path: root/docs/html/guide/topics/usb/adk.jd

page.title=Android Open Accessory Development Kit
@jd:body

<div id="qv-wrapper">
<div id="qv">
    <h2>In this document</h2>
    <ol>
      <li><a href="#getting-started">Getting Started with the ADK</a>
        <ol>
          <li><a href="#installing">Installing the Arduino software and necessary libraries</a></li>
          <li><a href="#installing-firmware">Installing the firmware to the ADK board</a></li>
          <li><a href="#running-demokit">Running the DemoKit Android application</a></li>
          <li><a href="#monitoring">Monitoring the ADK board</a></li>
        </ol>
      </li>
      <li><a href="#how">How an Accessory Communicates with an Android-powered Device
      in Accessory Mode</a>
        <ol>
          <li><a href="#wait">Wait for and detect connected devices</a></li>
          <li><a href="#determine">Determine the connected device's accessory mode support</a></li>
          <li><a href="#start">Attempt to start the device in accessory mode</a></li>
          <li><a href="#establish">Establish communication with the device</a></li>
        </ol>
      </li>
      <li><a href="#firmware">How the ADK board communicates with an Android-powered Device
      in Accessory Mode</a>
        <ol>
          <li><a href="#wait-adk">Wait for and detect connected devices</a></li>
          <li><a href="#determine-adk">Determine the connected device's accessory mode support</a></li>
          <li><a href="#start-adk">Attempt to start the device in accessory mode</a></li>
          <li><a href="#establish-adk">Establish communication with the device</a></li>
        </ol>
      </li>
    </ol>
</div>
</div>

  <p>The Android 3.1 platform (also backported to Android 2.3.4) introduces Android Open Accessory support, which allows external USB hardware (an Android USB accessory)
  to interact with an Android-powered device in a special "accessory" mode. When an Android-powered powered device is in accessory mode,
  the connected accessory acts as the USB host (powers the bus and enumerates devices) and the Android-powered device acts as the device. 
  Android USB accessories are specifically designed to attach to Android-powered devices and
  adhere to a simple protocol (Android accessory protocol) that allows them to detect Android-powered devices that support accessory mode. Accessories must also provide 
  500mA at 5V for charging power. Many previously released
  Android-powered devices are only capable of acting as a USB device and cannot initiate connections with external USB devices. Android Open Accessory support
  overcomes this limitation and allows you to build accessories that can interact with an assortment of Android-powered devices by allowing the accessory
  initiate the connection.</p>
  
  <p class="note"><strong>Note:</strong> Accessory mode is ultimately dependent on the device's hardware
  and not all devices will support accessory mode. Devices that support accessory mode can be filtered using a <code>&lt;uses-feature&gt;</code>
  element in your corresponding application's Android manifest. For more information, see the <a href="{@docRoot}guide/topics/usb/accessory.html#manifest">USB Accessory</a> Developer
  Guide.</p>
  
  <p>The Android Open Accessory Development Kit (ADK) provides an implementation of an Android USB accessory that is based on the
  <a href="http://www.arduino.cc/">Arduino open source electronics prototyping platform</a>, the accessory's hardware design files,
  code that implements
  the accessory's firmware, and the Android application that interacts with the
  accessory. The hardware design files and code are contained in the <a href="https://dl-ssl.google.com/android/adk/adk_release_0506.zip">ADK package download</a>.
  You can <a href="http://www.rt-net.jp/products/rt-adk">buy the hardware components</a> of the ADK if you do not already have them.
  The main hardware and software components of the ADK include:</p>

  <ul>
    <li>A USB micro-controller board that is based on the Arduino Mega2560 and Circuits@Home USB Host
    Shield designs (now referred to as the ADK board), which you will later implement as an Android USB accessory.
    The ADK board provides input and output pins that you can implement through the use of attachments called "shields."
    Custom firmware, written in C++, is installed on the board to define the board's functionality and interaction with
    the attached shield and Android-powered device. The hardware design files for the
    board are located in <code>hardware/</code> directory.</li>

    <li>An Android Demo Shield (ADK shield) that affixes atop the ADK board implements the
    input and output points on the board. These implementations include a joystick, LED
    outputs, and temperature and light sensors. You can create or buy your own shields or wire
    your own features to the ADK board to implement custom functionality. 
    The hardware design files for the shield are located in <code>hardware/</code>.</li>

