topics/usb/host.jd

page.title=USB Host
@jd:body

  <div id="qv-wrapper">
    <div id="qv">
      <h2>In this document</h2>

      <ol>
        <li><a href="#api">API Overview</a></li>

        <li><a href="#usage">Android Manifest Requirements</a></li>

        <li>
          <a href="#working-d">Working with devices</a>

          <ol>
            <li><a href="#discovering-d">Discovering a device</a></li>

            <li><a href="#permission-d">Obtaining permission to communicate with a device</a></li>

            <li><a href="#communicating-d">Communicating with a device</a></li>

            <li><a href="#terminating-d">Terminating communication with a device</a></li>
          </ol>
        </li>
      </ol>

      <h2>Related Samples</h2>

      <ol>
        <li><a href="{@docRoot}resources/samples/USB/AdbTest/index.html">AdbTest</a></li>

        <li><a href=
        "{@docRoot}resources/samples/USB/MissileLauncher/index.html">MissleLauncher</a></li>
      </ol>
    </div>
  </div>

  <p>When your Android-powered device is in USB host mode, it acts as the USB host, powers the bus,
  and enumerates connected USB devices. USB host mode is supported in Android 3.1 and higher.</p>

  <h2 id="api">API Overview</h2>

  <p>Before you begin, it is important to understand the classes that you need to work with. The
  following table describes the USB host APIs in the {@link android.hardware.usb} package.</p>

  <p class="table-caption"><strong>Table 1.</strong> USB Host APIs</p>

  <table>
    <tr>
      <th>Class</th>

      <th>Description</th>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbManager}</td>

      <td>Allows you to enumerate and communicate with connected USB devices.</td>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbDevice}</td>

      <td>Represents a connected USB device and contains methods to access its identifying
      information, interfaces, and endpoints.</td>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbInterface}</td>

      <td>Represents an interface of a USB device, which defines a set of functionality for the
      device. A device can have one or more interfaces on which to communicate on.</td>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbEndpoint}</td>

      <td>Represents an interface endpoint, which is a communication channel for this interface. An
      interface can have one or more endpoints, and usually has input and output endpoints for
      two-way communication with the device.</td>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbDeviceConnection}</td>

      <td>Represents a connection to the device, which transfers data on endpoints. This class
      allows you to send data back and forth sychronously or asynchronously.</td>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbRequest}</td>

      <td>Represents an asynchronous request to communicate with a device through a {@link
      android.hardware.usb.UsbDeviceConnection}.</td>
    </tr>

    <tr>
      <td>{@link android.hardware.usb.UsbConstants}</td>

      <td>Defines USB constants that correspond to definitions in linux/usb/ch9.h of the Linux
      kernel.</td>
    </tr>
  </table>

  <p>In most situations, you need to use all of these classes ({@link
  android.hardware.usb.UsbRequest} is only required if you are doing asynchronous communication)
  when communicating with a USB device. In general, you obtain a {@link
  android.hardware.usb.UsbManager} to retrieve the desired {@link android.hardware.usb.UsbDevice}.
  When you have the device, you need to find the appropriate {@link
  android.hardware.usb.UsbInterface} and the {@link android.hardware.usb.UsbEndpoint} of that
  interface to communicate on. Once you obtain the correct endpoint, open a {@link
  android.hardware.usb.UsbDeviceConnection} to communicate with the USB device.</p>

  <h2 id="manifest">Android Manifest Requirements</h2>

  <p>The following list describes what you need to add to your application's manifest file before
  working with the USB host APIs:</p>

  <ul>
    <li>Because not all Android-powered devices are guaranteed to support the USB host APIs,
    include a <code>&lt;uses-feature&gt;</code> element that declares that your application uses
    the <code>android.hardware.usb.host</code> feature.</li>

    <li>Set the minimum SDK of the application to API Level 12 or higher. The USB host APIs are not
    present on earlier API levels.</li>

    <li>If you want your application to be notified of an attached USB device, specify an
    <code>&lt;intent-filter&gt;</code> and <code>&lt;meta-data&gt;</code> element pair for the
    <code>android.hardware.usb.action.USB_DEVICE_ATTACHED</code> intent in your main activity. The
    <code>&lt;meta-data&gt;</code> element points to an external XML resource file that declares
    identifying information about the device that you want to detect.

