Java -> Arduino

In this part of the lab, you'll work with sample code that shows how to send messages from Java to your Arduino using serial communication. The Java program transmits (TX) data across the serial bus; the Arduino receives (RX) it and responds by changing the status of an LED. 

In the steps below, text highlighted in yellow denote values you will need to change to update your setup.

Step 1: Java program

Take the following code and place in a file named SimpleSerialTX.java.

/* 

* Adapted from Yu Hin Hau's code

* https://billwaa.wordpress.com/2012/05/18/engineering-arduino-to-java-communication-2/

* 

* Simple example of Java and Arduino Communication

* Sends messages using the serial bus.

* If sent message is: 0 -> LED off, 1 -> LED on, 2 -> LED blink once.

*/

import java.util.Scanner;

import jssc.SerialPort;

import jssc.SerialPortList;

import jssc.SerialPortException;

public class SimpleSerialTX

{

/**** SETTINGS -- change to match your setup! *******/

// change the following to match the serial port you use for the Arduino

final static String SERIAL_PORT_ADDRESS = "/dev/tty.usbmodem1421"; 

// change to match the baud rate you're using from Arduino

final static int BAUD_RATE = 19200;

/**** END SETTINGS *********************************/

//Declare Special Symbol Used in Serial Data Stream from Arduino

final static String start_char = "@";

final static String end_char = "#";

final static String sep_char = ":";

private static SerialPort serialPort;

private static Scanner reader;

public static void main(String[] args) 

{

//Initialize Port

serialPort = new SerialPort( SERIAL_PORT_ADDRESS );

//Create Scanner to read from Console

reader = new Scanner(System.in);

try

{

System.out.println( "about to open port" );

//Open Serial Port and set Parameters

serialPort.openPort();

serialPort.setParams( BAUD_RATE, 8, 1, 0);

// begin main loop

while ( true )

{

// wait for the user to enter a command, then send across the serial port

readFromConsole();

}

}

catch (SerialPortException ex)

{

System.out.println(ex);

}

}

/**

* Prompt and wait for the user to enter a command.

* TX (transmit) that command through the serial port.

**/

public static void readFromConsole()

{

System.out.print("Operation (0 = Off / 1 = On / 2 = Blink / q = Quit ): ");

try 

{

// if there is something to read from the command line

if(reader.hasNext())

{

/* Remember that through Serial, we are sending bytes through,

* therefore, we must convert the char input we got

* into byte form. In another word, the value reprsented in

* the system encoding, aka ASCII code. 0 is 48, 1 is 49,

* 2 is 50... luckily, we can type cast in Java. Mess around

* with this program to see the different ASCII representation

* for each of your character input!

*/

// read input from the command line

String input = reader.next();

// get the first character (ignore the rest)

char firstChar = input.charAt( 0 );

// // convert it to an array of chars

// char inputArray[] = input.toCharArray();

// if it's a 'q', quit

if ( firstChar == 'q')

{  

// close the port

serialPort.closePort();

// end the program

System.exit( 0 );

}

else

{

// convert to byte by casting

// and send the data across the port

serialPort.writeByte((byte)firstChar);

System.out.println((byte)firstChar + " <= sent!");

}

}

}

catch (SerialPortException ex)

{

System.out.println(ex);

}

}

}

• 1. First, you will need to update the values of the constants to match your setup. Notice the settings area at the top of the class file. 

     1. The SERIAL_PORT_ADDRESS should match the port you select in the Arduino application.

     2. The BAUD_RATE must match that of the Arduino sketch (you should not have to change this). 

  2. Next, you will compile your code. This class relies on the JSSC (Java Simple Serial Connector) library, which is included in the Arduino environment (versions 1.5.6+). Therefore, in order to compile your java class, you'll need to include this library in the class path.

  3. First, determine the path of the library; it is actually stored as a JAR within your Arduino application. To see the actual file, right click your Arduino application in the Finder and select Show Package Contents. Within the resulting directory structure, navigate to Contents -> Resources -> Java. Within this directory, you should see a file name jssc****.jar, where the **** indicates the version number. Right-click that file and select Get Info. In the resulting window, look for the path to the file (next to "Where").

• 1. You will need to use this path along with the -cp flag when compiling your java program. For example, using the above path, I would compile my code with the command:

  2. javac -cp /Applications/Arduino1.6.0.app/Contents/Resources/Java/jssc-2.8.0.jar *.java

  3. Confirm that your code compiled -- you should see a SimpleSerialTX.class class file.

Step 2: Arduino sketch

Copy and upload the following Arduino sketch. It waits to receive (RX) messages from the Serial bus and changes the status of the onboard LED accordingly. (0 -> LED off, 1 -> LED on, 2 -> LED blink once)

/* 

* Adapted from Yu Hin Hau's code

* https://billwaa.wordpress.com/2012/05/18/engineering-arduino-to-java-communication-2/

* 

* Simple example of Java and Arduino Communication

* Receives messages using the serial bus.

* If received message is: 0 -> LED off, 1 -> LED on, 2 -> LED blink once.

*/

// settings

int BAUD_RATE = 19200;

int LED = 13;

//Define special symbols

char start_char    = '@';

char end_char      = '#';

char sep_char      = ':';

// rx data

char operation     = 0; // to hold received data

void setup() 

{

        // for communication

Serial.begin(BAUD_RATE);

        // for LED output

pinMode(LED, OUTPUT);

}

/**

* At each loop iteration,

* see if there is a message to receive (rx) and store it to use as the next operation.

**/

void loop() 

{

// try to receive a message

if ( receiveMessage() )

{

// if so, use it to control the LED

doOp();

}

}

/**

* See if there is a message to receive. If so, store it in

* operation variable.

**/

boolean receiveMessage()

{

// check if there is a message 

boolean msgWaiting = (Serial.available() > 0) ? true : false;

// if there is one, read it and store in operation

if ( msgWaiting )

operation  = Serial.read();

// return true if we got a message

return msgWaiting;  

}

/*Remember that the data we sent 

* from the Java client are bytes casted as chars;

* therefore, the values that get sent

* through are ASCII numerical codes.

* We use this data to convert / compare it back with

* a char type value.

* 0 -> LED off, 1 -> LED on, 2 -> LED blink once

*/

void doOp()

{

//LED Off

if(operation == '0')

digitalWrite(LED, LOW);

//LED On

else if(operation == '1')

digitalWrite(LED, HIGH);

//LED Blink

else if(operation == '2')

{

digitalWrite(LED, HIGH);

delay(500);

digitalWrite(LED, LOW);

delay(500);

}

}

• 1. Notice the BAUD_RATE value matches the constant from the Java file -- it is important that they agree on the communication rate!

  2. Upload your sketch to the Arduino. 

  3. Do not open the Serial monitor, as it will interfere with the java program's ability to access the bus.

Step 3: sending messages from Java -> Arduino

Now, it's time to test out sending messages from Java. Remember that the Java program relies on the JSSC library. Therefore, you need to again include the JAR in the classpath; in addition, you will need to include the directory with the class file of your java program. Here, we assume the current directory, denoted by the dot (.), and append it to the path by using the OS-dependent path separator ( : for *nix systems, including Mac OS X).

• 1. Run your java program with the java command and -cp flag, as in

  2. java -cp /Applications/Arduino1.6.0.app/Contents/Resources/Java/jssc-2.8.0.jar:. SimpleSerialTX

  3. Test it out by sending messages as indicated. You should see your onboard LED respond!