    <li>A library based on the <a href=
    "http://www.circuitsathome.com/arduino_usb_host_shield_projects">Arduino USB Host Shield</a>
    library provides the logic for the USB micro-controller board to act as a USB Host. This allows
    the board to initiate transactions with USB devices. Describing how to
    use this entire library is out of the scope of this document. Where needed, this document
    points out important interactions with the library. For more information, see the source code
    for the Arduino USB Host Shield library in the
    <code>firmware/arduino_libs/USB_Host_Shield</code> directory.</li>

    <li>An Arduino sketch, <code>firmware/demokit/demokit.pde</code>, defines the firmware that
    runs on the ADK board and is written in C++. The sketch calls the Android accessory protocol library
    to interact with the Android-powered device. It also sends data from the ADK board and shield to the Android application
    and receives data from the Android application and outputs it to the ADK board and shield.</li>

    <li>The Android accessory protocol library, which is located in the
    <code>firmware/arduino_libs/AndroidAccessory</code> directory. This library defines how to
    enumerate the bus, find a connected Android-powered device that supports accessory mode, and
    how to setup communication with the device.</li>

    <li>Other third party libraries to support the ADK board's functionality:

      <ul>
        <li><a href="http://www.arduino.cc/playground/Main/CapSense">CapSense library</a></li>

        <li><a href="http://www.arduino.cc/playground/Learning/I2C">I2C / TWI (Two-Wire Interface)
        library</a></li>

        <li><a href="http://www.arduino.cc/playground/ComponentLib/Servo">Servo library</a></li>

        <li><a href="http://www.arduino.cc/playground/Code/Spi">Spi library</a></li>

        <li><a href="http://www.arduino.cc/en/Reference/Wire">Wire library</a></li>
      </ul>
    </li>

    <li>An Android application, DemoKit, that communicates with the ADK board and shield. The
    source for this project is in the <code>app/</code> directory.</li>
  </ul>

  <h2 id="getting-started">Getting Started with the ADK</h2>

  <p>The following sections describe how to install the Arduino software on your computer, use the Arduino software
  to install the ADK board's firmware, and install and run the accompanying Android application for the ADK board.
  Before you begin, download the following items to set up your development environment:</p>

  <ul>
    <li><a href="http://www.arduino.cc/en/Main/software">Arduino Software</a>: contains libraries
    and an IDE for coding and installing firmware to the ADK board.</li>

    <li><a href="http://www.arduino.cc/playground/Main/CapSense">CapSense library</a>: contains the
    libraries to sense human capacitance. This is needed for the capacative button that is located
    on the ADK shield.</li>

    <li><a href="">The ADK package</a>: contains the firmware for the ADK board and
    hardware design files for the ADK board and shield.</li>
  </ul>

<h3 id="installing">Installing the Arduino software and necessary libraries</h3>
<p>To install the Arduino software:</p>
  <ol>
    <li><a href="http://arduino.cc/en/Guide/HomePage">Download and install</a> the Arduino Software
    as described on the Arduino website.
    
    <p class="note"><strong>Note:</strong> If you are on a Mac, install the FTDI USB Serial Driver
    that is included in the Arduino package,
    even though the installation instructions say otherwise.</p>
    </li>

    <li><a href="https://dl-ssl.google.com/android/adk/adk_release_0506.zip">Download</a> and extract the ADK package to a directory of your choice. You should have
    an <code>app</code>, <code>firmware</code>, and <code>hardware</code> directories.</li>

    <li>Extract the CapSense download to a directory of your choice.</li>

    <li>Install the necessary libraries:
      <p>On Windows:</p>
      <ol type="a">
        <li>Copy the <code>firmware/arduino_libs/AndroidAccessory</code> and
        <code>firmware/arduino_libs/USB_Host_Shield</code> directories (the complete directories,
        not just the files within) to the <code>&lt;arduino_installation_root&gt;/libraries/</code>
        directory.</li>

        <li>Create a CapSense directory in the
        <code>&lt;arduino_installation_root&gt;/libraries/</code> directory</li>

        <li>Copy <code>CapSense.cpp</code> and <code>CapSense.h</code> from the unzipped CapSense
        download to the <code>CapSense</code> directory.</li>
      </ol>

      <p>On Mac:</p>
      <ol type="a">
        <li>Right-click on the Arduino application in Finder and select <strong>Show Package
        Contents</strong>.</li>