      <p>In the XML resource file, declare <code>&lt;usb-device&gt;</code> elements for the USB
      devices that you want to filter. The following list describes the attributes of
      <code>&lt;usb-device&gt;</code>. In general, use vendor and product ID if you want to filter
      for a specific device and use class, subclass, and protocol if you want to filter for a group
      of USB devices, such as mass storage devices or digital cameras. You can specify none or
      all of these attributes. Specifying no attributes matches every USB device, so only do this
      if your application requires it:</p>

      <ul>
        <li><code>vendor-id</code></li>

        <li><code>product-id</code></li>

        <li><code>class</code></li>

        <li><code>subclass</code></li>

        <li><code>protocol</code> (device or interface)</li>
      </ul>

      <p>Save the resource file in the <code>res/xml/</code> directory. The resource file name
      (without the .xml extension) must be the same as the one you specified in the
      <code>&lt;meta-data&gt;</code> element. The format for the XML resource file is in the
      <a href="#example">example</a> below.</p>
    </li>
  </ul>

  <h3 id="manifest-example">Manifest and resource file examples</h3>

  <p>The following example shows a sample manifest and its corresponding resource file:</p>
  <pre>
&lt;manifest ...&gt;
    &lt;uses-feature android:name="android.hardware.usb.host" /&gt;
    &lt;uses-sdk android:minSdkVersion="12" /&gt;
    ...
    &lt;application&gt;
        &lt;activity ...&gt;
            ...
            &lt;intent-filter&gt;
                &lt;action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" /&gt;
            &lt;/intent-filter&gt;

            &lt;meta-data android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"
                android:resource="@xml/device_filter" /&gt;
        &lt;/activity&gt;
    &lt;/application&gt;
&lt;/manifest&gt;
</pre>

  <p>In this case, the following resource file should be saved in
  <code>res/xml/device_filter.xml</code> and specifies that any USB device with the specified
  attributes should be filtered:</p>
  <pre>
&lt;?xml version="1.0" encoding="utf-8"?&gt;

&lt;resources&gt;
    &lt;usb-device vendor-id="1234" product-id="5678" class="255" subclass="66" protocol="1" /&gt;
&lt;/resources&gt;
</pre>

  <h2 id="working-d">Working with Devices</h2>

  <p>When users connect USB devices to an Android-powered device, the Android system can determine
  whether your application is interested in the connected device. If so, you can set up
  communication with the device if desired. To do this, your application has to:</p>

  <ol>
    <li>Discover connected USB devices by using an intent filter to be notified when the user
    connects a USB device or by enumerating USB devices that are already connected.</li>

    <li>Ask the user for permission to connect to the USB device, if not already obtained.</li>

    <li>Communicate with the USB device by reading and writing data on the appropriate interface
    endpoints.</li>
  </ol>

  <h3 id="discovering-d">Discovering a device</h3>

  <p>Your application can discover USB devices by either using an intent filter to be notified when
  the user connects a device or by enumerating USB devices that are already connected. Using an
  intent filter is useful if you want to be able to have your application automatically detect a
  desired device. Enumerating connected USB devices is useful if you want to get a list of all
  connected devices or if your application did not filter for an intent.</p>

  <h4 id="using-intents">Using an intent filter</h4>

  <p>To have your application discover a particular USB device, you can specify an intent filter to
  filter for the <code>android.hardware.usb.action.USB_DEVICE_ATTACHED</code> intent. Along with
  this intent filter, you need to specify a resource file that specifies properties of the USB
  device, such as product and vendor ID. When users connect a device that matches your device
  filter, the system presents them with a dialog that asks if they want to start your application.
  If users accept, your application automatically has permission to access the device until the
  device is disconnected.</p>

  <p>The following example shows how to declare the intent filter:</p>
  <pre>
&lt;activity ...&gt;
...
    &lt;intent-filter&gt;
        &lt;action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" /&gt;
    &lt;/intent-filter&gt;

    &lt;meta-data android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"
        android:resource="@xml/device_filter" /&gt;
&lt;/activity&gt;
</pre>

