Using the LEGO NXT

March 2007 David Schilling Gus Jansson

Overview

NXT HardwareProgramming LanguagesUsing the NXTDemo Robots

NXT – What you see

4 Inputs (digital & analog)3 Outputs (support for encoders)Screen (100x64 pixels)4 ButtonsSoundUSB and Bluetooth

Processors

Main processor: Atmel 32-bit ARM processor,

AT91SAM7S256 256 KB FLASH 64 KB RAM 48 MHz

Co-processor: Atmel 8-bit AVR processor, ATmega48 4 KB FLASH 512 Byte RAM 8 MHz

Bluetooth and USB communication

Bluetooth wireless communication CSR BlueCoreTM 4 v2.0 +EDR System Supports the Serial Port Profile (SPP) Internal 47 KByte RAM External 8 MBit FLASH 26 MHz

USB 2.0 Full speed com port (12 Mbit/s)

Graphics, Sound, etc

Display 100 x 64 pixel LCD black & white graphical displayLoudspeaker Sound output channel with 8-bit resolution Sample rate of 2-16 KHz

4 Buttons

Misc.

3 output ports 6-wire interface supporting input from encoders4 input ports 6-wire interface supporting I2C as well as analog 1 high speed port, IEC 61158 Type 4/EN

50170 compliant

JTag debugging pins if you want themLEGO has released technical specifications for all this

Online Documentation

http://mindstorms.lego.com/ - click on “NXTreme” button for information and downloads for advanced usersOpen source firmware!!!SDK – Software Developer KitHDK – Hardware Developer KitBDK – Bluetooth Developer Kit

Motors

Biggest improvement over RCX – rotation counters built in360 counts per rotationThey are relative, not absolute positionsMaximum speed is approx. 200 RPMMotors are much stronger than the other, older LEGO motors

Sensors

Digital ultrasonic sensor (3cm-100cm)Light sensor – can control LEDSound sensor – DB and DBATouch sensor - binary

Conversion cable

LEGO has a conversion cable that allows you to use the sensors and motors of the RCX on the NXTAnd vice versa, but you can’t use the ultrasonic sensor on the RCX, and the motors won’t have encoder support

Wires

The LEGO NXT wires have odd ends (like the DEC RJ-12 connector, but backwards)Wiring diagrams:

Analog input

3rd Party Sensors

Prices from $30 to $50 (so far)HiTechnic (www.HiTechnic.com) Packaging like LEGO sensors Authorized by LEGO

MindSensors (www.MindSensors.com) Circuit boards with connectors

HiTechnic SensorsColor SensorCompass SensorAcceleration SensorMore coming… Prototype board Gyro sensor (analog, not I2C digital) IR Link (to talk to RCX and other LEGO IR

devices) IR Seeker (for robo soccer, for example) No-contact touch sensor (electrostatic sensor) Motor Multiplexor Sensor Multiplexor

MindSensors Sensors

Acceleration SensorRCX to NXT communications linkPS2 controller interfaceMotor MultiplexorMagnetic CompassPneumatic Pressure Sensor

Bluetooth

One way to download programsCan send files from one NXT to anotherAllows up to four NXTs to talk with each other One master, three slaves

Can use a phone as a remote control for your NXTOther devices may be supported

Programming Languages

NXT-G codeNXC (and NBC)Robot CpbLuaJava and others

NXT-G code

LEGO’s development environmentGraphical programmingBased on National Instruments’ LabViewGreat for quick programsWonderful for kids to use … but a real pain for old geezers like me who grew up with text programming languages

NXC (and NBC)

NXC (Not eXactly C) is a language similar to NQC, which was the most popular way to program the RCXBuilt on NBC, the Next Byte Code ‘assembler’Uses the built in firmware, and p-code system

NXC uses BricxCC

Reasonably stable development environmentProvides support for all LEGO robotic platformsFree!