        <li>Copy the <code>firmware/arduino_libs/AndroidAccessory</code> and
        <code>firmware/arduino_libs/USB_Host_Shield</code> directories (the complete directories,
        not just the files within) to the <code>Contents/Resources/Java/libraries</code>
        directory inside the Arduino application.</li>

        <li>Create a <code>CapSense</code> directory in the
        <code>Contents/Resources/Java/libraries</code> directory.</li>

        <li>Copy <code>CapSense.cpp</code> and <code>CapSense.h</code> from the unzipped CapSense
        download to the <code>CapSense</code> directory.</li>
        
      </ol>
      
      <p>On Linux (Ubuntu):</p>
       <ol type="a">
        <li>Copy the <code>firmware/arduino_libs/AndroidAccessory</code> and
        <code>firmware/arduino_libs/USB_Host_Shield</code> directories (the complete directories,
        not just the files within) to the <code>&lt;arduino_installation_root&gt;/libraries/</code> directory.</li>

        <li>Create a <code>CapSense</code> directory in the
        <code>&lt;arduino_installation_root&gt;/libraries/</code> directory.</li>

        <li>Copy <code>CapSense.cpp</code> and <code>CapSense.h</code> from the unzipped CapSense
        download to the <code>CapSense</code> directory.</li>
        
        <li>Install the avr-libc library by entering <code>sudo apt-get install avr-libc</code> from a shell prompt.</li>
      </ol>

</li>
</ol>
      <p>You should now have three new directories in the Arduino libraries directory:
      <code>AndroidAccessory</code>, <code>USB_Host_Shield</code>, and <code>CapSense</code>.</p>

<h3 id="installing-firmware">Installing the firmware to the ADK board</h3>
<p>To install the firmware to the ADK board:</p>
<ol>
    <li>Connect the ADK board to your computer using the micro-USB port, which allows two-way communication and provides power to the ADK board.</li>

    <li>Launch Arduino.</li>

    <li>Click <strong>Tools &gt; Board &gt; Arduino Mega 2560</strong> to specify the ADK board's type.</li>

    <li>Select the appropriate USB port:
    <ul>
    <li>On Windows: click <strong>Tools &gt; Serial Port &gt; COM#</strong> to specify the port of
    communication. The COM port number varies depending on your computer. COM1 is usually reserved for serial port
    connections. You most likely want COM2 or COM3.  </li>  
    
    <li>On Mac: Click <strong>Tools &gt; Serial Port &gt; dev/tty.usbserial-###</strong> to specify the
    port of communication.</li>
    
    <li>On Linux (Ubuntu): Click <strong>Tools &gt; Serial Port &gt; dev/ttyUSB#</strong> to specify the
    port of communication.</li>
    </ul></li>

    <li>To open the firmware code (a sketch), click <strong>File &gt; Open</strong> and select
    <code>firmware/demokit/demokit.pde</code>.</li>

    <li>Click <strong>Sketch &gt; Compile/Verify</strong> to ensure that the sketch has no
    errors.</li>

    <li>Select <strong>File &gt; Upload to I/O Board</strong>. When Arduino outputs <strong>Done uploading.</strong>, the board 
    is ready to communicate with your Android-powered device.</li>
    
    </ol>
    

<h3 id="running-demokit">Running the DemoKit Android application</h3>

  <p>The DemoKit Android application runs on your Android-powered device and communicates with the
  ADK board. The ADK board receives commands such as lighting up the board's LEDs or sends data
  from the board such as joystick movement and temperature readings.</p>

  <p>To install and run the application in Eclipse:</p>

  <ol>
    <li><a href="http://code.google.com/android/add-ons/google-apis/installing.html">Install the Google APIs API Level 10 add-on library</a>, 
    which includes the Open Accessory library for 2.3.4 devices that support accessory mode.
    This library is also forward compatible with Android 3.1 or newer devices that support accessory mode. If you only care
    about Android 3.1 or newer devices, all you need is API Level 12. For more information
    on deciding which API level to use, see the <a href="{@docRoot}guide/topics/USB/accessory.html#choosing">USB Accessory</a> documentation.</li>
    <li>Click <strong>File &gt; New &gt; Project...</strong>, then select <strong>Android &gt; Android Project</strong></li>
    <li>In the <strong>Project name:</strong> field, type DemoKit.
    <li>Choose <strong>Create project from existing source</strong>, click <strong>Browse</strong>, select the
    <code>app</code> directory, and click <strong>Finish</strong>.</li>

