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page.title=Accessory Development Kit 2011 Guide
<div id="qv-wrapper">
<div id="qv">
<h2>In this document</h2>
<li><a href="#components">ADK Components</a></li>
<a href="#getting-started">Getting Started with the ADK</a>
<li><a href="#installing">Installing the Arduino software and necessary
<li><a href="#installing-firmware">Installing the firmware to the ADK board</a></li>
<li><a href="#running-demokit">Running the DemoKit Android application</a></li>
<li><a href="#monitoring">Monitoring the ADK board</a></li>
<a href="#firmware">How the ADK board implements the Android Accessory Protocol</a>
<li><a href="#wait-adk">Wait for and detect connected devices</a></li>
<li><a href="#determine-adk">Determine the connected device's accessory mode
<li><a href="#start-adk">Attempt to start the device in accessory mode</a></li>
<li><a href="#establish-adk">Establish communication with the device</a></li>
<li><a href="">ADK package</a></li>
<h2>See also</h2>
<li><a href="">Google I/O Session Video</a></li>
<li><a href="{@docRoot}guide/topics/connectivity/usb/accessory.html">USB Accessory Dev Guide</a></li>
<p>The Android Open Accessory Development Kit (ADK) is a reference implementation of an Android
Open Accessory, based on the <a href="">Arduino open source electronics
prototyping platform</a>. The accessory's hardware design files, code that implements the
accessory's firmware, and the Android application that interacts with the accessory are provided
as part of the kit to help hardware builders and software developers get started building their
own accessories. The hardware design files and firmware code are contained in the <a href=
"">ADK package download</a>.</p>
<p>A limited number of kits were produced and distributed at the Google I/O 2011 developer
conference. However, many hardware builders have reproduced and enhanced the original design and
these boards are available for purchase. The following list of distributors are currently
producing Android Open Accessory compatible development boards:</p>
<li>The <a href="">Arduino Store</a> provides the <a
href="">Arduino Mega ADK</a> (for EU nations or non-
EU nations) that is based on the ATmega2560 and supports the ADK firmware.</li>
<li><a href="">DIY
Drones</a> provides an Arduino-compatible board geared towards RC (radio controlled) and UAV
(unmanned aerial vehicle) enthusiasts.</li>
<li><a href="">mbed</a> provides a microcontroller and a library
to develop accessories that support the Android accessory protocol. For more information, see
<a href="">mbed with the Android ADK</a>.
<li><a href="">Microchip</a> provides a PIC based USB
microcontroller board.</li>
<li><a href="">Modern
Device</a> provides an Arduino-compatible board that supports the ADK firmware.</li>
<li><a href="">
RT Corp</a> provides an Arduino-compatible board based on the Android ADK board design.</li>
<li><a href="">
Seeed Studio</a> provides an Arduino-compatible board that supports the ADK firmware.</li>
<li><a href="">
SparkFun</a>'s IOIO board now has beta support for the ADK firmware.</li>
<li><a href="">Troido</a> has produced an
Arduino-compatible version of the ADK hardware.</li>
<p>We expect more hardware distributers to create a variety of kits, so please stay tuned for
further developments.</p>
<h2 id="components">ADK Components</h2>
<p>The main hardware and software components of the ADK include:</p>
<li>A USB micro-controller board that is based on the Arduino Mega2560 and Circuits@Home USB
Host Shield designs (now referred to as the ADK board), which you will later implement as an
Android USB accessory. The ADK board provides input and output pins that you can implement
through the use of attachments called "shields." Custom firmware, written in C++, is installed
on the board to define the board's functionality and interaction with the attached shield and
Android-powered device. The hardware design files for the board are located in
<code>hardware/</code> directory.</li>
<li>An Android Demo Shield (ADK shield) that affixes atop the ADK board implements the input
and output points on the board. These implementations include a joystick, LED outputs, and
temperature and light sensors. You can create or buy your own shields or wire your own features
to the ADK board to implement custom functionality. The hardware design files for the shield