  <p>The following example shows how to declare the corresponding resource file that specifies the
  USB devices that you're interested in:</p>
  <pre>
&lt;?xml version="1.0" encoding="utf-8"?&gt;

&lt;resources&gt;
    &lt;usb-device vendor-id="1234" product-id="5678" /&gt;
&lt;/resources&gt;
</pre><!-- REMEMBER TO ADD PROTOCOL, and device class and subclass, if applicable -->

  <p>In your activity, you can obtain the {@link android.hardware.usb.UsbDevice} that represents
  the attached device from the intent like this:</p>
  <pre>
UsbDevice device = (UsbDevice) intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
</pre>

  <h4>Enumerating devices</h4>

  <p>If your application is interested in inspecting all of the USB devices currently connected
  while your application is running, it can enumerate devices on the bus. Use the {@link
  android.hardware.usb.UsbManager#getDeviceList() getDeviceList()} method to get a hash map of all
  the USB devices that are connected. The hash map is keyed by the USB device's name if you want to
  obtain a device from the map.</p>
  <pre>
UsbManager manager = (UsbManager) getSystemService(Context.USB_SERVICE);
...
HashMap&lt;String, UsbDevice&gt; deviceList = manager.getDeviceList();
UsbDevice device = deviceList.get("deviceName");
</pre>

  <p>If desired, you can also just obtain an iterator from the hash map and process each device one
  by one:</p>
  <pre>
UsbManager manager = (UsbManager) getSystemService(Context.USB_SERVICE);
...
HashMap&lt;String, UsbDevice&gt; deviceList = manager.getDeviceList();
Iterator&lt;UsbDevice&gt; deviceIterator = deviceList.values().iterator();
while(deviceIterator.hasNext()){
    UsbDevice device = deviceIterator.next()
    //your code
}
</pre>

  <h3 id="permission-d">Obtaining permission to communicate with a device</h3>

  <p>Before communicating with the USB device, your applicaton must have permission from your
  users.</p>

  <p class="note"><strong>Note:</strong> If your application <a href="#using-intents">uses an
  intent filter</a> to discover USB devices as they're connected, it automatically receives
  permission if the user allows your application to handle the intent. If not, you must request
  permission explicitly in your application before connecting to the device.</p>

  <p>Explicitly asking for permission might be neccessary in some situations such as when your
  application enumerates USB devices that are already connected and then wants to communicate with
  one. You must check for permission to access a device before trying to communicate with it. If
  not, you will receive a runtime error if the user denied permission to access the device.</p>

  <p>To explicitly obtain permission, first create a broadcast receiver. This receiver listens for
  the intent that gets broadcast when you call {@link
  android.hardware.usb.UsbManager#requestPermission requestPermission()}. The call to {@link
  android.hardware.usb.UsbManager#requestPermission requestPermission()} displays a dialog to the
  user asking for permission to connect to the device. The following sample code shows how to
  create the broadcast receiver:</p>
  <pre>
private static final String ACTION_USB_PERMISSION =
    "com.android.example.USB_PERMISSION";
private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {

    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if (ACTION_USB_PERMISSION.equals(action)) {
            synchronized (this) {
                UsbDevice device = (UsbDevice)intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);

                if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
                    if(device != null){
                      //call method to set up device communication
                   }
                }
                else {
                    Log.d(TAG, "permission denied for device " + device);
                }
            }
        }
    }
};
</pre>

  <p>To register the broadcast receiver, add this in your <code>onCreate()</code> method in your
  activity:</p>
  <pre>
UsbManager mUsbManager = (UsbManager) getSystemService(Context.USB_SERVICE);
private static final String ACTION_USB_PERMISSION =
    "com.android.example.USB_PERMISSION";
...
mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0);
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
registerReceiver(mUsbReceiver, filter);
</pre>

  <p>To display the dialog that asks users for permission to connect to the device, call the {@link
  android.hardware.usb.UsbManager#requestPermission requestPermission()} method:</p>
  <pre>
UsbDevice device;
...
mUsbManager.requestPermission(device, mPermissionIntent);
</pre>

  <p>When users reply to the dialog, your broadcast receiver receives the intent that contains the
  {@link android.hardware.usb.UsbManager#EXTRA_PERMISSION_GRANTED} extra, which is a boolean
  representing the answer. Check this extra for a value of true before connecting to the
  device.</p>

