To start playing with Arduino, you just need to set up UDOO and connect the Android part to the SAM3X (Arduino-compatible) part. The following images show an UDOO, as seen from above. On the left, jumper 18 is highlighted. This jumper must be unplugged to enable the SAM3X. On the right, the USB port you are going to connect to is highlighted as well:

Once the board is ready, you can move to the software part.

For this quick example, we are going to command an LED connected to the UDOO. The LED will be connected to input 13 of the UDOO board. Every LED has two pins; the longer one is the anode and has to be connected to input 13, and the shorter one is the cathode and has to be connected to ground, the unnumbered input on the left of input 13:

The electronics setup is in place. Let's download the Arduino IDE from http://www.udoo.org/other-resources/.

The first time you run the Arduino IDE, you will be presented with an empty project file:

This empty Arduino sketch gives you a skeleton structure for our Arduino program:

To properly connect and program our Arduino, we need to select the board type and port. From the Arduino IDE Tools menu, select Board | Arduino Due (Programming Port):

The previous image shows the amount of different Arduino boards that are available on the market nowadays. UDOO is compatible with the Arduino Due, so we are choosing that board model. Once we have selected the proper board, we need to select the Port to use to connect to the UDOO:

As you can see in the image, the port name could be slightly different on different computers. The previous image shows a common configuration for an Apple MacBook Pro.

Once the IDE has been properly configured, we can start with the source code, as follows:

#include "variant.h"
#include <stdio.h>
#include <adk.h>

#define  LED_PIN  13

// Accessory descriptor. It's how Arduino identifies itself to Android.
char descriptionName[] = "ArduinoADK"; 
char modelName[] = "UDOO_ADK";           // Arduino Accessory name (Need to be the same defined in the Android App)
char manufacturerName[] = "Packt";     // Manufacturer (Need to be the same defined in the Android App)

char versionNumber[] = "1.0";            // version (Need to be the same defined in the Android App)
char serialNumber[] = "1";
char url[] = "http://www.packtpub.com";      // If there isn't any compatible app installed, Android suggest to visit this url

USBHost Usb;
ADK adk(&Usb, manufacturerName, modelName, descriptionName, versionNumber, url, serialNumber);

#define RCVSIZE 128
uint8_t buf[RCVSIZE];
uint32_t bytesRead = 0;

void setup() {
    Serial.begin(115200);   
    pinMode(LED_PIN, OUTPUT);
    delay(500);
    Serial.println("Starting...");
}

void loop() {
    Usb.Task();
     
    if (adk.isReady()) {
      adk.read(&bytesRead, RCVSIZE, buf);// read data into buf variable
      if (bytesRead > 0) {
        if (parseCommand(buf[0]) == 1) {// compare received data
          // Received "1" - turn on LED
          digitalWrite(LED_PIN, HIGH);
        } else if (parseCommand(buf[0]) == 0) {
          // Received "0" - turn off LED
          digitalWrite(LED_PIN, LOW); 
        }  
      }
    } else {
      digitalWrite(LED_PIN , LOW); // turn off light
    }  
    
    delay(10);
}

// the characters sent to Arduino are interpreted as ASCII, we decrease 48 to return to ASCII range.
uint8_t parseCommand(uint8_t received) {
  return received - 48;
}

We can quickly analyze the source code and can find out that:

Once the source code is in place, you can flash it to the Arduino using the IDE File | Upload menu.

The Android app will be super simple: a toggle button to turn the LED ON and OFF. You can create the initial app using the Android Studio wizard, creating an empty Activity to get things started. Once the skeleton is in place, you need to add a new dependency to your build.gradle:

dependencies {
    compile 'me.palazzetti:adktoolkit:0.3.0'
}

Emanuele Palazzetti, author of Getting started with UDOO by Packt Publishing, released a handy Android library, ADK Toolkit (https://github.com/palazzem/adk-toolkit), for easier communication between your Android apps and your Android ADK device, and we are going to take full advantage of this library.

You need to add some specific configurations in your Android Manifest. In your <activity> tag, add the following lines:

<intent-filter>
  <action android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"/>
</intent-filter>
<meta-data
  android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"
  android:resource="@xml/accessory_filter"/>

As you will have noted, the <meta-data> tag refers to an XML resource, named accessory_filter. Right now, it's missing. Let's create a accessory_filter.xml in the src/res/xml/ folder:

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <usb-accessory
    manufacturer="Packt"
    model="UDOO_ADK"
    version="1.0"/>
</resources>

This is the exact information we added to the Arduino sketch and will allow Android to properly identify our board.

Setup is over. Let's move on to the UI of our app. Having followed the wizard, you now have a single Activity with its own layout; chances are that its name is main.xml and it's located in src/res/layout. Once you have located the layout, we can add our button:

<ToggleButton 
 android:id="@+id/toggleButtonLED"
 android:textOn="Turn OFF" 
 android:textOff="Turn ON" 
 android:layout_width="500dp" 
 android:layout_height="200dp" 
 android:layout_centerVertical="true" 
 android:layout_centerHorizontal="true"
 android:textSize="50sp"
 android:onClick="blinkLED"/>

It's pretty straightforward: an ID, a couple of labels, and an onClick method to trigger when the button is tapped.

The method referred by the onClick will be placed into our Activity:

public void blinkLED(View v) {
    if (buttonLED.isChecked()) {
        adkManager.write("1");
    } else {
        adkManager.write("0");
    }
}

When the button is clicked, we send 1 if it's ON, or 0 if it's OFF. Fair enough, but where do we send this data? What's that adkManager?

The adkManager module comes with the ADK Toolkit. We create it and set it up in our Activity. This is the final result:

public class UDOOBlinkLEDActivity extends Activity {

    private ToggleButton buttonLED;

    private AdkManager adkManager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        buttonLED = (ToggleButton) findViewById(R.id.toggleButtonLED);

        adkManager = new AdkManager(this);
        registerReceiver(adkManager.getUsbReceiver(), adkManager.getDetachedFilter());
    }

    @Override
    public void onResume() {
        super.onResume();
        adkManager.open();
    }

    @Override
    public void onPause() {
        super.onPause();
        adkManager.close();
    }

    public void blinkLED(View v) {
        if (buttonLED.isChecked()) {
            adkManager.write("1");
        } else {
            adkManager.write("0");
        }
    }
}

Finally, our app is completed. Just upload it to our UDOO and you will have a huge button to turn your LED ON and OFF: