1 <html devsite> 2 <head> 3 <title>Android Open Accessory Protocol 1.0</title> 4 <meta name="project_path" value="/_project.yaml" /> 5 <meta name="book_path" value="/_book.yaml" /> 6 </head> 7 <body> 8 <!-- 9 Copyright 2017 The Android Open Source Project 10 11 Licensed under the Apache License, Version 2.0 (the "License"); 12 you may not use this file except in compliance with the License. 13 You may obtain a copy of the License at 14 15 http://www.apache.org/licenses/LICENSE-2.0 16 17 Unless required by applicable law or agreed to in writing, software 18 distributed under the License is distributed on an "AS IS" BASIS, 19 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 20 See the License for the specific language governing permissions and 21 limitations under the License. 22 --> 23 24 25 <p>Android USB accessories must adhere to the Android Open Accessory (AOA) 26 protocol, which defines how an accessory detects and sets up communication with 27 an Android-powered device. Accessories should carry out the following steps:</p> 28 29 <ol> 30 <li>Wait for and detect a connected device.</li> 31 <li>Determine the device's accessory mode support.</li> 32 <li>Attempt to start the device in accessory mode (if needed).</li> 33 <li>If the device supports AOA, establish communication with the device.</li> 34 </ol> 35 36 <p>The following sections explain how to implement these steps.</p> 37 38 <p class="note"><strong>Note:</strong> When developing a new accessory that 39 connects to an Android device over USB, use 40 <a href="aoa2.html">AOAv2</a>.</p> 41 42 <h2 id="wait-for-and-detect-connected-devices">Wait for and detect connected 43 devices</h2> 44 45 <p>Accessories should continuously check for connected Android-powered devices. 46 When a device is connected, the accessory should determine if the device 47 supports accessory mode.</p> 48 49 <h2 id="determine-accessory-mode-support">Determine accessory mode support</h2> 50 51 <p>When an Android-powered device connects, it can be in one of three states: 52 </p> 53 54 <ul> 55 <li>Supports Android accessory mode and is already in accessory mode.</li> 56 <li>Supports Android accessory mode but it is not in accessory mode.</li> 57 <li>Does not support Android accessory mode.</li> 58 </ul> 59 60 <p>During the initial connection, the accessory should check the vendor ID and 61 product ID of the connected device's USB device descriptor. The vendor ID 62 should match Google's ID (<code>0x18D1</code>). If the device is already in 63 accessory mode, the product ID should be <code>0x2D00</code> or 64 <code>0x2D01</code> and the accessory can 65 <a href="#establish-communication-with-the-device">establish communication with 66 the device</a> through bulk transfer endpoints using its own communication 67 protocol (the device does not need to be started in accessory mode).</p> 68 69 <p class="note"><strong>Note:</strong> <code>0x2D00</code> is reserved for 70 Android-powered devices that support accessory mode. <code>0x2D01</code> is 71 reserved for devices that support accessory mode as well as the Android Debug 72 Bridge (ADB) protocol, which exposes a second interface with two bulk endpoints 73 for ADB. You can use these endpoints for debugging the accessory application if 74 you are simulating the accessory on a computer. In general, do not use this 75 interface unless the accessory implements a passthrough to ADB on the device. 76 </p> 77 78 <p>If the vendor ID or the product ID found in USB device descriptor do not 79 match expected values, the accessory cannot determine if the device supports 80 Android accessory mode. The accessory should attempt to start the device in 81 accessory mode (detailed below) to determine device support.</p> 82 83 <h2 id="attempt-to-start-in-accessory-mode">Attempt to start in accessory 84 mode</h2> 85 86 <p>If the vendor and product IDs do not correspond to an Android-powered device 87 in accessory mode, the accessory cannot discern whether the device supports (but 88 is not in) accessory mode or if the device does not support accessory mode. This 89 can occur because devices that support accessory mode (but are not in accessory 90 mode) initially report the <em>device</em> manufacturer vendor and product IDs 91 instead of the <em>AOA</em> vendor and product IDs.