title: Using Android to Communicate with a USB HID Device
body: <p>I am new to USB and to Android so please forgive me if I don't explain myself clearly.</p>
  
  <p>I have a USB HID device that I can communicate with in Windows.  I am trying to establish communication using an Acer Iconia A500 tablet running Android 3.1.</p>
  
  <p>I am able to find the device, enumerate it, get its only available interface, get the only available endpoint (0), and determine what type of endpoint it is (transfer interrupt from device to host).</p>
  
  <p>My understanding of the USB spec is that all HID devices are required at a munimum to have a control endpoint (Endpoint 0) and an interrupt IN endpoint. But it seems that endpoint 0 here is the interrupt In endpoint, not the control endpoint.</p>
  
  <p>Yet in order for the device to enumerate it must successfully transfer its descriptor data across the control endpoint.  I deduce that the control endpoint therefore must be getting found (and used) because the host does, in fact, enumerate the device.</p>
  
  <p>This is as far as I am able to proceed, as stated above, the only interface/endpoint presented to me at the application level is an interrupt type going from device to host. No endpoint available to my app going from host to device, interrupt or control. So the device waits to be told what to do and the host waits for something to happen in the device.  Not very stimulating.</p>
  
  <p>Bear in mind that this device responds properly when connected to Windows, e.g. I am able to send a report containing 13 bytes of data that causes the device to light an LED.  So it seems to be complying with the USB HID spec.  As an act of desperation I have tried using this one endpoint as both a control endpoint and as a interrupt OUT endpoint, using controltransfer() and UsbRequest() to submit the data to the device, no response in either case.</p>
  
  <p>So my question is: "The control transfer endpoint is (?) being used to set up the device, why am I not able to find &amp; use it?" </p>
  
  <p>Thanks for any insight, below is the relevant code, I can include the rest in its entirety if needed:</p>
  
  <pre><code>private UsbManager mUsbManager;
  private UsbDevice mDevice;
  private UsbDeviceConnection mConnectionRead;
  private UsbDeviceConnection mConnectionWrite;
  private UsbEndpoint mEndpointRead;
  private UsbEndpoint mEndpointWrite;
  
      // check for existing devices
      for (UsbDevice device :  mUsbManager.getDeviceList().values())
      {
          //Need to filter for my device when other HIDs are also connected, but for now...           
          String devName = device.getDeviceName();
          if (DEBUG == 1){
          Toast.makeText(UsbHidDeviceTesterActivity.this, "My device got connected: " + devName, Toast.LENGTH_LONG).show();
          }
          //mDevice = device;
          setHIDDevice(device);
      }
  
  private boolean setHIDDevice(UsbDevice device)
  {    
      UsbInterface usbInterfaceRead = null;
      UsbInterface usbInterfaceWrite = null;
      UsbEndpoint ep1 = null;
      UsbEndpoint ep2 = null;
      boolean UsingSingleInterface = true;
  
      mDevice = device;
  
      //This HID device is using a single interface
      if (UsingSingleInterface)
      {
          //usbInterfaceRead = device.getInterface(0x00);//only 1 EP on this interface
          usbInterfaceRead = findInterface(device);
  
          //Try getting an interface at next index
          //usbInterfaceWrite = device.getInterface(0x01);//throws exception
  
          // Try using the same interface for reading and writing
          usbInterfaceWrite = usbInterfaceRead;
  
          int endPointCount = usbInterfaceWrite.getEndpointCount();
          if (DEBUG == 2)
          {
              Toast.makeText(UsbHidDeviceTesterActivity.this, "Endpoints: " + endPointCount, Toast.LENGTH_LONG).show();
              //Toast.makeText(UsbHidDeviceTesterActivity.this, "Interface: " + usbInterfaceRead, Toast.LENGTH_LONG).show();
          }
  
          if (endPointCount == 1)//only getting 1 endpoint
          {
              ep1 = usbInterfaceRead.getEndpoint(0);
              //As an act of desperation try equating ep2 to this read EP, so that we can later attempt to write to it anyway
              ep2 = usbInterfaceRead.getEndpoint(0);
          }
          else if (endPointCount == 2)
          {
              ep1 = usbInterfaceRead.getEndpoint(0);
              ep2 = usbInterfaceRead.getEndpoint(1);
          }
      }
  
      else        // ! UsingSingleInterface
      {
          usbInterfaceRead = device.getInterface(0x00);
          usbInterfaceWrite = device.getInterface(0x01);
          if ((usbInterfaceRead.getEndpointCount() == 1) &amp;&amp; (usbInterfaceWrite.getEndpointCount() == 1))
          {
              ep1 = usbInterfaceRead.getEndpoint(0);
              ep2 = usbInterfaceWrite.getEndpoint(0);
          }
          if (DEBUG == 3)
          {
              Toast.makeText(UsbHidDeviceTesterActivity.this, "Using Dual Interface", Toast.LENGTH_LONG).show();
          }
      }
  
      //because ep1 = ep2 this will now not cause a return unless no ep is found at all
      if ((ep1 == null) || (ep2 == null))
      {
          if (DEBUG == 4)
          {
              Toast.makeText(UsbHidDeviceTesterActivity.this, "One EP is null", Toast.LENGTH_LONG).show();
          }
          return false;
      }
  
