Android Open Accessory Protocol 2.0

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 first release of AOA.

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.

Detecting Android Open Accessory 2.0 Support

In order for an accessory to determine if a connected Android device supports accessories and at what protocol level, the accessory must send a getProtocol() command and check the result. Android devices supporting the initial version of the Android Open Accessory protocol return a 1, representing the protocol version number. Devices that support the new features described in this document must return 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 example from the Accessory Development Kit 2011 source code (<adk-src>/adk1/board/AndroidAccessory/AndroidAccessory.cpp) library demonstrates this protocol check:

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;
}

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:

  • accessory - An interface providing 2 bulk endpoints for communicating with an Android application.
  • audio - A new standard USB audio class interface for streaming audio from an Android device to an accessory.
  • adb - 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.

In AOA 1.0, there are only two USB product IDs:

  • 0x2D00 - accessory
  • 0x2D01 - accessory + adb

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

  • 0x2D02 - audio
  • 0x2D03 - audio + adb
  • 0x2D04 - accessory + audio
  • 0x2D05 - accessory + audio + adb

Audio Support

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.

To enable the audio support, the accessory must send a new USB control request:

**SET_AUDIO_MODE**
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

This command must be sent before sending the ACCESSORY_START command for entering accessory mode.

HID Support

AOA 2.0 allows the accessory to register one or more USB Human Interface Devices (HID) 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.

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.

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:

  • ACCESSORY_REGISTER_HID 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.
  • ACCESSORY_UNREGISTER_HID unregisters a HID device that was previously registered with ACCESSORY_REGISTER_HID.
  • ACCESSORY_SET_HID_REPORT_DESC 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.
  • ACCESSORY_SEND_HID_EVENT sends input events from the accessory to the Android device.

The code definitions for these new control requests are as follows:

/* 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

Interoperability with AOA 1.0 Features

The original AOA protocol 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.

Connecting AOA 2.0 without an Android App

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.