docs/html/tools/adk/aoa2.jd - LeafOS-Project/android_frameworks_base - Gitiles

 page.title=Android Open Accessory Protocol 2.0
 @jd:body

 <div id="qv-wrapper">
   <div id="qv">
     <h2>In this document</h2>
     <ol>
       <li><a href="#detecting">Detecting Android Open Accessory 2.0 Support</a></li>
       <li><a href="#audio-support">Audio Support</a></li>
       <li><a href="#hid">HID Support</a></li>
       <li><a href="#interop-aoa">Interoperability with AOA 1.0 Features</a></li>
       <li><a href="#no-app-conn">Connecting AOA 2.0 without an Android App</a></li>
     </ol>

     <h2>See also</h2>
     <ol>
       <li><a href="aoa.html">Android Open Accessory Protocol</a></li>
     </ol>
   </div>
 </div>

 <p>This document describes the changes to the Android Open Accessory (AOA) protocol since its
 initial release, and is a supplement to the documentation of the <a href="aoa.html">first
 release of AOA</a>.</p>

 <p>The Android Open Accessory Protocol 2.0 adds two new features: audio output (from the Android
 device to the accessory) and support for the accessory acting as one or more human interface devices
 (HID) to the Android device. The Android SDK APIs available to Android application developers
 remain unchanged.</p>

 <h2 id="detecting">Detecting Android Open Accessory 2.0 Support</h2>

 <p>In order for an accessory to determine if a connected Android device supports accessories and at
 what protocol level, the accessory must send a {@code getProtocol()} command and check the result.
 Android devices supporting the initial version of the Android Open Accessory protocol return a
 {@code 1}, representing the protocol version number. Devices that support the new features described
 in this document must return {@code 2} for the protocol version. Version 2.0 of the protocol is
 upwardly compatible, so accessories designed for the original accessory protocol still work
 with newer Android devices. The following code from the <a href="adk.html">Android Development Kit
 2011</a> {@code AndroidAccessory} library demonstrates this protocol check:</p>

 <pre>
 bool AndroidAccessory::switchDevice(byte addr)
 {
     int protocol = getProtocol(addr);
     if (protocol >= 1) {
         Serial.print("device supports protocol 1 or higher\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>

 <p>AOA 2.0 includes new USB product IDs, one for each combination of USB interfaces available when
 in accessory mode. The possible USB interfaces are:</p>

 <ul>
   <li><strong>accessory</strong> - An interface providing 2 bulk endpoints for communicating with an
 Android application.</li>
   <li><strong>audio</strong> -A new standard USB audio class interface for streaming audio
 from an Android device to an accessory.</li>
   <li><strong>adb</strong> - An interface intended only for debugging purposes while developing an
 accessory. Only enabled if the user has USB Debugging enabled in Settings on the Android device.
   </li>
 </ul>

 <p>In AOA 1.0, there are only two USB product IDs:</p>

 <ul>
   <li>{@code 0x2D00} - accessory</li>
   <li>{@code 0x2D01} - accessory + adb</li>
 </ul>

 <p>AOA 2.0 adds an optional USB audio interface and, therefore, includes product IDs for the new
 combinations of USB interfaces:</p>

 <ul>
   <li>{@code 0x2D02} - audio</li>
   <li>{@code 0x2D03} - audio + adb</li>
   <li>{@code 0x2D04} - accessory + audio</li>
   <li>{@code 0x2D05} - accessory + audio + adb</li>
 </ul>

 <h2 id="audio-support">Audio Support</h2>

 <p>AOA 2.0 includes optional support for audio output from an Android device to an accessory. This
 version of the protocol supports a standard USB audio class interface that is capable of 2 channel
 16-bit PCM audio with a bit rate of 44100 Khz. AOA 2.0 is currently limited to this output mode, but
 additional audio modes may be added in the future.</p>

 <p>To enable the audio support, the accessory must send a new USB control request:</p>

 <pre>
 <strong>SET_AUDIO_MODE</strong>
 requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
 request:        58
 value:          0 for no audio (default),
                 1 for 2 channel, 16-bit PCM at 44100 KHz
 index:          0
 data            none
 </pre>

 <p>This command must be sent <em>before</em> sending the {@code ACCESSORY_START} command for
 entering accessory mode.</p>

 <h2 id="hid">HID Support</h2>

 <p>AOA 2.0 allows the accessory to register one or more HID devices with
 an Android device. This approach reverses the direction of communication for typical USB HID
 devices like USB mice and keyboards. Normally, the HID device is a peripheral connected to a USB
 host like a personal computer. But in the case of the AOA protocol, the USB host acts as one or more
 input devices to a USB peripheral.</p>

