The Android Accessory Development Kit (ADK) for 2012 is the latest reference implementation of an Android Open Accessory device, designed to help Android hardware accessory builders and software developers create accessories for Android. The ADK 2012 is based on the Arduino open source electronics prototyping platform, with some hardware and software extensions that allow it to communicate with Android devices.
A limited number of these kits were produced and distributed at the Google I/O 2012 developer conference. If you did not receive one of these kits, fear not! The specifications and design files for the hardware were also released for use by manufacturers and hobbyists. You should expect to see kits with similar features available for purchase, or you can build one yourself!
One of the important new features demonstrated by this ADK is the ability to play audio over a USB connection. Be sure to check out the reference implementation of a USB audio dock in this ADK if you are interested in making audio-related USB accessories for Android.
The ADK 2012 is based on the Arduino open source electronics prototyping platform and is an open hardware design. The hardware design files and firmware source code are included with the ADK software download. The ADK contains two main physical hardware components:
The main hardware features of the ADK are as follows:
The ADK comes preloaded with an alarm clock firmware program that you can use immediately. A companion Android application, ADK 2012, is available on Google Play. The source code for both the Android application and the ADK firmware (an Arduino sketch) can be downloaded from this page.
The ADK 2012 also comes with additional parts to help you develop accessories with it, including:
An alarm clock program comes preloaded on the ADK. This firmware program allows you to use the ADK as an alarm clock.
To use the ADK as an alarm clock:
Note: You may need to hold your finger in place for 1-2 seconds.
To use the ADK 2012 companion application for the alarm clock:
Note: Your device must support Android USB accessory mode. Devices that support this mode include Google Nexus devices.
Note: When using the ADK with a USB connection to an Android device, make sure the AC power adapter is plugged in. A USB connection to a computer does not provide sufficient voltage to power both the ADK and a USB connection to a device.
The ADK 2012 comes with audio output capabilities, including an amplifier and speaker. You can use it to play audio from your Android device using either a Bluetooth or USB connection.
To play audio over Bluetooth:
To play audio over USB, you must use a device running Android 4.1 (API Level 16) or higher:
Note: Your device must support Android USB accessory mode. Devices that support this mode include Google Nexus devices.
Note: When using the ADK with a USB connection to an Android device, make sure the AC power adapter is plugged in. A USB connection to a computer does not provide sufficient voltage to power both the ADK and a USB connection to a device.
The ADK 2012 is a hardware platform and a set of software libraries for prototyping Android accessories. This section discusses how to set up a development environment for programming the ADK to make it do what you want and to test software for building your own Android accessories.
The support software and hardware specifications for the ADK 2012 are available from the Android source repository. Follow the instructions below to obtain the source material for the ADK.
To download the ADK 2012 software, source code and hardware design specifications.
Note: Developers using Windows must use a Linux compatibility package, such as cygwin, to install and run {@code repo}. Within your compatibility environment, you must install {@code curl}, {@code git} and {@code python} to be able to download and use the {@code repo} tool.
$> mkdir android-accessories $> cd android-accessories $> repo init -u https://android.googlesource.com/accessories/manifest $> repo sync
After successfully completing this process, you should have the source code and tools for working with the ADK 2012:
The ADK 2012 comes with an integrated development environment (IDE) that you use to develop software and program the ADK 2012 accessory. The following instructions explain how to setup and run the ADK 2012 IDE.
To set up and run the ADK 2012 IDE:
After you have successfully installed and run the ADK 2012 IDE, you must configure it to use the ADK 2012 library.
To configure the ADK 2012 IDE for use with the ADK libraries:
An alternative, make file-based build and upload system is also available for the ADK 2012. This system is command line based and intended for production environments where using an IDE environment to load software onto accessory hardware may be inconvenient or undesirable.
To setup the environment:
$> ./setup
To build a program for your accessory:
$> ./build
To load the program on your accessory hardware:
$> ./flash
The essential feature of any Android accessory is its ability to connect and communicate with an Android device. Creating a fast and reliable connection between your accessory and Android devices is the first order of business when building software for an accessory. This section describes the connection and communication essentials used in the ADK 2012 so that you can apply them to developing your own Android accessories.
The ADK 2012 app and hardware accessory use a Bluetooth Serial Port Profile (SPP) connection to communicate. This connection allows two way communication between the ADK accessory and Android devices.