    <li>For Build Target, select <strong>Google APIs</strong> (Platform 2.3.3, API Level 10).
    <p class="note"><strong>Note:</strong> Even though the add-on is labeled as 
    <strong>2.3.3</strong>, the newest Google API add-on library for API level 10 adds
    USB Open Accessory API support for 2.3.4 devices.</li>

    <li>Click <strong>Finish</strong>.</li>
    <li>Install the application to your device.</li>

    <li>Connect the ADK board (USB-A) to your Android-powered device (micro-USB). Ensure that the power cable to the
    accessory is plugged in or that the micro-USB port on the accesory is connected to your computer
    for power (this also allows you to <a href="monitoring">monitor the ADK board</a>). When
    connected, accept the prompt that asks for whether or not to open the
    DemoKit application to connect to the accessory. If the prompt does not show up, connect and reconnect the accessory.</li>
  </ol>
  <p>You can now interact with the ADK board by moving the color LED or servo sliders (make sure the servos are connected) 
  or by pressing the relay buttons in the application. On the ADK shield, you can press the
  buttons and move the joystick to see their outputs displayed in the application.</p>

<h3 id="monitoring">Monitoring the ADK Board</h3>
<p>The ADK firmware consists of a few files that you should be looking at if you want to build your own accessory.
The files in the <code>firmware/arduino_libs/AndroidAccessory</code> directory are the most important files and have the logic to detect and connect
to Android-powered devices that support accessory mode. Feel free to add debug statements (Arduino <code>Serial.print()</code> statements) to the code located in the
<code>arduino_libraries_directory/AndroidAccessory</code> directory and <code>firmware/demokit/demokit.pde</code> sketch and re-upload the sketch to the ADK board to discover more
about how the firmware works.</p>

<p>You can view the debug statements in the Arduino Serial Monitor by clicking
<strong>Tools > Serial Monitor</strong> and setting the baud to 115200.
The following sections about how accessories communicate with Android-powered
devices describe much of what you should be doing in your own accessory.</p>
</p>

  <h2 id="how">How an Accessory Communicates with an Android-powered Device in Accessory Mode</h2>

  <p>When you connect an accessory to an Android-powered device, the accessory's firmware must
  carry out some standard steps to set up communication with the Android-powered device. If you are building an
  accessory along with an application, this section goes over some general steps that your firmware
  should carry out.</p>

  <p>In general, an accessory should carry out the following steps:</p>

  <ol>
    <li>Wait for and detect connected devices</li>

    <li>Determine the device's accessory mode support</li>

    <li>Attempt to start the device in accessory mode if needed</li>

    <li>Establish communication with the device if it supports the Android accessory protocol</li>
  </ol>

  <h3 id="wait"><p>Wait for and detect connected devices</h3>Your accessory should have logic to
  continuously check for connected Android-powered devices. When a device is connected, your accessory should
  determine if the device supports accessory mode.</p>

  <h3 id="determine"><p>Determine the device's accessory mode support</h3>
  <p>When an Android-powered device is connected, it can be in one of three states:</p>

  <ol type="a">
  
    <li>The attached device supports Android accessory mode and is already in accessory mode.</li>
    <li>The attached device supports Android accessory mode, but it is not in accessory mode.</li>
    <li>The attached device does not support Android accessory mode.</li>

    
  </ol>

  <p>During the initial connection, the accessory should check the vendor and product IDs of the connected device's USB
  device descriptor. The vendor ID should match Google's ID (0x18D1) and the product ID should be
  0x2D00 or 0x2D01 if the device is already in accessory mode (case A). If so, the accessory can now <a href=
  "#establish">establish communication with the device</a> through bulk transfer endpoints with its
  own communication protocol. There is no need to start the device in accessory mode.</p>

  <p class="note"><strong>Note:</strong> 0x2D00 is reserved for Android-powered devices that
  support accessory mode. 0x2D01 is reserved for devices that support accessory mode as well as the
  ADB (Android Debug Bridge) protocol, which exposes a second interface with two bulk endpoints for
  ADB. You can use these endpoints for debugging the accessory application if you are simulating
  the accessory on a computer. In general, do not use this interface unless your accessory is
  implementing a passthrough to ADB on the device.</p>