are located in <code>hardware/</code>.</li>
<li>A library based on the <a href=
"">Arduino USB Host Shield</a>
library provides the logic for the USB micro-controller board to act as a USB Host. This allows
the board to initiate transactions with USB devices. Describing how to use this entire library
is beyond the scope of this document. Where needed, this document points out important
interactions with the library. For more information, see the source code for the Arduino USB
Host Shield library in the <code>arduino_libs/USB_Host_Shield</code> directory.</li>
<li>An Arduino sketch, <code>arduino_libs/AndroidAccessory/examples/demokit/demokit.pde</code>,
defines the firmware that
runs on the ADK board and is written in C++. The sketch calls the Android accessory protocol
library to interact with the Android-powered device. It also sends data from the ADK board and
shield to the Android application and receives data from the Android application and outputs it
to the ADK board and shield.</li>
<li>The Android accessory protocol library, which is located in the
<code>arduino_libs/AndroidAccessory</code> directory. This library defines how to
enumerate the bus, find a connected Android-powered device that supports accessory mode, and
how to setup communication with the device.</li>
<li>Other third party libraries to support the ADK board's functionality:
<li><a href="">CapSense library</a></li>
<li><a href="">I2C / TWI (Two-Wire Interface)
<li><a href="">Servo library</a></li>
<li><a href="">Spi library</a></li>
<li><a href="">Wire library</a></li>
<li>An Android application, DemoKit, that communicates with the ADK board and shield. The
source for this project is in the <code>app/</code> directory.</li>
<h2 id="getting-started">Getting Started with the ADK</h2>
<p>The following sections describe how to install the Arduino software on your computer, use the
Arduino IDE to install the ADK board's firmware, and install and run the accompanying
Android application for the ADK board. Before you begin, download the following items to set up
your development environment:</p>
<li><a href="">Arduino 1.0 or higher</a>: contains
libraries and an IDE for coding and installing firmware to the ADK board.</li>
<li><a href="">CapSense library v.04</a>:
contains the libraries to sense human capacitance. This library is needed for the capacitive
button that is located on the ADK shield.</li>
<li><a href="">ADK software
package</a>: contains the firmware for the ADK board and hardware design files for the ADK
board and shield.</li>
<h3 id="installing">Installing the Arduino software and necessary libraries</h3>
<p>To install the Arduino software:</p>
<a href="">Download and install</a> the Arduino 1.0 or
higher as described on the Arduino website.
<p class="note"><strong>Note:</strong> If you are on a Mac, install the FTDI USB Serial
Driver that is included in the Arduino package, even though the installation instructions say
<li><a href="">Download</a> and
extract the ADK package to a directory of your choice. You should have an <code>app</code>,
<code>arduino_libs</code>, and <code>hardware</code> directories.</li>
<li><a href="">Download</a> and extract
the CapSense package to a directory of your choice.</li>
<li>Install the necessary libraries:
<p>On Windows:</p>
<ol type="a">
<li>Copy the <code>arduino_libs/AndroidAccessory</code> and
<code>arduino_libs/USB_Host_Shield</code> directories (the complete directories,
not just the files within) to the <code>&lt;arduino_installation_root&gt;/libraries/</code>
<li>Copy the extracted <code>CapSense/</code> library directory and its contents to the
<code>&lt;arduino_installation_root&gt;/libraries/</code> directory.</li>
<p>On Mac:</p>
<ol type="a">
<li>Create, if it does not already exist, an <code>Arduino</code>
directory inside your user account's <code>Documents</code> directory, and within
that, a <code>libraries</code> directory.</li>
<li>Copy the <code>arduino_libs/AndroidAccessory</code> and
<code>arduino_libs/USB_Host_Shield</code> directories (the
complete directories, not just the files within) to your
<code>Documents/Arduino/libraries/</code> directory.</li>
<li>Copy the extracted <code>CapSense/</code> library directory and its contents to the
<code>Documents/Arduino/libraries/</code> directory.