 <p>HID support in AOA 2.0 is simply a proxy for standard HID events. The implementation makes no
 assumptions about the content or type of events and merely passes it through to the input system,
 so an AOA 2.0 accessory can act as any HID device (mouse, keyboard, game controller, etc.). It
 can be used for something as simple as the play/pause button on a media dock, or something as
 complicated as a docking station with a mouse and full QWERTY keyboard.</p>

 <p>The AOA 2.0 protocol adds four new USB control requests to allow the accessory to act as one or
 more HID input devices to the Android device.  Since HID support is done entirely through
 control requests on endpoint zero, no new USB interface is needed to provide this support. The
 control requests are as follows:</p>

 <ul>
   <li><strong>ACCESSORY_REGISTER_HID</strong> registers a new HID device with the Android device.
 The accessory provides an ID number that is used to identify the HID device for the other three
 calls. This ID is valid until USB is disconnected or until the accessory sends
 ACCESSORY_UNREGISTER_HID to unregister the HID device.</li>
   <li><strong>ACCESSORY_UNREGISTER_HID</strong> unregisters a HID device that was previously
 registered with ACCESSORY_REGISTER_HID.</li>
   <li><strong>ACCESSORY_SET_HID_REPORT_DESC</strong> sends a report descriptor for a HID device to
 the Android device. This request is used to describe the capabilities of the HID device, and must
 be sent before reporting any HID events to the Android device. If the report descriptor is larger
 than the maximum packet size for endpoint zero, multiple ACCESSORY_SET_HID_REPORT_DESC commands are
 sent in order to transfer the entire descriptor.</li>
   <li><strong>ACCESSORY_SEND_HID_EVENT</strong> sends input events from the accessory to the Android
 device.</li>
 </ul>

 <p>The code definitions for these new control requests are as follows:</p>

 <pre>
 /* Control request for registering a HID device.
  * Upon registering, a unique ID is sent by the accessory in the
  * value parameter. This ID will be used for future commands for
  * the device
  *
  *  requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
  *  request:        ACCESSORY_REGISTER_HID_DEVICE
  *  value:          Accessory assigned ID for the HID device
  *  index:          total length of the HID report descriptor
  *  data            none
  */
 #define ACCESSORY_REGISTER_HID         54

 /* Control request for unregistering a HID device.
  *
  *  requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
  *  request:        ACCESSORY_REGISTER_HID
  *  value:          Accessory assigned ID for the HID device
  *  index:          0
  *  data            none
  */
 #define ACCESSORY_UNREGISTER_HID         55

 /* Control request for sending the HID report descriptor.
  * If the HID descriptor is longer than the endpoint zero max packet size,
  * the descriptor will be sent in multiple ACCESSORY_SET_HID_REPORT_DESC
  * commands. The data for the descriptor must be sent sequentially
  * if multiple packets are needed.
  *
  *  requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
  *  request:        ACCESSORY_SET_HID_REPORT_DESC
  *  value:          Accessory assigned ID for the HID device
  *  index:          offset of data in descriptor
  *                      (needed when HID descriptor is too big for one packet)
  *  data            the HID report descriptor
  */
 #define ACCESSORY_SET_HID_REPORT_DESC         56

 /* Control request for sending HID events.
  *
  *  requestType:    USB_DIR_OUT | USB_TYPE_VENDOR
  *  request:        ACCESSORY_SEND_HID_EVENT
  *  value:          Accessory assigned ID for the HID device
  *  index:          0
  *  data            the HID report for the event
  */
 #define ACCESSORY_SEND_HID_EVENT         57
 </pre>

 <h2 id="interop-aoa">Interoperability with AOA 1.0 Features</h2>

 <p>The original <a href="aoa.html">AOA protocol</a> provided support for an Android application to
 communicate directly with a USB host (accessory) over USB. AOA 2.0 keeps that support, but adds new
 features to allow the accessory to communicate with the Android operating system itself
 (specifically the audio and input systems). The design of the AOA 2.0 makes it is possible to build
 an accessory that also makes use of the new audio and/or HID support in addition to the original
 feature set. Simply use the new features described in this document in addition to the original AOA
 protocol features.</p>

 <h2 id="no-app-conn">Connecting AOA 2.0 without an Android App</h2>

 <p>It is possible to design an accessory (for example, an audio dock) that uses the new audio and
 HID support, but does not need to communicate with an application on the Android device. In that
 case, the user would not want to see the dialog prompts related to finding and associating the newly
 attached accessory with an Android application that can communicate with it. To prevent these
 dialogs from appearing after the device and accessory are connected, the accessory can simply not
 send the manufacturer and model names to the Android device. If these strings are not provided to
 the Android device, then the accessory is able to make use of the new audio and HID support in AOA
 2.0 without the system attempting to find an application to communicate with the accessory. Also,
 if these strings are not provided, the accessory USB interface is not present in the Android
 device USB configuration after the device enters accessory mode.</p>
	page.title=Android Open Accessory Protocol 2.0
	@jd:body