  <p>If the vendor and product ID do not match, there is no way to distinguish between states b and
  c, so the accessory <a href="#start">attempts to start the device in accessory mode</a> to
  figure out if the device is supported.</p>

  <h3 id="start">Attempt to start the device in accessory mode</h3>

  <p>If the vendor and product IDs do not correspond to an Android-powered device in accessory mode, the accessory
  cannot discern whether the device supports accessory mode and is not in that state, or if the
  device does not support accessory mode at all. This is because devices that support accessory mode but aren't in it
  initially report the device's manufacturer vendor ID and product ID, and not the special Google ones.
  In either case, the accessory should try to start the device
  into accessory mode to figure out if the device supports it. The following steps explain how to do this:</p>
  
  <ol>
  <li>Send a 51 control request ("Get Protocol") to figure out if the device supports
  the Android accessory protocol. A non-zero number is returned if the protocol is supported, which
  represents the version of the protocol that the device supports (currently, only version 1
  exists). This request is a control request on endpoint 0 with the following characteristics:
  <pre>
requestType:    USB_DIR_IN | USB_TYPE_VENDOR
request:        51
value:          0
index:          0
data:           protocol version number (16 bits little endian sent from the device to the accessory)
</pre>
</li>

  <li>If the device returns a proper protocol version, send identifying string information to the device.
  This information allows the device to figure out an appropriate application for this accessory and also present the user
  with a URL if an appropriate application does not exist. These requests
  are control requests on endpoint 0 (for each string ID) with the following characteristics:
  <pre>
requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
request:        52
value:          0
index:          string ID
data            zero terminated UTF8 string sent from accessory to device
</pre>

  <p>The following string IDs are supported, with a maximum size of 256 bytes for each string (must
  be zero terminated with \0).</p>
<pre>
manufacturer name:  1
model name:         2
description:        3
version:            4
URI:                5
serial number:      6
</pre>
</li>

  <li>When the identifying strings are sent, request the device start up in accessory mode.
  This request is a control request on endpoint 0 with the following
  characteristics:
  <pre>
requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
request:        53
value:          0
index:          0
data:           none
</pre>
</li>
</ol>
  <p>After sending the final control request, the connected USB device should re-introduce itself on the bus
  in accessory mode and the accessory can re-enumerate the connected devices. The algorithm jumps back to
  <a href="#determine">determining the device's accessory mode support</a> to check for the
  vendor and product ID. The vendor ID and product ID of the device will be different if the device
  successfully switched to accessory mode and will now correspond to Google's vendor and product
  IDs instead of the device manufacturer's IDs. The accessory can now <a href="#establish">establish communication with the device</a>.</p>

  <p>If at any point these steps fail, the device does not support Android accessory mode and the
  accessory should wait for the next device to be connected.</p>
  

  <h3 id="establish">Establish communication with the device</h3>

  <p>If an Android-powered device in accessory mode is detected, the accessory can query the device's interface and
  endpoint descriptors to obtain the bulk endpoints to communicate with the device. An
  Android-powered device that has a product ID of 0x2D00 has one interface with two bulk endpoints for
  input and output communication. A device with product ID of 0x2D01 has two interfaces with two bulk endpoints
  each for input and output communication. The first interface is for standard communication while the second
  interface is for ADB communication. To communicate on an interface, all you need to do is find the first bulk input and output endpoints,
  set the device's configuration to a value of 1 with a SET_CONFIGURATION (0x09) device request, then communicate using the endpoints.</p>

  

  <h2 id="firmware">How the ADK board communicates with an Android-powered Device in Accessory Mode</h2>

  <p>If you have access to the ADK board and shield, the following sections describe the firmware code that you installed onto the ADK board. The firmware demonstrates a practical
  example of how to communicate with an Android-powered device. Even if you do not have the ADK board and shield, reading through how the hardware detects
  and interacts with devices in accessory mode is still useful if you want to port the code over for your own accessories.</p>

  <p>The important pieces of the firmware are the <code>accessory/demokit/demokit/demokit.pde</code> sketch, which is the code that
  receives and sends data to the DemoKit application running on the Android-powered device. The
  code to detect and set up communication with the Android-powered device is contained in the
  <code>accessory/arduino_libs/AndroidAccessory/AndroidAccessory.h</code> and <code>accessory/arduino_libs/AndroidAccessory/AndroidAccessory.cpp</code>
  files. This code includes most of the logic that will help you implement your own accessory's firmware.
  It might be useful to have all three of these files open in a text editor as you read through these next sections.</p>

