xref: /frameworks/base/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="#components">ADK Components</a></li>
        <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="#accessory-protocol">Implementing the Android Accessory Protocol</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 implements the Android Accessory Protocol</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>

      <h2>Download</h2>
      <ol>
        <li><a href="https://dl-ssl.google.com/android/adk/adk_release_0512.zip">ADK package</a></li>
      </ol>

      <h2>See also</h2>

      <ol>
        <li><a href="http://www.youtube.com/watch?v=s7szcpXf2rE">Google I/O Session Video</a></li>
        <li><a href="{@docRoot}guide/topics/usb/accessory.html">USB Accessory Dev Guide</a></li>
      </ol>

      <h2>Where to buy</h2>

      <ol>

      <li><a href="http://store.arduino.cc/">
        Arduino Store</a></li>

      <li><a href="https://store.diydrones.com/ProductDetails.asp?ProductCode=BR-PhoneDrone">
        DIY Drones</a></li>

      <li><a href=
        "http://mbed.org/order/">
        mbed</a></li>

      <li><a href=
        "http://www.microchip.com/android">
        Microchip</a></li>

      <li><a href="http://shop.moderndevice.com/products/freeduino-usb-host-board">
        Modern Device</a></li>

      <li><a href=
        "http://www.rt-net.jp/shop/index.php?main_page=product_info&cPath=3_4&products_id=1">
        RT Corp</a></li>

      <li><a href="http://www.seeedstudio.com/depot/seeeduino-adk-main-board-p-846.html">
        Seeed Studio</a></li>

      <li><a href="http://www.sparkfun.com/products/10748">
        SparkFun</a></li>

      <li><a href="http://troido.de/de/shoplsmallgbuy-android-stufflsmallg">
        Troido</a></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 USB 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 to 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 following list of distributers are currently producing Android Open Accessory compatible
  development boards:</p>

  <ul>

    <li>The <a href="http://store.arduino.cc/">Arduino Store</a> provides the Arduino Mega ADK
    (in <a href="http://store.arduino.cc/eu/index.php?main_page=product_info&cPath=11_12&products_id=144">EU nations</a>
    or <a href="http://store.arduino.cc/ww/index.php?main_page=product_info&cPath=11_12&products_id=144">non-EU nations</a>)
    that is based on the ATmega2560 and supports the ADK firmware.</li>

    <li><a href="https://store.diydrones.com/ProductDetails.asp?ProductCode=BR-PhoneDrone">DIY
    Drones</a> provides an Arduino-compatible board geared towards RC (radio controlled) and UAV
    (unmanned aerial vehicle) enthusiasts.</li>

    <li><a href="http://mbed.org/order/">mbed</a> provides a microcontroller and a library
    to develop accessories that support the Android accessory protocol. For more information, see
    <a href="http://mbed.org/cookbook/mbed-with-Android-ADK">mbed with the Android ADK</a>.
    </li>

    <li><a href="http://www.microchip.com/android">Microchip</a> provides a PIC based USB
    microcontroller board.</li>

    <li><a href="http://shop.moderndevice.com/products/freeduino-usb-host-board">Modern
    Device</a> provides an Arduino-compatible board that supports the ADK firmware.</li>

    <li><a href="http://www.rt-net.jp/shop/index.php?main_page=product_info&cPath=3_4&products_id=1">
    RT Corp</a> provides an Arduino-compatible board based on the Android ADK board design.</li>

    <li><a href="http://www.seeedstudio.com/depot/seeeduino-adk-main-board-p-846.html">
    Seeed Studio</a> provides an Arduino-compatible board that supports the ADK firmware.</li>

    <li><a href="http://www.sparkfun.com/products/10748">
    SparkFun</a>'s IOIO board now has beta support for the ADK firmware.</li>

  </ul>

  <p>We expect more hardware distributers to create a variety of kits, so please stay tuned for
  further developments.</p>

  <h2 id="components">ADK Components</h2>
  <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 firmware code are contained in the <a href=
  "https://dl-ssl.google.com/android/adk/adk_release_0512.zip">ADK package download</a>.</p>
  <p>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>

        <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>
    </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="https://dl-ssl.google.com/android/adk/adk_release_0512.zip">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_0512.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>Create, if it does not already exist, an <code>Arduino</code>
        directory inside your user account's <code>Documents</code> directory, and within
        that, a <code>libraries</code> directory.</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 your
        <code>Documents/Arduino/libraries/</code> directory.</li>

        <li>Create a <code>CapSense</code> directory in your
        <code>Documents/Arduino/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; Verify/Compile</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>

    <li>Choose <strong>Create project from existing source</strong>, click <strong>Browse</strong>,
    select the <code>app</code> directory, click <strong>Open</strong> to close that dialog and then
    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.</p>
    </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 &gt;
  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>

  <h2 id="accessory-protocol">Implementing the Android Accessory Protocol</h2>

  <p>An Android USB accessory must adhere to Android Accessory Protocol, which defines how
  an accessory detects and sets up communication with an Android-powered device. 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>

  <p>The following sections go into depth about how to implement these steps.</p>

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

  <p>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">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 Android Open Accessory 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:  0
model name:         1
description:        2
version:            3
URI:                4
serial number:      5
</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 implements the Android Accessory protocol</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 implement the Android Accessory protocol. 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="#accessory-protocol">Implementing the Android Accessory Protocol</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">establish communication with the device</a>. If not, the board <a href=
  "#start">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-adk">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 ADK board
  reads and writes data.</p>