasable PRogrammable Read-Only Memory (EEPROM), which means it can be erased and rewritten multiple times using the proper electrical signals. These days, the term EEPROM normally is used only to refer to the more traditional type of EEPROM (which is still used in smaller sizes due to some of its other advantages). In-circuit programming is great because it minimizes equipment costs and prototyping turnaround time.

The flagship Arduino module is known as the Duemilanove. It is a 2.7" x 2.1" circuit board that has 14 digital I/O (input/output) pins and 6 analog input pins. It also has a built-in USB port, uses an Atmel Atmega 328 processor and screams along at 16MHz. Modules are powered by batteries or from a wall wart, with a recommended range of 7–12 volts DC.

Setting Up the Arduino

Arduinos are programmed in a language called Processing. The Arduino integrated development environment (IDE) manages compilation of the Processing source code into machine code that is then uploaded to the Arduino board. Veteran programmers quickly will note Processing's remarkable similarity in format and syntax to the C language.

The Arduino IDE runs on 32-bit or 64-bit Linux notebooks and Netbooks. Obviously, Netbooks are cool because they are tremendously portable. Windows versions of the IDE are available. So, you'll always have the capability to program an Arduino, even if you get in a spot and don't have your Linux notebook close at hand.

Download the latest Arduino software from the Web site. Various Java packages, gcc-avr and avr-libc need to be installed, along with the Arduino integrated development environment. See Resources for a good tutorial on getting everything working in a 64-bit Ubuntu environment.

Open a terminal, and move to the directory where you installed the Arduino IDE. On the command line, start the IDE:

rreilly>  ./arduino

The main Arduino code editor screen will appear. From the drop-down File menu, select New to get a blank code window. Type in your program. You also can copy code from another source, such as Web examples or from the sample code bundled with the IDE in the Examples directory.

Figure 3. Arduino IDE and Editor Window

The examples offer standard routines to read various input sensors and control output pins. The Arduino Web forums and reference pages contain all kinds of code snippets that you can use instead of having to write everything from scratch. Just like open source in the Linux world, Arduino users are encouraged to develop and share their code.