<p>On Linux (Ubuntu):</p>
<ol type="a">
<li>Copy the <code>firmware/arduino_libs/AndroidAccessory</code> and
<code>firmware/arduino_libs/USB_Host_Shield</code> directories (the complete directories,
not just the files within) to the <code>&lt;arduino_installation_root&gt;/libraries/</code>
<li>Copy the extracted <code>CapSense/</code> library directory and its contents to the
<code>&lt;arduino_installation_root&gt;/libraries/</code> directory.</li>
<li>Install the avr-libc library by entering <code>sudo apt-get install avr-libc</code>
from a shell prompt.</li>
<p>You should now have three new directories in the Arduino <code>libraries/</code> directory:
<code>AndroidAccessory</code>, <code>USB_Host_Shield</code>, and <code>CapSense</code>.</p>
<h3 id="installing-firmware">Installing the firmware to the ADK board</h3>
<p>To install the firmware to the ADK board:</p>
<li>Connect the ADK board to your computer using the micro-USB port, which allows two-way
communication and provides power to the ADK board.</li>
<li>Launch the Arduino IDE.</li>
<li>Click <strong>Tools &gt; Board &gt; Arduino Mega 2560</strong> to specify the ADK board's
<li>Select the appropriate USB port:
<li>On Windows: click <strong>Tools &gt; Serial Port &gt; COM#</strong> to specify the port
of communication. The COM port number varies depending on your computer. COM1 is usually
reserved for serial port connections. You most likely want COM2 or COM3.</li>
<li>On Mac: Click <strong>Tools &gt; Serial Port &gt; dev/tty.usbserial-###</strong> to
specify the port of communication.</li>
<li>On Linux (Ubuntu): Click <strong>Tools &gt; Serial Port &gt; dev/ttyUSB#</strong> to
specify the port of communication.</li>
<li>To open the Demokit sketch (firmware code), click <strong>File &gt; Examples &gt;
AndroidAccessory &gt; demokit</strong>.</li>
<li>Click <strong>Sketch &gt; Verify/Compile</strong> to ensure that the sketch has no
<li>Select <strong>File &gt; Upload</strong>. When Arduino outputs <strong>Done
uploading.</strong>, the board is ready to communicate with your Android-powered device.</li>
<h3 id="running-demokit">Running the DemoKit Android application</h3>
<p>The DemoKit Android application runs on your Android-powered device and communicates with the
ADK board. The ADK board receives commands such as lighting up the board's LEDs or sends data
from the board such as joystick movement and temperature readings.</p>
<p>To install and run the application in Eclipse:</p>
<li><a href="">Install the
Google APIs API Level 10 add-on library</a>, which includes the Open Accessory library for
2.3.4 devices that support accessory mode. This library is also forward compatible with Android
3.1 or newer devices that support accessory mode. If you only care about Android 3.1 or newer
devices, all you need is API Level 12. For more information on deciding which API level to use,
see the <a href="{@docRoot}guide/topics/connectivity/usb/accessory.html#choosing">USB Accessory</a>
<li>Click <strong>File &gt; New &gt; Project...</strong>, then select <strong>Android &gt;
Android Project</strong></li>
<li>In the <strong>Project name:</strong> field, type DemoKit.</li>
<li>Choose <strong>Create project from existing source</strong>, click <strong>Browse</strong>,
select the <code>app</code> directory, click <strong>Open</strong> to close that dialog and then
click <strong>Finish</strong>.</li>
<li>For Build Target, select <strong>Google APIs</strong> (Platform 2.3.3, API Level 10).