Robot C

Carnegie Mellon’s programming language for various robotic platforms (including VEX, RCX, as well as NXT)Requires new firmwareNot free (free trial, $30 download)

pbLua

New firmware, and languageSmall portable language for embedded systemsBy Ralph Hemple, the author of pbForth for the RCXCompiles on brick!Use any terminal device to talk to it

Other languages

Java and several other languages are reportedly in the works as wellMS Robotic Studio also supports the NXT Download a small client program to

NXT Actual control is on a PC

Programming with NXC

Assuming you are comfortable with C…

How to use the motorsHow to read the sensorsThreading model

NXC Programs

NXC is based on CRestrictions because it’s based on the built-in P-Code system There’s no stack Limited memory Limited number of tasks (256) And subroutines, which are also

‘tasks’

NXC program skeleton

#include "NXCDefs.h"

task main() { }

NXC Sensors

#include "NXCDefs.h"

task main() { SetSensorType( IN_1,

SENSOR_TYPE_TOUCH ); SetSensorMode( IN_1,

SENSOR_MODE_BOOL ); }

NXC Sensors

#include "NXCDefs.h"

task main() { SetSensor( IN_1, SENSOR_TOUCH ); }

NXC Sensors

To read a sensor use

x = Sensor( IN_1 );

NXC Sample program#include "NXCDefs.h“task main() { SetSensor( IN_1, SENSOR_TOUCH ); while( true ) { if( Sensor( IN_1 ) ) { PlayToneEx(440, 100, 3, false); TextOut( 0, LCD_LINE1, "TOUCHING!"); while( Sensor( IN_1 ) ) ; } TextOut( 0, LCD_LINE1, "---------"); while( !Sensor( IN_1 ) ) ; }}

NXC Motors

Simple motor commands are available OnFwd(), OnRev(), Off(), Float()

NXC Motors

To use the built-in rotation sensors, you need to use the new motor commandsEasiest to use is RotateMotor()

RotateMotor

RotateMotor( port, speed, angle ); port is OUT_A, OUT_B, OUT_C, or

combinations such as OUT_AB, OUT_ABC

speed is a number -100 to 100 angle is the number of degrees you

want the motor to turn (positive or negative)

Mimicking a Servo Motor

Read the current motor angle Current =

MotorRotationCount( OUT_A );

Calculate how far you need to turn Delta = Target – Current;

Turn that amount RotateMotor( OUT_A, speed, Delta );

Displaying Text

The LCD display is useful for Debugging Setting parameters Runtime messages Graphics Games … and more

Displaying Text

TextOut( x, y, text );NumOut( x, y, number );

LCD display

The origin is the bottom leftSo TextOut( 0,0, “hi” ) will display in the bottom left cornerUse the LCD_LINEn macros if you like to make it easier

Text Example

y = 42;TextOut(0, LCD_LINE5, "Answer:" );NumOut( 8*6, LCD_LINE5, y );

// characters are 6 pixels wide

Graphics

There are also commands to draw lines, circles, rectangles, and set pointsYou can display bitmaps from a fileRIC files – contain drawing commandsOne problem is there isn’t an easy way to clear areas of the screen It’s easy to clear the whole screen You can display a 1x1 blank bitmap

Tasks and Subroutines

Multiple tasks are possible, but don’t work like you might expectScheduling is different – tasks start when a ‘dependant’ task finishes There is no easy way of stopping a

task

Use Precedes() or Follows() in a task to define task dependencies

Taskstask FooTask() { // will start executing when main() finishes}

task main() { // program starts here Precedes( FooTask );}

task BarTask() { Follows( main ); // will also start executing when main()

finishes}

Subroutines

Essentially a task that can be calledIt suspends the calling task until it returnsDon’t use task keyword to define theseCan pass in parameters or return a value

Subroutine Examplevoid TestSub( int x, int y, short i ) { x = y + i; }task main() { TestSub( 1, 2, 3 ); }

NXC Help

Preliminary help file is a PDFThere are many samples and a tutorial online

Demo Robots

Dog6-legged walkerCraneCrate Dump truckOthers?