NXT & LeJOS & Wii Nunchuck

the NXT controller, the Wii Nunchuck and the adapter

Finally !

It’s been a while since I first thought about trying LeJOS, the JVM for NXT… I don’t recall why I didn’t do it when I first bought the NXT set, more than 2 years ago…

The thing is that, after some frustration with the URBI NXT installation (very nice project, but somehow didn’t manage to make the latest version work…) I felt that I had to do something “new” with the NXT… and I must admit that I’m so pleased with the simplicity and power of the LeJOS JVM that it feels like a pity I didn’t install it earlier !

The installation is quite straight forward, you need to install stuff on your PC and flash the NXT to replace the original firmware (no worries it can be flashed back to the initial state).

It’s also great that the bluetooth communication works “out of the box” so no more wires to program your box and communicate with it !

First project: interact with a Wii Nunchuck

This is famously easy to communicate with as it’s using the I2C protocol, but the tricky bit is that you need to slightly physically alter the nunchuck, as the NXT can’t take the I2C lines down, due to the pull-up resistors inside the nunchuck being to low (R1 and R2 are 1.8KOhms rather than the 82KOhms recommended by the NXT specifications).

Here’s a link nicely describing the hack (it’s in German, but the pictures are quite straight forward 🙂 )

Now my approach was much simpler

1. get rid of the SMD R1 and R2
2. put back the nunchuck without anything else
3. add 100KOhms pull-up resistors directly on the cables (where the nunchuck interfaces with the NXT cable) – I didn’t have 33KO ones, and 100 is closer anyway to the NXT specs
4. no diode as I didn’t have any, and was already using the nunchuck with a 5V Arduino… hope it won’t damage it too quickly 🙂

And here’s the Java module, please feel free to re-use it in your programs

import lejos.nxt.I2CPort;
import lejos.nxt.I2CSensor;

/**
* https://trandi.wordpress.com
*/

public class WiiNunchuck extends I2CSensor {

private static final int NUNCHUCK_ADDR = 0x52; // 7bit addressing.  It would be 0xA4 in 8bit
private static final int NUNCHUCK_MEM_ADDR = 0x40;

// array to store RAW nunchuck data
private byte[] _buff = new byte[6];
private byte[] _initBuff = new byte[2];
private int joy_x_axis;
private int joy_y_axis;
private int accel_x_axis;
private int accel_y_axis;
private int accel_z_axis;
private boolean z_button = false;
private boolean c_button = false;
public WiiNunchuck(I2CPort port){
super(port);
setAddress(NUNCHUCK_ADDR);
}

public boolean updateData() {
// ??? somehow it gets disabled....
getPort().i2cEnable(I2CPort.LEGO_MODE);

// INITIALIZE - tell the nunchuck we're talking to it
_initBuff[0] = 0x00;

sendData(NUNCHUCK_MEM_ADDR, _initBuff, 1);
try{ Thread.sleep(10);}catch(InterruptedException e){}

// send a request for data
sendData(0x00, (byte)0x00);

if(getData(0x00, _buff, 6) != 0) return false;

try{ Thread.sleep(10);}catch(InterruptedException e){}

for(int i = 0; i < 6; i++) _buff[i] = nunchuk_decode_byte(_buff[i]);

// transform into something meaningful
joy_x_axis = _buff[0];
joy_y_axis = _buff[1];
accel_x_axis = _buff[2];
accel_y_axis = _buff[3];
accel_z_axis = _buff[4];

// byte nunchuck_buf[5] contains bits for z and c buttons
z_button = (_buff[5] & 0x01) == 0;
c_button = (_buff[5] & 0x02) == 0;

// it also contains the least significant bits for the accelerometer data so we have to check each bit of byte outbuf[5]
if ((_buff[5] & 0x03) > 0) accel_x_axis += 2;
if ((_buff[5] & 0x04) > 0) accel_x_axis += 1;
if ((_buff[5] & 0x05) > 0) accel_y_axis += 2;
if ((_buff[5] & 0x06) > 0) accel_y_axis += 1;
if ((_buff[5] & 0x07) > 0) accel_z_axis += 2;
if ((_buff[5] & 0x08) > 0) accel_z_axis += 1;
joy_x_axis = updateRange(joy_x_axis);
joy_y_axis = updateRange(joy_y_axis);
accel_x_axis = updateRange(accel_x_axis);
accel_y_axis = updateRange(accel_y_axis);
accel_z_axis = updateRange(accel_z_axis);

return true;
}

/**
* Transforms a 0..127 -128 .. 0 range into a 128..0..-128 one
*/

private static int updateRange(int init){
int result = init;
boolean negative = result < 0;
if(negative) result = -result;
result = - result + 128;
if(negative) result = -result;
return result;
}

// Encode data to format that most wiimote drivers except only needed if you use one of the regular wiimote drivers
private static byte nunchuk_decode_byte (byte x){
return (byte)((x ^ 0x17) + 0x17);
}

public int getAccelX() {
return accel_x_axis;
}

public int getAccelY() {
return accel_y_axis;
}

public int getAccelZ() {
return accel_z_axis;
}

public int getJoyX() {
return joy_x_axis;
}

public int getJoyY() {
return joy_y_axis;
}

public boolean isC_button() {
return c_button;
}

public boolean isZ_button() {
return z_button;
}

}

It feels so good now that I can leverage the power of Java to program it… no more awkward syntax or C-like string manipulation… it’s comparable to the pleasure of programming the FEZ Domino, but even better as I’m really a Java guy and feel much more comfortable with it than with C#.