Android-Arduino Communication via USB OTG

USB On-the-go capability in Android devices has now become more available in the market. And why wouldn’t it be? This feature, nicknamed OTG, enables you to use your flash drives, keyboards, mice and even printers with just your phone! What’s more interesting is it can also enable you to communicate and control your microprocessors using your Android device without the need for additional modules – just a cable. In this article, we will see how this communication can become possible. To demonstrate, we will control the behavior of an LED and send messages to another very popular item in the electronics world – the Arduino.

The shorter end of the stick is Arduino’s side. In your Arduino, simply upload this code:

int ledPin = 13;

void setup(){  
  Serial.begin(9600);
  Serial.setTimeout(200);
  pinMode(ledPin, OUTPUT);  
}  

void loop(){
  if (Serial.available()){
    String c = Serial.readString();
    if (c.equals("TONLED")) digitalWrite(ledPin, HIGH); 
    else if (c.equals("TOFFLED")) digitalWrite(ledPin, LOW);
    else Serial.print(c);
  }
}

In the above sketch, we are simply waiting for the data arriving at our serial line and performing actions based on the data received. For instance, turning on the LED ledPin requires a TONLED message from our Android device. You’ve probably noticed that there are no special libraries or methods in our Arduino sketch.  That’s a great thing because it tells us that the system is not exclusive to Arduino and will work with any microcontroller that supports serial communication.

Let’s now move on to Android’s side. The first step is to create an Android project and add the necessary components. In the project we created, we added extra components for user convenience. For learning and testing purposes, only the following are necessary:

• Text Field – used to get input data by the user, which will be sent to and echoed by the Arduino
• Toggle Button – used to control the behavior of the LED
• Start Button – used to open the serial port
• Send Button – used to send messages to Arduino
• Text View – used to display logs

To simplify the setup and processes, we will use the UsbSerial library by felHR85. There are a lot of libraries you can choose from. In case you have other preferences, feel free to modify and adapt to your preferred library.

In the build.gradle of your project, add jitpack. Jitpack is a very awesome tool that enables us to get a Git project into our build.

allprojects {
    repositories {
        jcenter()
        maven { url "https://jitpack.io" }
    }
}

Now, add the dependency to your module’s build.gradle.

compile 'com.github.felHR85:UsbSerial:4.3'

Moving on to our main activity, there are some variables that we wish to declare globally for convenience.

private static final String ACTION_USB_PERMISSION = "com.android.example.USB_PERMISSION";

UsbDevice device;
UsbDeviceConnection connection;
UsbManager usbManager;
UsbSerialDevice serialPort;
PendingIntent pendingIntent;

The next items that we will present here will not be discussed thoroughly, but you can refer to Android's official documentation for details.

Before trying to start the communication, you must seek permission from the user. To do this, create a broadcast receiver. This receiver listens for the intent that gets broadcasted when you call requestPermission(). Only when granted can we proceed to opening the connection and setting parameters for the Serial communication.

private final BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        if (intent.getAction().equals(ACTION_USB_PERMISSION)) {
            boolean granted = intent.getExtras().getBoolean(UsbManager.EXTRA_PERMISSION_GRANTED);
            if (granted) {
                connection = usbManager.openDevice(device);
                serialPort = UsbSerialDevice.createUsbSerialDevice(device, connection);
                if (serialPort != null) {
                    if (serialPort.open()) {
                        serialPort.setBaudRate(9600);
                        serialPort.setDataBits(UsbSerialInterface.DATA_BITS_8);
                        serialPort.setStopBits(UsbSerialInterface.STOP_BITS_1);
                        serialPort.setParity(UsbSerialInterface.PARITY_NONE);
                        serialPort.setFlowControl(UsbSerialInterface.FLOW_CONTROL_OFF);
                        serialPort.read(mCallback); 
                    } else {
                        Log.d("SERIAL", "PORT NOT OPEN");
                    }
                } else {
                    Log.d("SERIAL", "PORT IS NULL");
                }
            } else {
                Log.d("SERIAL", "PERMISSION NOT GRANTED");
            }
        } else if (intent.getAction().equals(UsbManager.ACTION_USB_DEVICE_ATTACHED)) {
            onClickStart(startButton);
        } else if (intent.getAction().equals(UsbManager.ACTION_USB_DEVICE_DETACHED)) {
            //can add something to close the connection
        }
    };
};