Note: The implementation of the ADK hardware allows the use of other profiles and multiple connections. However, the basic communication between the ADK 2012 accessory and the Android application uses SPP.
In order to enable Bluetooth communications, the {@code clock.ino} sketch for the ADK 2012 accessory calls a {@code btStart()} method during the {@code setup()} method to enable radio frequency communications and start listening for Bluetooth connections:
ADK L;
void setup() {
L.adkInit();
L.btStart();
}
...
void btStart(){
uint8_t i, dlci;
int f;
L.btEnable(adkBtConnectionRequest, adkBtLinkKeyRequest, adkBtLinkKeyCreated,
adkBtPinRequest, NULL);
dlci = L.btRfcommReserveDlci(RFCOMM_DLCI_NEED_EVEN);
if(!dlci) dbgPrintf("BTADK: failed to allocate DLCI\n");
else{
//change descriptor to be valid...
for(i = 0, f = -1; i < sizeof(sdpDescrADK); i++){
if(sdpDescrADK[i] == MAGIX){
if(f == -1) f = i;
else break;
}
}
if(i != sizeof(sdpDescrADK) || f == -1){
dbgPrintf("BTADK: failed to find a single marker in descriptor\n");
L.btRfcommReleaseDlci(dlci);
return;
}
sdpDescrADK[f] = dlci >> 1;
dbgPrintf("BTADK has DLCI %u\n", dlci);
L.btRfcommRegisterPort(dlci, btAdkPortOpen, btAdkPortClose, btAdkPortRx);
L.btSdpServiceDescriptorAdd(sdpDescrADK, sizeof(sdpDescrADK));
}
}
Notice that the {@code sdpDescrADK} object contains a Universally Unique Identifier (UUID) in the variable {@code BT_ADK_UUID}. This identifier must match the device UUID provided in the {@link android.bluetooth.BluetoothSocket} connection request in the Android application code.
Once Bluetooth is enabled with the code shown above, the accessory listens for connection requests. The ADK library handles listening and connection details, so the accessory calls {@code ADK::adkEventProcess()} once during each loop execution:
void loop(void)
{
...
L.adkEventProcess(); //let the adk framework do its thing
...
}
If a Bluetooth connection has been established, any commands are routed to the {@code btAdkPortRx()} callback method (which was registered with the ADK library as part of the {@code btStart()} method) and processed accordingly. The ADK accessory sends messages back through the Bluetooth connection using the {@code ADK::btRfcommPortTx()} method. For more details, review the implementations of these methods in the {@code clock.ino} sketch.
In the ADK 2012 Android app, the code for handling Bluetooth connections is encapsulated in in a {@code BTConnection} class. In this class, the application requests access to the Bluetooth adapter and negotiates a connection with the ADK 2012 accessory. Here is a summary of the relevant code:
mAdapter = BluetoothAdapter.getDefaultAdapter(); BluetoothDevice device = mAdapter.getRemoteDevice(address); mSocket = device.createInsecureRfcommSocketToServiceRecord(ADK_UUID); mSocket.connect();
Note the {@code ADK_UUID} parameter in the second line. This identifier must match the identifier output by the accessory (the {@code BT_ADK_UUID} variable mentioned earlier), otherwise the protocol negotiation fails and the {@link android.bluetooth.BluetoothSocket} is not created. Once a connection is established, you obtain {@link java.io.InputStream} and {@link java.io.OutputStream} objects from the socket to communicate with the accessory:
mInStream = mSocket.getInputStream(); mOutStream = mSocket.getOutputStream();
Review the {@code BTConnection.java} file provided in the ADK 2012 software download for more implementation details.
The ADK 2012 app and hardware accessory can also use a USB connection to communicate, similar to the original ADK.
The ADK library takes care of most of the implementation details for a USB connection, the accessory code must make a few calls to initialize USB connectivity, including setting the accessory identification strings:
ADK L;
void setup() {
L.adkInit();
L.usbSetAccessoryStringVendor(...);
L.usbSetAccessoryStringName(...);
L.usbSetAccessoryStringLongname(...);
L.usbSetAccessoryStringVersion(...);
L.usbSetAccessoryStringUrl(...);
L.usbSetAccessoryStringSerial(...);
L.usbStart();
}
Note: The identification strings must match the USB accessory filter settings specified in the connecting Android application,otherwise the application cannot connect with the accessory.
