page.title=Custom Accessories
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    <h2>In this document</h2>
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<p>An accessory for Android can be anything: keyboard, thermometer, robot,
lighting control, or anything else you can imagine. All Android accessories
connect to an Android device in some way, so when building an accessory you must
consider the type of connections your accessory will use. This page provides a
quick overview of your options for connecting your Android accessory and
a list of resources to help you get started.</p>

<h2 id="connecting-over-usb">Connecting over USB</h2>
<p>An accessory that connects to an Android device through a USB cable must
support the Android Open Accessory (AOA) protocol, which specifies how an
accessory can establish communication with an Android device via USB.
Due to the low power output of Android devices, AOA requires the accessory to
act as a USB host, meaning the connecting accessory must power the bus.</p>

<p>AOA has two versions that support different types of communication:</p>
<ul>
<li><strong>AOAv1</strong>. Supports generic accessory communication and adb
debugging. Available in Android 3.1 (API Level 12) and higher and supported
through an
<a href="https://developers.google.com/android/add-ons/google-apis/">Add-On
Library</a> in Android 2.3.4 (API Level 10) and higher.</li>
<li><strong>AOAv2</strong>. Supports audio streaming and human interface
device (HID) capabilities. Available in Android 4.1 (API Level 16).</li>
</ul>

<p>If you use the general accessory protocol to communicate with your accessory
(rather than the adb or audio protocol), you must provide an Android application
that can detect the connection of your USB accessory and establish communication.
</p>

<h3 id="next-steps_0">Next steps</h3>
<p>To get started building an Android accessory that uses a USB connection:
</p>
<ul>
<li>Select a hardware platform or build a hardware device that can support USB
host mode.</li>
<li>Review <a href="protocol.html">AOA</a> specifications to
understand how to implement this protocol on your accessory hardware.
Implementing <a href="aoa2.html">AOAv2</a> is
recommended for all new Android USB accessories.</li>
<li>Review the ADK 2012
<a href="http://developer.android.com/tools/adk/adk2.html#src-download">firmware
source code</a> (<code>&lt;adk-src&gt;/adk2012/board/library/ADK2/</code>),
which demonstrates an implementation of an accessory using a USB connection for
general data communications and audio streaming.</li>
<li>When planning to build an Android application that communicates with your
accessory via USB, review the ADK 2012 Android
<a href="http://developer.android.com/tools/adk/adk2.html#src-download">application
source code</a> (<code>&lt;adk-src&gt;/adk2012/app/</code>).</li>
</ul>

<h2 id="connecting-over-bluetooth">Connecting over Bluetooth</h2>
<p>An accessory that connects with Android devices over a Bluetooth connection
can use connection profiles supported by Android, including the Simple Serial
Protocol (SSP) and Advanced Audio Distribution Profile (A2DP) profile. An
accessory that uses Bluetooth to connect to Android devices must support
Bluetooth communications and at least one of the supported connection profiles.
</p>
<p>Users must enable Bluetooth on their Android device and pair with your
accessory to use the accessory. You can also provide a secondary Android
application that handles specialized communication such as data input or control
outputs to interface with your accessory.</p>

<h3 id="next-steps_1">Next steps</h3>
<p>To get started building an Android accessory that uses a Bluetooth connection:
</p>
<ul>
<li>Select a hardware platform or build an hardware device that can support
Bluetooth communications and an Android supported connection profile, such as
SSP or A2DP.</li>
<li>Review the ADK 2012
<a href="http://developer.android.com/tools/adk/adk2.html#src-download">firmware
source code</a> (<code>&lt;adk-src&gt;/adk2012/board/library/ADK2/</code>),
which includes an example implementation of general data communications and
audio streaming using a Bluetooth connection.</li>
<li>When planning to build an Android application that communicates with your
accessory via Bluetooth, review the ADK 2012 Android
<a href="http://developer.android.com/tools/adk/adk2.html#src-download">application
source code</a> (<code>&lt;adk-src&gt;/adk2012/app/</code>).</li>
</ul>

<p class="note"><strong>Note:</strong> The ADK 2012 source code includes an open
source Bluetooth stack built for the Texas Instruments CC2564 chip but is
designed to work with any Bluetooth chip that supports a standard
Host/Controller Interface (HCI).</p>

<h2 id="audio-over-usb">Connecting audio over USB</h2>
<p>An accessory that connects with Android over USB can use AOAv2 (supported on
Android 4.1 (API Level 16) and higher. After an Android device connects to an
accessory that supports this protocol, the Android system treats it as a
standard audio output device and routes all audio to that accessory. No
secondary software application is required on the Android device.</p>

<p class="note"><strong>Note:</strong> Due to the low power output of Android
devices, AOA requires accessories to act as a USB host, meaning the connecting
accessory must power the bus.</p>

<h3 id="next-steps_2">Next steps</h3>
<p>To get started building an audio accessory that uses a USB connection, see
<a href="#next-steps_0">next steps for USB connections.</a></p>

<p>AOAv2 also supports the
<a href="aoa2.html#hid-support">human interface device</a>
(HID) protocol through a USB connection, enabling accessories such as audio
docks to provide hardware play back controls such as pause, fast-forward or
volume buttons.</p>