<p class="note"><strong>Note:</strong> Even though the add-on is labeled as
<strong>2.3.3</strong>, the newest Google API add-on library for API level 10 adds USB Open
Accessory API support for 2.3.4 devices.</p>
<li>Click <strong>Finish</strong>.</li>
<li>Install the application to your device.</li>
<li>Connect the ADK board (USB-A) to your Android-powered device (micro-USB). Ensure that the
power cable to the accessory is plugged in or that the micro-USB port on the accessory is
connected to your computer for power (this also allows you to <a href="#monitoring">monitor the
ADK board</a>). When connected, accept the prompt that asks for whether or not to open the
DemoKit application to connect to the accessory. If the prompt does not show up, connect and
reconnect the accessory.</li>
<p>You can now interact with the ADK board by moving the color LED or servo sliders (make sure
the servos are connected) or by pressing the relay buttons in the application. On the ADK shield,
you can press the buttons and move the joystick to see their outputs displayed in the
<h3 id="monitoring">Monitoring the ADK Board</h3>
<p>The ADK firmware consists of a few files that you should be looking at if you want to build
your own accessory. The files in the <code>arduino_libs/AndroidAccessory</code>
directory are the most important files and have the logic to detect and connect to
Android-powered devices that support accessory mode. Feel free to add debug statements (Arduino
<code>Serial.println()</code> statements) to the code located in the
<code>&lt;arduino_installation_root&gt;/libraries/AndroidAccessory</code> directory and
<code>demokit.pde</code> sketch and re-upload the sketch to the ADK board to
discover more about how the firmware works.</p>
<p>You can view the debug statements in the Arduino Serial Monitor by clicking <strong>Tools &gt;
Serial Monitor</strong> and setting the baud to 115200. The following sections about how
accessories communicate with Android-powered devices describe much of what you should be doing in
your own accessory.</p>
<h2 id="firmware">How the ADK board implements the Android Accessory protocol</h2>
<p>If you have access to the ADK board and shield, the following sections describe the firmware
code that you installed onto the ADK board. The firmware demonstrates a practical example of how
to implement the Android Accessory protocol. Even if you do not have the ADK board and shield,
reading through how the hardware detects and interacts with devices in accessory mode is still
useful if you want to port the code over for your own accessories.</p>
<p>The important pieces of the firmware are the
<code>arduino_libs/AndroidAccessory/examples/demokit/demokit/demokit.pde</code> sketch, which is
the code that receives and sends data to the DemoKit application running on the Android-powered
device. The code to detect and set up communication with the Android-powered device is contained
in the <code>arduino_libs/AndroidAccessory/AndroidAccessory.h</code> and
<code>arduino_libs/AndroidAccessory/AndroidAccessory.cpp</code> files. This code
includes most of the logic that will help you implement your own accessory's firmware. It might
be useful to have all three of these files open in a text editor as you read through these next
<p>The following sections describe the firmware code in the context of the algorithm described in
<a href="#accessory-protocol">Implementing the Android Accessory Protocol</a>.</p>
<h3 id="wait-adk">Wait for and detect connected devices</h3>
<p>In the firmware code (<code>demokit.pde</code>), the <code>loop()</code> function runs
repeatedly and calls <code>AndroidAccessory::isConnected()</code> to check for any connected
devices. If there is a connected device, it continuously updates the input and output streams
going to and from the board and application. If nothing is connected, it continuously checks for
a device to be connected:</p>
AndroidAccessory acc("Google, Inc.",
"DemoKit Arduino Board",
void loop()
if (acc.isConnected()) {
//communicate with Android application
//set the accessory to its default state
<h3 id="determine-adk">Determine the connected device's accessory mode support</h3>
<p>When a device is connected to the ADK board, it can already be in accessory mode, support
accessory mode and is not in that mode, or does not support accessory mode. The
<code>AndroidAccessory::isConnected()</code> method checks for these cases and responds
accordingly when the <code>loop()</code> function calls it. This function first checks to see if
the device that is connected hasn't already been handled. If not, it gets the connected device's
device descriptor to figure out if the device is already in accessory mode by calling
<code>AndroidAccessory::isAccessoryDevice()</code>. This method checks the vendor and product ID
of the device descriptor. A device in accessory mode has a vendor ID of 0x18D1 and a product ID
of 0x2D00 or 0x2D01. If the device is in accessory mode, then the ADK board can <a href=
"#establish">establish communication with the device</a>. If not, the board <a href=
"#start">attempts to start the device in accessory mode</a>.</p>
bool AndroidAccessory::isConnected(void)
byte err;
if (!connected &amp;&amp;
usb.getUsbTaskState() &gt;= USB_STATE_CONFIGURING &amp;&amp;
usb.getUsbTaskState() != USB_STATE_RUNNING) {
Serial.print("\nDevice addressed... ");
Serial.print("Requesting device descriptor.");
err = usb.getDevDescr(1, 0, 0x12, (char *) devDesc);
if (err) {
Serial.print("\nDevice descriptor cannot be retrieved. Program Halted\n");
if (isAccessoryDevice(devDesc)) {
Serial.print("found android accessory device\n");
connected = configureAndroid();
} else {
Serial.print("found possible device. switching to serial mode\n");
} else if (usb.getUsbTaskState() == USB_DETACHED_SUBSTATE_WAIT_FOR_DEVICE) {
connected = false;
return connected;
<h3 id="start-adk">Attempt to start the device in accessory mode</h3>
<p>If the device is not already in accessory mode, then the ADK board must determine whether or
not it supports it by sending control request 51 to check the version of the USB accessory
protocol that the device supports (see <code>AndroidAccessory::getProtocol()</code>). Protocol
version 1 is supported by Android 2.3.4 (API Level 10) and higher. Protocol version 2 is
supported by Android 4.1 (API Level 16) and higher. Versions greater than 2 may supported in
the future.