	<div id="qv-wrapper">
	<div id="qv">
	<h2>In this document</h2>
	<ol>
	<li><a href="#detecting">Detecting Android Open Accessory 2.0 Support</a></li>
	<li><a href="#audio-support">Audio Support</a></li>
	<li><a href="#hid">HID Support</a></li>
	<li><a href="#interop-aoa">Interoperability with AOA 1.0 Features</a></li>
	<li><a href="#no-app-conn">Connecting AOA 2.0 without an Android App</a></li>
	</ol>

	<h2>See also</h2>
	<ol>
	<li><a href="aoa.html">Android Open Accessory Protocol</a></li>
	</ol>
	</div>
	</div>

	<p>This document describes the changes to the Android Open Accessory (AOA) protocol since its
	initial release, and is a supplement to the documentation of the <a href="aoa.html">first
	release of AOA</a>.</p>

	<p>The Android Open Accessory Protocol 2.0 adds two new features: audio output (from the Android
	device to the accessory) and support for the accessory acting as one or more human interface devices
	(HID) to the Android device. The Android SDK APIs available to Android application developers
	remain unchanged.</p>

	<h2 id="detecting">Detecting Android Open Accessory 2.0 Support</h2>

	<p>In order for an accessory to determine if a connected Android device supports accessories and at
	what protocol level, the accessory must send a {@code getProtocol()} command and check the result.
	Android devices supporting the initial version of the Android Open Accessory protocol return a
	{@code 1}, representing the protocol version number. Devices that support the new features described
	in this document must return {@code 2} for the protocol version. Version 2.0 of the protocol is
	upwardly compatible, so accessories designed for the original accessory protocol still work
	with newer Android devices. The following code from the <a href="adk.html">Android Development Kit
	2011</a> {@code AndroidAccessory} library demonstrates this protocol check:</p>

	<pre>
	bool AndroidAccessory::switchDevice(byte addr)
	{
	int protocol = getProtocol(addr);
	if (protocol >= 1) {
	Serial.print("device supports protocol 1 or higher\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>

	<p>AOA 2.0 includes new USB product IDs, one for each combination of USB interfaces available when
	in accessory mode. The possible USB interfaces are:</p>

	<ul>
	<li><strong>accessory</strong> - An interface providing 2 bulk endpoints for communicating with an
	Android application.</li>
	<li><strong>audio</strong> -A new standard USB audio class interface for streaming audio
	from an Android device to an accessory.</li>
	<li><strong>adb</strong> - An interface intended only for debugging purposes while developing an
	accessory. Only enabled if the user has USB Debugging enabled in Settings on the Android device.
	</li>
	</ul>

	<p>In AOA 1.0, there are only two USB product IDs:</p>

	<ul>
	<li>{@code 0x2D00} - accessory</li>
	<li>{@code 0x2D01} - accessory + adb</li>
	</ul>

	<p>AOA 2.0 adds an optional USB audio interface and, therefore, includes product IDs for the new
	combinations of USB interfaces:</p>

	<ul>
	<li>{@code 0x2D02} - audio</li>
	<li>{@code 0x2D03} - audio + adb</li>
	<li>{@code 0x2D04} - accessory + audio</li>
	<li>{@code 0x2D05} - accessory + audio + adb</li>
	</ul>

	<h2 id="audio-support">Audio Support</h2>

	<p>AOA 2.0 includes optional support for audio output from an Android device to an accessory. This
	version of the protocol supports a standard USB audio class interface that is capable of 2 channel
	16-bit PCM audio with a bit rate of 44100 Khz. AOA 2.0 is currently limited to this output mode, but
	additional audio modes may be added in the future.</p>

	<p>To enable the audio support, the accessory must send a new USB control request:</p>

	<pre>
	<strong>SET_AUDIO_MODE</strong>
	requestType: USB_DIR_OUT \| USB_TYPE_VENDOR
	request: 58
	value: 0 for no audio (default),
	1 for 2 channel, 16-bit PCM at 44100 KHz
	index: 0
	data none
	</pre>

	<p>This command must be sent <em>before</em> sending the {@code ACCESSORY_START} command for
	entering accessory mode.</p>

	<h2 id="hid">HID Support</h2>

	<p>AOA 2.0 allows the accessory to register one or more HID devices with
	an Android device. This approach reverses the direction of communication for typical USB HID
	devices like USB mice and keyboards. Normally, the HID device is a peripheral connected to a USB
	host like a personal computer. But in the case of the AOA protocol, the USB host acts as one or more
	input devices to a USB peripheral.</p>

	<p>HID support in AOA 2.0 is simply a proxy for standard HID events. The implementation makes no
	assumptions about the content or type of events and merely passes it through to the input system,
	so an AOA 2.0 accessory can act as any HID device (mouse, keyboard, game controller, etc.). It
	can be used for something as simple as the play/pause button on a media dock, or something as
	complicated as a docking station with a mouse and full QWERTY keyboard.</p>