Once USB is enabled with code shown above, the accessory listens for connection requests. The ADK library handles listening and connection details, so the accessory calls {@code ADK::adkEventProcess()} once during each loop execution:
void loop(void)
{
...
L.adkEventProcess(); //let the adk framework do its thing
...
}
The accessory must then check for a live USB connection to process commands and send messages. Here is a summary of the relevant code:
void loop() {
if (L.accessoryConnected()) {
int recvLen = L.accessoryReceive(msg, sizeof(msg));
if (recvLen > 0) {
... // process message
}
L.accessorySend(outmsg, outmsgLen);
}
L.adkEventProcess();
}
For more details, review the implementations of these methods in the {@code clock.ino} sketch.
In the ADK 2012 Android app, the code for handling USB connections is encapsulated in a {@code UsbConnection} class. This class sets up a {@link android.content.BroadcastReceiver} to listen for USB events and then attempts to connect when a matching connection event is received. Here is a summary of the relevant code:
import com.android.future.usb.UsbAccessory; import com.android.future.usb.UsbManager; mUSBManager = UsbManager.getInstance(this); UsbAccessory acc = mUSBManager.getAccessoryList()[0]; if (!mUSBManager.hasPermission(acc)) return;
The ADK 2012 app uses the support library to implement the USB accessory connections, in order to support devices running Android 2.3.4 (API Level 10). If you only need to support Android 3.1 (API Level 12) and higher devices, you can replace the first 4 lines the following code:
import android.hardware.usb.UsbAccessory
import android.hardware.usb.UsbManager
mUSBManager = (UsbManager) getSystemService(Context.USB_SERVICE);
UsbAccessory acc = (UsbAccessory)
intent.getParcelableExtra(UsbManager.EXTRA_ACCESSORY);
Note that the app only receives events when the USB accessory identification information matches the information in the {@code res/xml/usb_accessory_filter.xml} file, referenced by the application’s manifest statement:
<meta-data
android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"
android:resource="@xml/usb_accessory_filter" />
Connections from other USB devices are not received by the ADK 2012 accessory.
Once the connection is established, the app can communicate with the accessory through file input and output streams, as shown in the following example code:
ParcelFileDescriptor mFD = mUSBManager.openAccessory(acc);
if (mFD != null) {
FileDescripter fd = mFD.getFileDescriptor();
mIS = new FileInputStream(fd); // use this to receive messages
mOS = new FileOutputStream(fd); // use this to send commands
}
Review the {@code UsbConnection.java} file provided in the ADK 2012 source code for more implementation details.
One of the important new features introduced with the ADK 2012 is the ability to play audio over a USB connection. This innovation was introduced as an update to Android Open Accessory (AOA) protocol 2.0 and is available on devices running Android 4.1 (API Level 16) and higher.
The ADK 2012 provides a reference implementation of this functionality for accessory developers. No software application is required to be installed on the connected Android device, accessory developers only need to support AOA v2. This implementation demonstrates audio output of 16bit, 44.1kHz stereo PCM source data compressed into a single channel due to the audio hardware available on the accessory.
Using the audio output features provided by the ADK library requires only a few function calls. The first few calls are in the accessory {@code setup()} routine, which prepare the accessory for USB connections and audio output, as summarized in the code example below:
ADK L;
void setup() {
L.audioInit();
L.usbh_init()
L.usbStart();
}
For more information about the {@code ADK::audioInit()} function, see the {@code libraries/ADK/Audio.c} library file. For more information about the {@code ADK::usbh_init()} function, see the {@code libraries/ADK/Usbh.c} library file.
After completing this setup, the {@code loop()} function calls {@code ADK::adkEventProcess()} to handle audio output and other ADK functions:
void loop(void)
{
...
L.adkEventProcess(); //let the adk framework do its thing
...
}
This call executes task queuing for the ADK and as part of the execution process, the task queue executes {@code usbh_work()} in {@code libraries/ADK/Usbh.c}, which handles audio output requests. Review the implementation of this function for details. For additional implementation details on audio output, see the {@code libraries/ADK/accessory.c} library file.