If the appropriate protocol version is returned, the board sends control request 52 (one
for each string with <code>AndroidAcessory:sendString()</code>) to send it's identifying
information, and tries to start the device in accessory mode with control request 53. The
<code>AndroidAccessory::switchDevice()</code> method takes care of this:</p>
bool AndroidAccessory::switchDevice(byte addr)
int protocol = getProtocol(addr);
if (protocol >= 1) {
Serial.print("device supports protocol 1\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);
return true;
</pre>If this method returns false, the board waits until a new device is connected. If it is
successful, the device displays itself on the USB bus as being in accessory mode when the ADK board
re-enumerates the bus. When the device is in accessory mode, the accessory then <a href=
"#establish-adk">establishes communication with the device</a>.
<h3 id="establish-adk">Establish communication with the device</h3>
<p>If a device is detected as being in accessory mode, the accessory must find the proper bulk
endpoints and set up communication with the device. When the ADK board detects an Android-powered
device in accessory mode, it calls the <code>AndroidAccessory::configureAndroid()</code>
if (isAccessoryDevice(devDesc)) {
Serial.print("found android acessory device\n");
connected = configureAndroid();
<p>which in turn calls the <code>findEndpoints()</code> function:</p>
bool AndroidAccessory::configureAndroid(void)
byte err;
EP_RECORD inEp, outEp;
if (!findEndpoints(1, &amp;inEp, &amp;outEp))
return false;
<p>The <code>AndroidAccessory::findEndpoints()</code> function queries the Android-powered
device's configuration descriptor and finds the bulk data endpoints in which to communicate with
the USB device. To do this, it first gets the device's first four bytes of the configuration
descriptor (only need descBuff[2] and descBuff[3]), which contains the information about the
total length of data returned by getting the descriptor. This data is used to determine whether
or not the descriptor can fit in the descriptor buffer. This descriptor also contains information
about all the interfaces and endpoint descriptors. If the descriptor is of appropriate size, the
method reads the entire configuration descriptor and fills the entire descriptor buffer with this
device's configuration descriptor. If for some reason the descriptor is no longer attainable, an
error is returned.</p>
bool AndroidAccessory::findEndpoints(byte addr, EP_RECORD *inEp, EP_RECORD *outEp)
int len;
byte err;
uint8_t *p;
err = usb.getConfDescr(addr, 0, 4, 0, (char *)descBuff);
if (err) {
Serial.print("Can't get config descriptor length\n");
return false;
len = descBuff[2] | ((int)descBuff[3] &lt;&lt; 8);
if (len &gt; sizeof(descBuff)) {
Serial.print("config descriptor too large\n");
/* might want to truncate here */
return false;
err = usb.getConfDescr(addr, 0, len, 0, (char *)descBuff);
if (err) {
Serial.print("Can't get config descriptor\n");
return false;
<p>Once the descriptor is in memory, a pointer is assigned to the first position of the buffer
and is used to index the buffer for reading. There are two endpoint pointers (input and output)
that are passed into <code>AndroidAccessory::findEndpoints()</code> and their addresses are set
to 0, because the code hasn't found any suitable bulk endpoints yet. A loop reads the buffer,
parsing each configuration, interface, or endpoint descriptor. For each descriptor, Position 0
always contains the size of the descriptor in bytes and position 1 always contains the descriptor
type. Using these two values, the loop skips any configuration and interface descriptors and
increments the buffer with the <code>descLen</code> variable to get to the next descriptor.</p>