	<p>The AOA 2.0 protocol adds four new USB control requests to allow the accessory to act as one or
	more HID input devices to the Android device. Since HID support is done entirely through
	control requests on endpoint zero, no new USB interface is needed to provide this support. The
	control requests are as follows:</p>

	<ul>
	<li><strong>ACCESSORY_REGISTER_HID</strong> registers a new HID device with the Android device.
	The accessory provides an ID number that is used to identify the HID device for the other three
	calls. This ID is valid until USB is disconnected or until the accessory sends
	ACCESSORY_UNREGISTER_HID to unregister the HID device.</li>
	<li><strong>ACCESSORY_UNREGISTER_HID</strong> unregisters a HID device that was previously
	registered with ACCESSORY_REGISTER_HID.</li>
	<li><strong>ACCESSORY_SET_HID_REPORT_DESC</strong> sends a report descriptor for a HID device to
	the Android device. This request is used to describe the capabilities of the HID device, and must
	be sent before reporting any HID events to the Android device. If the report descriptor is larger
	than the maximum packet size for endpoint zero, multiple ACCESSORY_SET_HID_REPORT_DESC commands are
	sent in order to transfer the entire descriptor.</li>
	<li><strong>ACCESSORY_SEND_HID_EVENT</strong> sends input events from the accessory to the Android
	device.</li>
	</ul>

	<p>The code definitions for these new control requests are as follows:</p>

	<pre>
	/* Control request for registering a HID device.
	* Upon registering, a unique ID is sent by the accessory in the
	* value parameter. This ID will be used for future commands for
	* the device
	*
	* requestType: USB_DIR_OUT \| USB_TYPE_VENDOR
	* request: ACCESSORY_REGISTER_HID_DEVICE
	* value: Accessory assigned ID for the HID device
	* index: total length of the HID report descriptor
	* data none
	*/
	#define ACCESSORY_REGISTER_HID 54

	/* Control request for unregistering a HID device.
	*
	* requestType: USB_DIR_OUT \| USB_TYPE_VENDOR
	* request: ACCESSORY_REGISTER_HID
	* value: Accessory assigned ID for the HID device
	* index: 0
	* data none
	*/
	#define ACCESSORY_UNREGISTER_HID 55

	/* Control request for sending the HID report descriptor.
	* If the HID descriptor is longer than the endpoint zero max packet size,
	* the descriptor will be sent in multiple ACCESSORY_SET_HID_REPORT_DESC
	* commands. The data for the descriptor must be sent sequentially
	* if multiple packets are needed.
	*
	* requestType: USB_DIR_OUT \| USB_TYPE_VENDOR
	* request: ACCESSORY_SET_HID_REPORT_DESC
	* value: Accessory assigned ID for the HID device
	* index: offset of data in descriptor
	* (needed when HID descriptor is too big for one packet)
	* data the HID report descriptor
	*/
	#define ACCESSORY_SET_HID_REPORT_DESC 56

	/* Control request for sending HID events.
	*
	* requestType: USB_DIR_OUT \| USB_TYPE_VENDOR
	* request: ACCESSORY_SEND_HID_EVENT
	* value: Accessory assigned ID for the HID device
	* index: 0
	* data the HID report for the event
	*/
	#define ACCESSORY_SEND_HID_EVENT 57
	</pre>

	<h2 id="interop-aoa">Interoperability with AOA 1.0 Features</h2>

	<p>The original <a href="aoa.html">AOA protocol</a> provided support for an Android application to
	communicate directly with a USB host (accessory) over USB. AOA 2.0 keeps that support, but adds new
	features to allow the accessory to communicate with the Android operating system itself
	(specifically the audio and input systems). The design of the AOA 2.0 makes it is possible to build
	an accessory that also makes use of the new audio and/or HID support in addition to the original
	feature set. Simply use the new features described in this document in addition to the original AOA
	protocol features.</p>

	<h2 id="no-app-conn">Connecting AOA 2.0 without an Android App</h2>

	<p>It is possible to design an accessory (for example, an audio dock) that uses the new audio and
	HID support, but does not need to communicate with an application on the Android device. In that
	case, the user would not want to see the dialog prompts related to finding and associating the newly
	attached accessory with an Android application that can communicate with it. To prevent these
	dialogs from appearing after the device and accessory are connected, the accessory can simply not
	send the manufacturer and model names to the Android device. If these strings are not provided to
	the Android device, then the accessory is able to make use of the new audio and HID support in AOA
	2.0 without the system attempting to find an application to communicate with the accessory. Also,
	if these strings are not provided, the accessory USB interface is not present in the Android
	device USB configuration after the device enters accessory mode.</p>