  <p>The following sections describe the firmware code in the context of the
  algorithm described in <a href="#how">How an Accessory Communicates with an Android-powered
  Device in Accessory Mode</a>.</p>

  <h3 id="wait-adk">Wait for and detect connected devices</h3>

  <p>In the firmware code (<code>demokit.pde</code>), the
  <code>loop()</code> function runs repeatedly and calls
  <code>AndroidAccessory::isConnected()</code> to check for any connected devices. If there is a
  connected device, it continuously updates the input and output streams going to and from the
  board and application. If nothing is connected, it continuously checks for a device to be connected:</p>
  <pre>
...

AndroidAccessory acc("Google, Inc.",
                     "DemoKit",
                     "DemoKit Arduino Board",
                     "1.0",
                     "http://www.android.com",
                     "0000000012345678");
                     
...
void loop()
{
...
    if (acc.isConnected()) {  
        //communicate with Android application 
    }
    else{
        //set the accessory to its default state
    }
...
}
</pre>

  <h3 id="determine-adk">Determine the connected device's accessory mode support</h3>

  <p>When a device is connected to the ADK board, it can already be in accessory mode,
  support accessory mode and is not in that mode, or does not support accessory mode. The
  <code>AndroidAccessory::isConnected()</code> method checks for these cases and responds
  accordingly when the <code>loop()</code> function calls it. This function first checks to see if
  the device that is connected hasn't already been handled. If not, it gets the connected device's
  device descriptor to figure out if the device is already in accessory mode by calling
  <code>AndroidAccessory::isAccessoryDevice()</code>. This method checks the vendor and product ID
  of the device descriptor. A device in accessory mode has a vendor ID of 0x18D1 and a product ID
  of 0x2D00 or 0x2D01. If the device is in accessory mode, then the ADK board can
  <a href="#establish-a">establish communication with the device</a>. If not, the board <a href=
  "start-a">attempts to start the device in accessory mode</a>.</p>
  <pre>
bool AndroidAccessory::isConnected(void)
{
    USB_DEVICE_DESCRIPTOR *devDesc = (USB_DEVICE_DESCRIPTOR *) descBuff;
    byte err;

    max.Task();
    usb.Task();

    if (!connected &amp;&amp;
        usb.getUsbTaskState() &gt;= USB_STATE_CONFIGURING &amp;&amp;
        usb.getUsbTaskState() != USB_STATE_RUNNING) {
        Serial.print("\nDevice addressed... ");
        Serial.print("Requesting device descriptor.");

        err = usb.getDevDescr(1, 0, 0x12, (char *) devDesc);
        if (err) {
            Serial.print("\nDevice descriptor cannot be retrieved. Program Halted\n");
            while(1);
        }

        if (isAccessoryDevice(devDesc)) {
            Serial.print("found android accessory device\n");

            connected = configureAndroid();
        } else {
            Serial.print("found possible device. switching to serial mode\n");
            switchDevice(1);
        }
    } else if (usb.getUsbTaskState() == USB_DETACHED_SUBSTATE_WAIT_FOR_DEVICE) {
        connected = false;
    }

    return connected;
}
</pre>

  <h3 id="start-adk">Attempt to start the device in accessory mode</h3>

  <p>If the device is not already in accessory mode, then the ADK board must
  determine whether or not it supports it by sending control request 51 to check the version of the
  USB accessory protocol that the device supports (see
  <code>AndroidAccessory::getProtocol()</code>). Protocol version 1 is the only version for now, but this can
  be an integer greater than zero in the future. If
  the appropriate protocol version is returned, the board sends control request 52 (one for each
  string with <code>AndroidAcessory:sendString()</code>) to send it's identifying information, and
  tries to start the device in accessory mode with control request 53. The
  <code>AndroidAccessory::switchDevice()</code> method takes care of this:</p>
  <pre>
bool AndroidAccessory::switchDevice(byte addr)
{
    int protocol = getProtocol(addr);
    if (protocol == 1) {
        Serial.print("device supports protocol 1\n");
    } else {
        Serial.print("could not read device protocol version\n");
        return false;
    }