  <h3 id="communicating-d">Communicating with a device</h3>

  <p>Communication with a USB device can be either synchronous or asynchronous. In either case, you
  should create a new thread on which to carry out all data transmissions, so you don't block the
  UI thread. To properly set up communication with a device, you need to obtain the appropriate
  {@link android.hardware.usb.UsbInterface} and {@link android.hardware.usb.UsbEndpoint} of the
  device that you want to communicate on and send requests on this endpoint with a {@link
  android.hardware.usb.UsbDeviceConnection}. In general, your code should:</p>

  <ul>
    <li>Check a {@link android.hardware.usb.UsbDevice} object's attributes, such as product ID,
    vendor ID, or device class to figure out whether or not you want to communicate with the
    device.</li>

    <li>When you are certain that you want to communicate with the device, find the appropriate
    {@link android.hardware.usb.UsbInterface} that you want to use to communicate along with the
    appropriate {@link android.hardware.usb.UsbEndpoint} of that interface. Interfaces can have one
    or more endpoints, and commonly will have an input and output endpoint for two-way
    communication.</li>

    <li>When you find the correct endpoint, open a {@link android.hardware.usb.UsbDeviceConnection}
    on that endpoint.</li>

    <li>Supply the data that you want to transmit on the endpoint with the {@link
    android.hardware.usb.UsbDeviceConnection#bulkTransfer bulkTransfer()} or {@link
    android.hardware.usb.UsbDeviceConnection#controlTransfer controlTransfer()} method. You should
    carry out this step in another thread to prevent blocking the main UI thread. For more
    information about using threads in Android, see <a href=
    "{@docRoot}guide/topics/fundamentals/processes-and-threads.html#Threads">Processes and
    Threads</a>.</li>
  </ul>

  <p>The following code snippet is a trivial way to do a synchronous data transfer. Your code
  should have more logic to correctly find the correct interface and endpoints to communicate on
  and also should do any transferring of data in a different thread than the main UI thread:</p>
  <pre>
private Byte[] bytes
private static int TIMEOUT = 0;
private boolean forceClaim = true;

...

UsbInterface intf = device.getInterface(0);
UsbEndpoint endpoint = intf.getEndpoint(0);
UsbDeviceConnection connection = mUsbManager.openDevice(device);
connection.claimInterface(intf, forceClaim);
connection.bulkTransfer(endpoint, bytes, bytes.length, TIMEOUT); //do in another thread
</pre>

  <p>To send data asynchronously, use the {@link android.hardware.usb.UsbRequest} class to {@link
  android.hardware.usb.UsbRequest#initialize initialize} and {@link
  android.hardware.usb.UsbRequest#queue queue} an asynchronous request, then wait for the result
  with {@link android.hardware.usb.UsbDeviceConnection#requestWait requestWait()}.</p>

  <p>For more information, see the <a href=
  "{@docRoot}resources/samples/USB/AdbTest/index.html">AdbTest sample</a>, which shows how to do
  asynchronous bulk transfers, and the <a href=
  "{@docRoot}resources/samples/USB/MissileLauncher/index.html">MissleLauncher sample</a>, which
  shows how to listen on an interrupt endpoint asynchronously.</p>

  <h3 id="terminating-d">Terminating communication with a device</h3>

  <p>When you are done communicating with a device or if the device was detached, close the {@link
  android.hardware.usb.UsbInterface} and {@link android.hardware.usb.UsbDeviceConnection} by
  calling {@link android.hardware.usb.UsbDeviceConnection#releaseInterface releaseInterface()} and
  {@link android.hardware.usb.UsbDeviceConnection#close() close()}. To listen for detached events,
  create a broadcast receiver like below:</p>
  <pre>
BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();

      if (UsbManager.ACTION_USB_DEVICE_DETACHED.equals(action)) {
            UsbDevice device = (UsbDevice)intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
            if (device != null) {
                // call your method that cleans up and closes communication with the device
            }
        }
    }
};
</pre>

  <p>Creating the broadcast receiver within the application, and not the manifest, allows your
  application to only handle detached events while it is running. This way, detached events are
  only sent to the application that is currently running and not broadcast to all applications.</p>