      // Determine which endpoint is the read, and which is the write
      if (ep1.getType() == UsbConstants.USB_ENDPOINT_XFER_INT)//I am getting a return of 3, which is an interrupt transfer
      {
          if (ep1.getDirection() == UsbConstants.USB_DIR_IN)//I am getting a return of 128, which is a device-to-host endpoint
          {
              mEndpointRead = ep1;
              if (DEBUG == 5)
              {
                  Toast.makeText(UsbHidDeviceTesterActivity.this, "EP1 type: " + ep1.getType(), Toast.LENGTH_LONG).show();
              }
          }
          if (ep1.getDirection() == UsbConstants.USB_DIR_OUT)//nope
          {
              mEndpointWrite = ep1;
              if (DEBUG == 6)
              {
                  Toast.makeText(UsbHidDeviceTesterActivity.this, "EP1 is a write", Toast.LENGTH_LONG).show();
              }
          }
      }
  
      if (ep2.getType() == UsbConstants.USB_ENDPOINT_XFER_INT)
      {
          if (ep2.getDirection() == UsbConstants.USB_DIR_IN)
          {
              //Try treating it as a write anyway             
              //mEndpointRead = ep2;
              mEndpointWrite = ep2;
          }
          else if (ep2.getDirection() == UsbConstants.USB_DIR_OUT)
          {
              //usbEndpointWrite = ep2;
              mEndpointWrite = ep2;
          }
      }
  
      //check that we should be able to read and write
      if ((mEndpointRead == null) || (mEndpointWrite == null))
      {
          return false;
      }
      if (device != null)
      {
          UsbDeviceConnection connection = mUsbManager.openDevice(device);
          if (connection != null &amp;&amp; connection.claimInterface(usbInterfaceRead, true))
          {
              Log.d(TAG, "open SUCCESS");
              mConnectionRead = connection;
              // Start the read thread
              //Comment out while desperately attempting to write on this connection/interface
              //Thread thread = new Thread(this);
              //thread.start();
  
          }
          else
          {
              Log.d(TAG, "open FAIL");
              mConnectionRead = null;
          }
       }
      if (UsingSingleInterface)
      {
          mConnectionWrite = mConnectionRead;
      }
      else //! UsingSingleInterface
      {
          mConnectionWrite = mUsbManager.openDevice(device);
          mConnectionWrite.claimInterface(usbInterfaceWrite, true);
      }
      return true;
  }
  
  // searches for an interface on the given USB device
   private UsbInterface findInterface(UsbDevice device) {
      Log.d(TAG, "findInterface " + device);
      int count = device.getInterfaceCount();
      if (DEBUG == 7)
      {
          Toast.makeText(UsbHidDeviceTesterActivity.this, "Interface count: " + count, Toast.LENGTH_LONG).show();
      }
  
      for (int i = 0; i &lt; count; i++) {
          UsbInterface intf = device.getInterface(i);
          String InterfaceInfo = intf.toString();
          Log.d(TAG, "Interface: " + InterfaceInfo);
          //Class below is 3 for USB_HID
          if (intf.getInterfaceClass() == 3 &amp;&amp; intf.getInterfaceSubclass() == 0 &amp;&amp;
                  intf.getInterfaceProtocol() == 0) {
              return intf;
          }
          //....try just returning the interface regardless of class/subclass
          //return intf;
      }
  
      return null;
  } 
   private boolean sendControlTransfer(byte[] dataToSend)
   {
      synchronized (this)
      { 
      if (mConnectionRead != null)
       { 
          //byte[] message = new byte[13];  // or 14?
          byte[] message = dataToSend;
           if (DEBUG == 9)
           {
               Toast.makeText(UsbHidDeviceTesterActivity.this, "Sending Control Transfer", Toast.LENGTH_LONG).show();
           } 
  
           //first field ox21 is bin 00100001 which splits into 0 01 00001 for direction(1bit)/type(2b)/recipient(5b)
           //To set direction as 'host to Device' we need 0, To set type to HID we need 11 (3), and for recipient we want 00001
           //second field 0x09 is class specific request code, 0x09 is listed as 'reserved for future use'
           //third field 0x200 is value
           //int transfer = mConnectionRead.controlTransfer(0x21, 0x9, 0x200, 0, message, message.length, 0);
           //try with type set to HID
           int transfer = mConnectionRead.controlTransfer(0xC1, 0x9, 0x200, 0, message, message.length, 0);
           if (DEBUG == 10)
           {
               Toast.makeText(UsbHidDeviceTesterActivity.this, "Transfer returned " + transfer, Toast.LENGTH_LONG).show();
           }
       } 
      }
      return true;
   }
  
  
  private boolean sendInterruptTransfer(byte[] dataToSend)
  { 
      int bufferDataLength = mEndpointWrite.getMaxPacketSize();//The write endpoint is null unless we just copy the read endpoint
      if (DEBUG == 12)
      {
          Toast.makeText(UsbHidDeviceTesterActivity.this, "Max Packet Size: " + bufferDataLength, Toast.LENGTH_LONG).show();
      }
  
      ByteBuffer buffer = ByteBuffer.allocate(bufferDataLength + 1);
      UsbRequest request = new UsbRequest();
      buffer.put(dataToSend);
  
      request.initialize(mConnectionWrite, mEndpointWrite);
      request.queue(buffer, bufferDataLength);
  
      try
      {
          /* only use requestwait on a read
          if (request.equals(mConnectionWrite.requestWait()))
          {
              return true;
          }
          */
      }
      catch (Exception ex)
      {
          // An exception has occurred
          if (DEBUG == 13)
          {
              Toast.makeText(UsbHidDeviceTesterActivity.this, "Caught Write Exception", Toast.LENGTH_LONG).show();
          }
      }
  
      return true;
  }   
  </code></pre>