The Android Open Accessory protocol allows communication between Android devices and USB accessories. It defines how Android apps declare support for accessories and how data is transferred over USB. Accessories identify themselves by sending strings to the Android device, then enter accessory mode to allow two-way communication over a USB interface using intents and file descriptors.

1. Android Open Accessory
USB device to accessory protocol

2. What is it?
● USB protocol that allows communication between Android devices and accessories in
accessory mode
● AOAv1 - supported since API 12, with lib since API 10, supports basic communication
● AOAv2 - supported since API 16, adds audio streaming and HID capabilities
● Device is USB host and accessory is USB client

3. How does it work? Android app
● App declares itself to handle an accessory or accessory family
● App handles specific intents
● UsbAccessory object is attached to that intent in a Bundle
● Once it is obtained, transfer of data between USB device and Android device is possible

4. How does it work? Accessory
● Accessory sends series of specific USB control commands:
○ Get AOA version - device returns integer, currently 0 for not supported, 1 or 2 if supported
○ Accessory identifies itself by sending its vendor, model, manufacturer URI and serial number, which
should match what Android application declared as supported devices
○ If Android device returns valid status on all of those, it sends “go to AOA mode” command
● Android device disconnects from USB and reconnects in AOA mode with static PID and VID
● At this stage, if everything went well, Android OS sends the intent to the application with
UsbAccessory object which is describing the accessory

5. Declaring support for AOA
res/accessory_filter.xml:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<usb-accessory manufacturer="Tikal" model="Shaul haGadol" version="1.0"/>
</resources>

6. Manifest
<activity android:name=".UsbTest" android:label="@string/app_name" android:launchMode="singleTop">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category
android:name="android.intent.category.LAUNCHER" />
<action
android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED" />
</intent-filter>
<meta-data
android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"
android:resource="@xml/accessory_filter" />
</activity>

7. In Activity
if (getIntent().getAction().equals("android.hardware.usb.action.USB_ACCESSORY_ATTACHED")) {
UsbAccessory accessory = UsbManager.getAccessory(getIntent());
FileDescriptor fd = null;
try {
fd = UsbManager.getInstance(this).openAccessory(accessory).getFileDescriptor();
} catch (IllegalArgumentException e) {
finish();
} catch (NullPointerException e) {
finish();
}
mFout = new FileOutputStream(fd);
mFin = new FileInputStream(fd);
}

8. Accessory side, get AOA version
rc = libusb_control_transfer(handle, //handle
USB_CMD_BLOCK_READ, //bmRequestType
ACCESSORY_GET_PROTOCOL, //bRequest
0, //wValue
0, //wIndex
ioBuffer, //data
2, //wLength
0 //timeout
);
libusb_release_interface(handle, 0);
libusb_close(handle);
int aoaVersion = ioBuffer[1] << 8 | ioBuffer[0];

9. Accessory side, identify device
response = libusb_control_transfer(
device.handle,
USB_CMD_VENDOR_SPECIFIC,
ACCESSORY_SEND_STRING,
0,
ACCESSORY_STRING_MANUFACTURER,
(unsigned char*)manufacturer,
strlen(manufacturer),
0);

10. Accessory side, put in AOA mode
response = libusb_control_transfer(
device.handle,
USB_CMD_VENDOR_SPECIFIC,
ACCESSORY_START,
0,
0,
NULL,
0,
0);

11. Accessory side, open device
rc = libusb_get_device_descriptor(device, &desc);
if (rc < 0) {
continue;
}
if ((libusb_open(device, &handle)) < 0) {
fprintf(stdout, "Problem acquiring handlen");
return;
}
libusb_claim_interface(handle, 0);

12. Accessory side, transfer data
while (libusb_bulk_transfer(sourceHandle, IN, buffer, BUFFER_LEN, &transferred, 1000000 * 10) ==
0) {
rc1 = libusb_bulk_transfer(targetHandle, OUT, buffer, transferred, &transferred,
1000000 * 10);
if(rc1 == 0) {
buffCnt++;
if(buffCnt % 200 == 0) {
double mb = (buffCnt*BUFFER_LEN) / (1024 * 1024);
fprintf(stdout, "%lu s sent %.2f mbn", time(NULL) -
before, mb);
}
} else {
fprintf(stdout, "error sending buffer rc=%dn", rc1);
return -1;
}
}

13. Accessory side, constants
#define USB_ACCESSORY_PRODUCT_ID 0x2D00
#define USB_ACCESSORY_ADB_PRODUCT_ID 0x2D01
#define ACCESSORY_STRING_MANUFACTURER 0
#define ACCESSORY_STRING_MODEL 1
#define ACCESSORY_STRING_DESCRIPTION 2
#define ACCESSORY_STRING_VERSION 3
#define ACCESSORY_STRING_URI 4
#define ACCESSORY_STRING_SERIAL 5
#define ACCESSORY_GET_PROTOCOL 51
#define ACCESSORY_SEND_STRING 52
#define ACCESSORY_START 53
#define USB_CMD_BLOCK_READ 0xc0
#define USB_CMD_VENDOR_SPECIFIC 0x40

14. Additional info
● https://source.android.com/devices/accessories/protocol
● https://www.youtube.com/watch?v=kReFemy4UmU
● https://opensourceforu.com/2014/11/the-android-open-accessory-protocol-
all-about/
● http://wiki.p2pfoundation.net/Android_Open_Accessory
● http://jjmilburn.github.io/2017/05/08/Android-AOA-Support/