On your onCreate method, declare the intent and register your broadcast receiver to start and stop the serial connection.

pendingIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0);
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
registerReceiver(broadcastReceiver, filter);

In our application, we created a start button to start the connection when pressed. In the method that corresponds to the onClick action of our button, we add the following:

public void onClickStart(View view) {

    if (!isSerialStarted) {
        usbManager = (UsbManager) getSystemService(Context.USB_SERVICE);

        HashMap<String, UsbDevice> usbDevices = usbManager.getDeviceList();
        if (!usbDevices.isEmpty()) {
            boolean keep = true;
            for (Map.Entry<String, UsbDevice> entry : usbDevices.entrySet()) {
                device = entry.getValue();
                int deviceVID = device.getVendorId();

                if (deviceVID == 1027 || deviceVID == 9025) { //Arduino Vendor ID
                    usbManager.requestPermission(device, pendingIntent); 
                    keep = false;
                } else {
                    connection = null;
                    device = null;
                }
                if (!keep)
                    break;
            }
        }
    }
}

The code above searches for vendor IDs 1027 or 9025 – the vendor ID’s associated to FTDI or Arduino. The vendor ID equal to 9025 is the more popular and more common value based on other articles in the internet, but mine has an ID of 1027. The easiest way to know is to just print the vendor IDs detected by the Android device.  If the vendor ID matches the expected ID for our device, we will call the requestPermission() method. With this, the intent will be broadcasted and picked up by our receiver, starting and opening the connection.

Once communication is opened, we can start sending and receiving data. To receive from Arduino, simply add the codes below. Note that we are appending the data received to the text view.

private UsbSerialInterface.UsbReadCallback mCallback = new UsbSerialInterface.UsbReadCallback() {
    //Defining a Callback which triggers whenever data is read.
    @Override
    public void onReceivedData(byte[] arg0) {
        String data = null;
        try {
            data = new String(arg0, "UTF-8");
            data.concat("/n");
            tvAppend(displayView, data);
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
    }
};

private void tvAppend(final TextView tv, final CharSequence text) {
    runOnUiThread(new Runnable() {
        @Override public void run() {
            if (text != null) {
                tv.append(text); 
            }
        }
    });
}

Sending data is easier. We only need to get user input from the text field, and send it to the connected device.

public void onClickSend(View view) {
    String textInput = inputView.getText().toString();
    serialPort.write(textInput.getBytes());
}

To control the LED in Arduino, simply add the code below. You are free to change TONLED and TOFFLED to whatever names you want. Just don’t forget to adjust the Arduino code as well.

public void onClickToggle(View view) {
    if (isLedON == false) {
        isLedON = true;
        tvAppend(displayView, "\nLED TURNED ON\n");
        serialPort.write("TONLED".getBytes());
    } else {
        isLedON = false;
        serialPort.write("TOFFLED".getBytes());
        tvAppend(displayView, "\nLED TURNED OFF\n");
    }
}

You can close the connection using:

serialPort.close();

We are almost done. In your manifest file, add the following so that your application will be notified of an attached USB device. 

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="yourpackage.com.name">

    <uses-feature android:name="android.hardware.usb.host" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
            <intent-filter>
                <action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" />
            </intent-filter>
            <meta-data
                android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"
                android:resource="@xml/device_filter" />
            <intent-filter>
                <action android:name="android.hardware.usb.action.USB_DEVICE_DETACHED" />
            </intent-filter>
        </activity>
    </application>
</manifest>

Create an xml folder inside the res folder and add device_filter.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <usb-device
        vendor-id="9025"/>
</resources>

And… were done! Additional tips, we can add a checker to confirm that the serial connection is already open. This saves us from crashes due to attempts to send serial data while the connection is still closed. We can also add clear buttons, or place the text view inside a scroll view then automatically scroll to end of page using: 

mScrollView.smoothScrollTo(0, displayView.getBottom());

That’s it. If you want to extend your phone’s sensors, or if you want to add storage, wireless and messaging capability, camera and orientation sensors in your microprocessor project with just one device, USB On-the-Go, may be your way to go.

A demo application, SimpleArDroid by YMSoftLabs can be downloaded from Google Play. Here's a video of how the system works.

References:
UsbSerial: A serial port driver library for Android v3.0
USBSerial
Communicate with Your Arduino Through Android