    sendString(addr, ACCESSORY_STRING_MANUFACTURER, manufacturer);
    sendString(addr, ACCESSORY_STRING_MODEL, model);
    sendString(addr, ACCESSORY_STRING_DESCRIPTION, description);
    sendString(addr, ACCESSORY_STRING_VERSION, version);
    sendString(addr, ACCESSORY_STRING_URI, uri);
    sendString(addr, ACCESSORY_STRING_SERIAL, serial);

    usb.ctrlReq(addr, 0, USB_SETUP_HOST_TO_DEVICE | USB_SETUP_TYPE_VENDOR | USB_SETUP_RECIPIENT_DEVICE,
                ACCESSORY_START, 0, 0, 0, 0, NULL);
    return true;
}
</pre>If this method returns false, the board waits until a new device is connected. If it is
successful, the device displays itself on the USB bus as being in accessory mode when the ADK board
re-enumerates the bus. When the device is in accessory mode, the accessory then <a href=
"establish-a">establishes communication with the device</a>.

  <h3 id="establish-adk">Establish communication with the device</h3>

  <p>If a device is detected as being in accessory mode, the accessory must find the proper bulk
  endpoints and set up communication with the device. When the ADK board detects an
  Android-powered device in accessory mode, it calls the
  <code>AndroidAccessory::configureAndroid()</code> function:</p>
  <pre>
...
if (isAccessoryDevice(devDesc)) {
            Serial.print("found android acessory device\n");

            connected = configureAndroid();
        }
...
</pre>

  <p>which in turn calls the <code>findEndpoints()</code> function:</p>
  <pre>
...
bool AndroidAccessory::configureAndroid(void)
{
    byte err;
    EP_RECORD inEp, outEp;

    if (!findEndpoints(1, &amp;inEp, &amp;outEp))
        return false;
...
</pre>

  <p>The <code>AndroidAccessory::findEndpoints()</code> function queries the Android-powered device's
  configuration descriptor and finds the bulk data endpoints in which to communicate with the USB
  device. To do this, it first gets the device's first four bytes of the configuration
  descriptor (only need descBuff[2] and descBuff[3]), which contains the information about the
  total length of data returned by getting the descriptor. This data is used to determine whether
  or not the descriptor can fit in the descriptor buffer. This descriptor also contains information
  about all the interfaces and endpoint descriptors. If the descriptor is of appropriate size, the
  method reads the entire configuration descriptor and fills the entire descriptor buffer with this
  device's configuration descriptor. If for some reason the descriptor is no longer attainable,
  an error is returned.</p>
  <pre>
...

bool AndroidAccessory::findEndpoints(byte addr, EP_RECORD *inEp, EP_RECORD *outEp)
{
    int len;
    byte err;
    uint8_t *p;

    err = usb.getConfDescr(addr, 0, 4, 0, (char *)descBuff);
    if (err) {
        Serial.print("Can't get config descriptor length\n");
        return false;
    }


    len = descBuff[2] | ((int)descBuff[3] &lt;&lt; 8);
    if (len &gt; sizeof(descBuff)) {
        Serial.print("config descriptor too large\n");
            /* might want to truncate here */
        return false;
    }

    err = usb.getConfDescr(addr, 0, len, 0, (char *)descBuff);
    if (err) {
        Serial.print("Can't get config descriptor\n");
        return false;
    }
    
...
</pre>

  <p>Once the descriptor is in memory, a pointer is assigned to the first position of the buffer
  and is used to index the buffer for reading. There are two endpoint pointers (input and output)
  that are passed into <code>AndroidAccessory::findEndpoints()</code> and their addresses are set
  to 0, because the code hasn't found any suitable bulk endpoints yet. A loop reads the buffer, parsing
  each configuration, interface, or endpoint descriptor. For each descriptor,
  Position 0 always contains the size of the descriptor in bytes and position 1 always contains the
  descriptor type. Using these two values, the loop skips any configuration and interface
  descriptors and increments the buffer with the <code>descLen</code> variable to get to the next
  descriptor.</p>

  <p class="note"><strong>Note:</strong> An Android-powered device in accessory mode can
  potentially have two interfaces, one for the default communication to the device and the other
  for ADB communication. The default communication interface is always indexed first, so finding
  the first input and output bulk endpoints will return the default communication endpoints, which
  is what the <code>demokit.pde</code> sketch does. If you are writing your own firmware, the logic
  to find the appropriate endpoints for your accessory might be different.</p>