<p class="note"><strong>Note:</strong> An Android-powered device in accessory mode can
potentially have two interfaces, one for the default communication to the device and the other
for ADB communication. The default communication interface is always indexed first, so finding
the first input and output bulk endpoints will return the default communication endpoints, which
is what the <code>demokit.pde</code> sketch does. If you are writing your own firmware, the logic
to find the appropriate endpoints for your accessory might be different.</p>
<p>When it finds the first input and output endpoint descriptors, it sets the endpoint pointers
to those addresses. If the findEndpoints() function finds both an input and output endpoint, it
returns true. It ignores any other endpoints that it finds (the endpoints for the ADB interface,
if present).</p>
p = descBuff;
inEp-&gt;epAddr = 0;
outEp-&gt;epAddr = 0;
while (p &lt; (descBuff + len)){
uint8_t descLen = p[0];
uint8_t descType = p[1];
switch (descType) {
Serial.print("config desc\n");
Serial.print("interface desc\n");
if (!inEp-&gt;epAddr &amp;&amp; (epDesc-&gt;bEndpointAddress &amp; 0x80))
ep = inEp;
else if (!outEp-&gt;epAddr)
ep = outEp;
ep = NULL;
if (ep) {
ep-&gt;epAddr = epDesc-&gt;bEndpointAddress &amp; 0x7f;
ep-&gt;Attr = epDesc-&gt;bmAttributes;
ep-&gt;MaxPktSize = epDesc-&gt;wMaxPacketSize;
ep-&gt;sndToggle = bmSNDTOG0;
ep-&gt;rcvToggle = bmRCVTOG0;
Serial.print("unkown desc type ");
Serial.println( descType, HEX);
p += descLen;
if (!(inEp-&gt;epAddr &amp;&amp; outEp-&gt;epAddr))
Serial.println("can't find accessory endpoints");
return inEp-&gt;epAddr &amp;&amp; outEp-&gt;epAddr;
<p>Back in the <code>configureAndroid()</code> function, if there were endpoints found, they are
appropriately set up for communication. The device's configuration is set to 1 and the state of
the device is set to "running", which signifies that the device is properly set up to communicate
with your USB accessory. Setting this status prevents the device from being re-detected and
re-configured in the <code>AndroidAccessory::isConnected()</code> function.</p>
bool AndroidAccessory::configureAndroid(void)
byte err;
EP_RECORD inEp, outEp;
if (!findEndpoints(1, &amp;inEp, &amp;outEp))
return false;
memset(&amp;epRecord, 0x0, sizeof(epRecord));
epRecord[inEp.epAddr] = inEp;
if (outEp.epAddr != inEp.epAddr)
epRecord[outEp.epAddr] = outEp;
in = inEp.epAddr;
out = outEp.epAddr;
Serial.print("inEp: ");
Serial.println(inEp.epAddr, HEX);
Serial.print("outEp: ");
Serial.println(outEp.epAddr, HEX);
epRecord[0] = *(usb.getDevTableEntry(0,0));
usb.setDevTableEntry(1, epRecord);
err = usb.setConf( 1, 0, 1 );
if (err) {
Serial.print("Can't set config to 1\n");
return false;
usb.setUsbTaskState( USB_STATE_RUNNING );
return true;
<p>Lastly, methods to read and write to the appropriate endpoints are needed. The
<code>demokit.pde</code> sketch calls these methods depending on the data that is read from the
Android-powered device or sent by the ADK board. For instance, moving the joystick on the ADK
shield writes data that is read by the DemoKit application running on the Android-powered device.
Moving sliders on the DemoKit application is read by the <code>demokit.pde</code> sketch and
changes the state of the accessory, such as lighting up or changing the color of the LED
int AndroidAccessory::read(void *buff, int len, unsigned int nakLimit) {
return usb.newInTransfer(1, in, len, (char *)buff, nakLimit); }
int AndroidAccessory::write(void *buff, int len) {
usb.outTransfer(1, out, len, (char *)buff);
return len; }
<p>See the <code>demokit.pde</code> sketch for information about how the ADK board
reads and writes data.</p>