AOA is a protocol Android devices use to talk to accessories. This is a brief introduction

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/