NXC_tutorial.pdf

Programming

LEGO NXT Robots

using NXC

(beta 30 or higher)

(Version 2.2, June 7, 2007)

by Daniele Benedettelli

with revisions by John Hansen

Preface

As happened for good old Mindstorms RIS, CyberMaster, and Spybotics, to unleash the full power of

Mindstorms NXT brick, you need a programming environment that is more handy than NXT-G, the National

Instruments Labview-like graphical language that comes with NXT retail set.

NXC is a programming language, invented by John Hansen, which was especially designed for the Lego robots.

If you have never written a program before, don't worry. NXC is really easy to use and this tutorial will lead you

on your first steps towards it.

To make writing programs even easier, there is the Bricx Command Center (BricxCC). This utility helps you to

write your programs, to download them to the robot, to start and stop them, browse NXT flash memory, convert

sound files for use with the brick, and much more. BricxCC works almost like a text processor, but with some

extras. This tutorial will use BricxCC (version 3.3.7.16 or higher) as integrated development environment (IDE).

You can download it for free from the web at the address

http://bricxcc.sourceforge.net/

BricxCC runs on Windows PCs (95, 98, ME, NT, 2K, XP, Vista). The NXC language can also be used on other

platforms. You can download it from the web page

http://bricxcc.sourceforge.net/nxc/

Most of this tutorial should also apply to other platforms, except that you loose some of the tools included in

BricxCC and the color-coding.

The tutorial has been updated to work with beta 30 of NXC and higher versions. Some of the sample programs

will not compile with versions older than beta 30.

As side note, my webpage is full of Lego Mindstorms RCX and NXT related content, including a PC tool to

communicate with NXT:

http://daniele.benedettelli.com

Acknowledgements

Many thanks go to John Hansen, whose work is priceless!

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Contents

Preface ___________________________________________________________________ 2

Acknowledgements ____________________________________________________________________ 2

Contents __________________________________________________________________ 3

I. Writing your first program __________________________________________________ 5

Building a robot ______________________________________________________________________ 5

Starting Bricx Command Center __________________________________________________________ 5

Writing the program ___________________________________________________________________ 6

Running the program __________________________________________________________________ 7

Errors in your program _________________________________________________________________ 8

Changing the speed ____________________________________________________________________ 8

Summary ____________________________________________________________________________ 9

II. A more interesting program _______________________________________________ 10

Making turns ________________________________________________________________________ 10

Repeating commands _________________________________________________________________ 10

Adding comments ____________________________________________________________________ 11

Summary ___________________________________________________________________________ 12

III. Using variables_________________________________________________________ 13

Moving in a spiral ____________________________________________________________________ 13

Random numbers ____________________________________________________________________ 14

Summary ___________________________________________________________________________ 15

IV. Control structures_______________________________________________________ 16

The if statement______________________________________________________________________ 16

The do statement _____________________________________________________________________ 17

Summary ___________________________________________________________________________ 17

V. Sensors ________________________________________________________________ 18

Waiting for a sensor __________________________________________________________________ 18

Acting on a touch sensor _______________________________________________________________ 19

Light sensor_________________________________________________________________________ 19

Sound sensor ________________________________________________________________________ 20

Ultrasonic sensor_____________________________________________________________________ 21

Summary ___________________________________________________________________________ 22

VI. Tasks and subroutines ___________________________________________________ 23

Tasks ______________________________________________________________________________ 23

Subroutines _________________________________________________________________________ 24

Defining macros _____________________________________________________________________ 25

Summary ___________________________________________________________________________ 26

VII. Making music _________________________________________________________ 28

Playing sound files ___________________________________________________________________ 28

Playing music _______________________________________________________________________ 28

Summary ___________________________________________________________________________ 30

VIII. More about motors ____________________________________________________ 31

Stopping gently ______________________________________________________________________ 31

Advanced commands _________________________________________________________________ 31

PID control _________________________________________________________________________ 33

Summary ___________________________________________________________________________ 34

IX. More about sensors _____________________________________________________ 35

Sensor mode and type _________________________________________________________________ 35

The rotation sensor ___________________________________________________________________ 36

Putting multiple sensors on one input _____________________________________________________ 37

Summary ___________________________________________________________________________ 38

X. Parallel tasks ___________________________________________________________ 39

A wrong program ____________________________________________________________________ 39

Critical sections and mutex variables _____________________________________________________ 39

Using semaphores ____________________________________________________________________ 40

Summary ___________________________________________________________________________ 41

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XI. Communication between robots ___________________________________________ 42

Master – Slave messaging ______________________________________________________________ 42

Sending numbers with acknowledgement __________________________________________________ 43

Direct commands ____________________________________________________________________ 45

Summary ___________________________________________________________________________ 45

XII. More commands _______________________________________________________ 46

Timers _____________________________________________________________________________ 46

Dot matrix display____________________________________________________________________ 46

File system _________________________________________________________________________ 47

Summary ___________________________________________________________________________ 50

XIII. Final remarks ________________________________________________________ 51

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I. Writing your first program

In this chapter I will show you how to write an extremely simple program. We are going to program a robot to

move forwards for 4 seconds, then backwards for another 4 seconds, and then stop. Not very spectacular but it

will introduce you to the basic idea of programming. And it will show you how easy this is. But before we can

write a program, we first need a robot.

Building a robot

The robot we will use throughout this tutorial is Tribot, the first rover you have been instructed to build once got

NXT set out of the box. The only difference is that you must connect right motor to port A, left motor to port C

and the grabber motor to port B.

Make sure to have correctly installed Mindstorms NXT Fantom Drivers that come with your set.

Starting Bricx Command Center

We write our programs using Bricx Command Center. Start it by double clicking on the icon BricxCC. (I assume

you already installed BricxCC. If not, download it from the web site (see the preface), and install it in any

directory you like. The program will ask you where to locate the robot. Switch the robot on and press OK . The

program will (most likely) automatically find the robot. Now the user interface appears as shown below (without

the text tab).

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