  <p>When it finds the first input and output endpoint descriptors, it sets the endpoint pointers
  to those addresses. If the findEndpoints() function finds both an input and output endpoint, it
  returns true. It ignores any other endpoints that it finds (the endpoints for the ADB interface, if
  present).</p>
  <pre>
...
    p = descBuff;
    inEp-&gt;epAddr = 0;
    outEp-&gt;epAddr = 0;
    while (p &lt; (descBuff + len)){
        uint8_t descLen = p[0];
        uint8_t descType = p[1];
        USB_ENDPOINT_DESCRIPTOR *epDesc;
        EP_RECORD *ep;

        switch (descType) {
        case USB_DESCRIPTOR_CONFIGURATION:
            Serial.print("config desc\n");
            break;

        case USB_DESCRIPTOR_INTERFACE:
            Serial.print("interface desc\n");
            break;

        case USB_DESCRIPTOR_ENDPOINT:
            epDesc = (USB_ENDPOINT_DESCRIPTOR *)p;
            if (!inEp-&gt;epAddr &amp;&amp; (epDesc-&gt;bEndpointAddress &amp; 0x80))
                ep = inEp;
            else if (!outEp-&gt;epAddr)
                ep = outEp;
            else
                ep = NULL;

            if (ep) {
                ep-&gt;epAddr = epDesc-&gt;bEndpointAddress &amp; 0x7f;
                ep-&gt;Attr = epDesc-&gt;bmAttributes;
                ep-&gt;MaxPktSize = epDesc-&gt;wMaxPacketSize;
                ep-&gt;sndToggle = bmSNDTOG0;
                ep-&gt;rcvToggle = bmRCVTOG0;
            }
            break;

        default:
            Serial.print("unkown desc type ");
            Serial.println( descType, HEX);
            break;
        }

        p += descLen;
    }

    if (!(inEp-&gt;epAddr &amp;&amp; outEp-&gt;epAddr))
        Serial.println("can't find accessory endpoints");

    return inEp-&gt;epAddr &amp;&amp; outEp-&gt;epAddr;
}

...
</pre>

  <p>Back in the <code>configureAndroid()</code> function, if there were endpoints found, they are
  appropriately set up for communication. The device's configuration is set to 1 and the state of the device is set to "running", which
  signifies that the device is properly set up to communicate with your USB accessory. Setting this
  status prevents the device from being re-detected and re-configured in the
  <code>AndroidAccessory::isConnected()</code> function.</p>
  <pre>
bool AndroidAccessory::configureAndroid(void)
{
    byte err;
    EP_RECORD inEp, outEp;

    if (!findEndpoints(1, &amp;inEp, &amp;outEp))
        return false;

    memset(&amp;epRecord, 0x0, sizeof(epRecord));

    epRecord[inEp.epAddr] = inEp;
    if (outEp.epAddr != inEp.epAddr)
        epRecord[outEp.epAddr] = outEp;

    in = inEp.epAddr;
    out = outEp.epAddr;

    Serial.print("inEp: ");
    Serial.println(inEp.epAddr, HEX);
    Serial.print("outEp: ");
    Serial.println(outEp.epAddr, HEX);

    epRecord[0] = *(usb.getDevTableEntry(0,0));
    usb.setDevTableEntry(1, epRecord);

    err = usb.setConf( 1, 0, 1 );
    if (err) {
        Serial.print("Can't set config to 1\n");
        return false;
    }

    usb.setUsbTaskState( USB_STATE_RUNNING );

    return true;
}
</pre>

  <p>Lastly, methods to read and write to the appropriate endpoints are needed. The
  <code>demokit.pde</code> sketch calls these methods depending on the data that is read from the
  Android-powered device or sent by the ADK board. For instance, moving the joystick
  on the ADK shield writes data that is read by the DemoKit application running on the
  Android-powered device. Moving sliders on the DemoKit application is read by the
  <code>demokit.pde</code> sketch and changes the state of the accessory, such as lighting up or
  changing the color of the LED lights.</p>
  <pre>
int AndroidAccessory::read(void *buff, int len, unsigned int nakLimit) { 
  return usb.newInTransfer(1, in, len, (char *)buff, nakLimit); } 
  
int AndroidAccessory::write(void *buff, int len) { 
  usb.outTransfer(1, out, len, (char *)buff); 
  return len; }
  
</pre>

  <p>See the <code>firmware/demokit/demokit.pde</code> file for information about how the Demo Shield reads and writes data.</p>