</p> 92 93 <p>The accessory should try to start the device in accessory mode to determine 94 if the device supports that mode:</p> 95 96 <ol> 97 <li>Send a 51 control request ("Get Protocol") to determine if the device 98 supports the Android accessory protocol. If the device supports the protocol, 99 it returns a non-zero number that represents the supported protocol version. 100 The control request is on endpoint 0 with the following characteristics: 101 102 <pre class="devsite-click-to-copy"> 103 requestType: USB_DIR_IN | USB_TYPE_VENDOR 104 request: 51 105 value: 0 106 index: 0 107 data: protocol version number (16 bits little endian sent from the 108 device to the accessory) 109 </pre> 110 </li> 111 112 <li>If the device returns a supported protocol version, send a control request 113 with identifying string information to the device. This information allows the 114 device to determine an appropriate application for the accessory (or present a 115 URL to the user if an appropriate application does not exist). The control 116 request is on endpoint 0 (for each string ID) with the following 117 characteristics: 118 119 <pre class="devsite-click-to-copy"> 120 requestType: USB_DIR_OUT | USB_TYPE_VENDOR 121 request: 52 122 value: 0 123 index: string ID 124 data zero terminated UTF8 string sent from accessory to device 125 </pre> 126 127 <p>The following string IDs are supported, with a maximum size of 256 bytes 128 for each string (must be zero-terminated with <code>\0</code>).</p> 129 130 <pre class="devsite-click-to-copy"> 131 manufacturer name: 0 132 model name: 1 133 description: 2 134 version: 3 135 URI: 4 136 serial number: 5 137 </pre> 138 </li> 139 140 <li>Send a control request to ask the device to start in accessory mode. The 141 control request is on endpoint 0 with the following characteristics: 142 143 <pre class="devsite-click-to-copy"> 144 requestType: USB_DIR_OUT | USB_TYPE_VENDOR 145 request: 53 146 value: 0 147 index: 0 148 data: none 149 </pre> 150 </li> 151 </ol> 152 153 <p>After completing these steps, the accessory should wait for the connected USB 154 device to re-introduce itself on the bus in accessory mode, then re-enumerate 155 connected devices. The algorithm 156 <a href="#determine-accessory-mode-support">determines accessory mode support</a> 157 by checking the vendor and product IDs, which should be correct (e.g. correspond 158 to Google's vendor and product IDs instead of the device manufacturer's IDs) if 159 the device successfully switched to accessory mode. If IDs are correct, the 160 accessory moves to <a href="#establish-communication-with-the-device">establish 161 communication with the device</a>.</p> 162 163 <p class="note"><strong>Note:</strong> AOA does not currently support 164 simultaneous AOA and MTP connections. To switch from AOA to MTP, the accessory 165 must first disconnect the USB device (either physically or in an electrically 166 equivalent way) then reconnect using MTP.</p> 167 168 <p>If any step fails, the accessory determines the device does not support 169 Android accessory mode and waits for the next device to connect.</p> 170 171 172 <h2 id="establish-communication-with-the-device">Establish communication with 173 the device</h2> 174 175 <p>If the accessory detects an Android-powered device in accessory mode, the 176 accessory can query the device interface and endpoint descriptors to obtain the 177 bulk endpoints for communicating with the device.</p> 178 179 <p>The number of interfaces and bulk endpoints depends on the product ID. An 180 Android-powered device with a product ID of:</p> 181 182 <ul> 183 <li><code>0x2D00</code> has one interface with two bulk endpoints for input and 184 output communication.</li> 185 <li><code>0x2D01</code> has two interfaces with two bulk endpoints each for 186 input and output communication. The first interface handles standard 187 communication and the second interface handles ADB communication. To use an 188 interface, locate the first bulk input and output endpoints, set the 189 device configuration to a value of 1 with a <code>SET_CONFIGURATION</code> 190 (<code>0x09</code>) device request, then communicate using the endpoints.</li> 191 </ul> 192 193 </body> 194 </html> 195
