XBee Enabled Joystick pIII

If you are arriving at part III first, check out the first two parts.

Xbee Enabled Joystick, part I
Xbee Enabled Joystick, part II

I finally got around to demonstrating this project. It has been a long time in the making. Throwing an Arduino with an XBee shield onto my homemade RC boat, Das Boot seemed like the simplest method to demonstrate the 'XBee Enabled Joystick'.

Setup

The setup is the same as before in the first two parts. I only changed the sample rate on the joystick XBee to 0x021, which translates to 33ms, or about 30Hz. Below you can find sample code I used to demonstrate this project. I'll admit, the code was quick and dirty, but demonstrates some principles of parsing the XBee sample packet.

Enjoy the video!

Schematic

I'm including the schematic for the transmitting XBee (the black box seen in the video with the blinking LEDs). Really, it is quite simple.

Board **Updated

This is the board layout for the transmitting XBee (black box). All the joystick outputs are routed to the ADC and GPIO pins of the XBee, like the schematic shows.

Top

Bottom

Example Code

// Simple sketch that takes in XBee sample data and converts
// it to usuable PWM servo control signals.  This example will
// control 2 servos, throttle and rudder (steering).

#include <Servo.h>

#define BUFFER_SIZE   50
#define THROTTLE_PIN  9
#define RUDDER_PIN    10
#define THROTTLE_ADC  2
#define RUDDER_ADC    1
#define SERVO_MIN     900
#define SERVO_MAX     2100

#define XBEE_START    0x7E
#define XBEE_CMDID    0x83
#define XBEE_ADC_RES  10
#define XBEE_ADC_CTR  (0x0001 << ((XBEE_ADC_RES) - 1))
#define XBEE_NUM_ADC  5

#define SERIAL_TIMEOUT  10000
#define SERIAL_BAUD     115200

Servo throttle;  // Throttle servo
Servo rudder;    // Rudder servo, steers the vehicle

int rxbuffer[BUFFER_SIZE];
int data_count = 0;
unsigned int buttons = 0;
unsigned long axis[5];

// Simple function that indefinitely waits for a byte over serial
unsigned char get_byte();
// Same as above, but can be given a timeout.  If it takes too long
// to receive a byte, the functions returns 0
unsigned char get_byte(int timeout);
// Returns the length of the sensor packet sent from the XBee
int get_length();
// Parses the packet from the XBee.  Return 1 if a valid packet
// has been received.
int get_packet();

unsigned char get_byte()
{
  while (!Serial.available()) {;}
  return Serial.read();
}

unsigned char get_byte(int timeout)
{
  int i = 0;
  while (!Serial.available() && i < timeout) {timeout++;}
  if (Serial.available()) {
    return Serial.read();
  } else {
    return 0;
  }
}

int get_length()
{
  int length = 0;
  // Get two bytes from the serial stream, which equal the
  // length of the packet
  length = (get_byte() << 8) | get_byte();
  return length;
}

int get_packet()
{
  int i, j = 0;
  int length = 0;
  int num_samples = 0;
  int channel_ind_msb = 0;
  int channel_ind_lsb = 0;
  unsigned char checksum = 0;
  unsigned char adc_msb = 0, adc_lsb = 0;
  char c = 0;
  // First step is to read in the length
  length = get_length();
  // Next figure out if this is the command we're looking for
  if (get_byte() != XBEE_CMDID) {
    return 0;
  }
  checksum += XBEE_CMDID;
  
  // The next couple bytes tell us a little more about this packet
  // Right now, we won't do anything with them, other than add them
  // to our checksum
  checksum += get_byte();  // Source Address MSB
  checksum += get_byte();  // Source Address LSB
  checksum += get_byte();  // Source Address RSSI
  checksum += get_byte();  // Source Address Options
  
  // The next byte contains the number of samples
  num_samples = get_byte();
  checksum += num_samples;
  
  // Repeat the following steps for for each sample.  Hopefully, 
  // for this program this will always be 1.
  for (i=0; i < num_samples; i++) {
    
    // Find out which lines are active
    channel_ind_msb = get_byte();
    checksum += channel_ind_msb;
    channel_ind_lsb = get_byte();
    checksum += channel_ind_lsb;
    
    // If any digital lines are active, read them in
    if ((channel_ind_msb & 0x01) || channel_ind_lsb) {
      // Right now, not doing anything with this data
      checksum += get_byte();
      checksum += get_byte();
    }
        
    // Now, read in all the ADC values
    for (j=0; j < 5; j++) {
      if (channel_ind_msb & 0x01 << (j+1))
      {
        // There is a reading for this ADC
        adc_msb = get_byte();
        checksum += adc_msb;
        adc_lsb = get_byte();
        checksum += adc_lsb;
        // Join the bytes into an int
        axis[j] = (adc_msb << 8) | adc_lsb;
      }
    }
    
  }
  // return 1 if there is a correct checksum
  if ((0xFF - checksum) == get_byte()) 
  {
    return 1;
  } else {
    return 0;
  }
}

void setup() {
  int i=0;
  // Start serial port at SERIAL_BAUD bps.
  // For this example, it should be 115200:
  Serial.begin(SERIAL_BAUD);
  //Set up some pins as PWM for a servo
  rudder.attach(RUDDER_PIN, SERVO_MIN, SERVO_MAX);
  throttle.attach(THROTTLE_PIN, SERVO_MIN, SERVO_MAX);
  // Zero out the joystick axis values
  for (i=0; i < XBEE_NUM_ADC; i++)
  {
    axis[i] = 0;
  }
}

void loop() {
  // Let's parse us an XBee sample packet!
  if (get_byte(SERIAL_TIMEOUT) == XBEE_START) {
    if (get_packet())
    {
      // Only command the servos if a valid
      // packet has been received.
      rudder.write((axis[RUDDER_ADC]*180)/1024);
      throttle.write((axis[THROTTLE_ADC]*180)/1024);
    }
  }
}