1 page.title=Android Open Accessory Protocol 1.0 2 @jd:body 3 4 <!-- 5 Copyright 2015 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18 --> 19 <p>Android USB accessories must adhere to the Android Open Accessory (AOA) 20 protocol, which defines how an accessory detects and sets up communication with 21 an Android-powered device. Accessories should carry out the following steps:</p> 22 23 <ol> 24 <li>Wait for and detect a connected device.</li> 25 <li>Determine the device's accessory mode support.</li> 26 <li>Attempt to start the device in accessory mode (if needed).</li> 27 <li>If the device supports AOA, establish communication with the device.</li> 28 </ol> 29 30 <p>The following sections explain how to implement these steps.</p> 31 32 <p class="note"><strong>Note:</strong> When developing a new accessory that 33 connects to an Android device over USB, use 34 <a href="aoa2.html">AOAv2</a>.</p> 35 36 <h2 id="wait-for-and-detect-connected-devices">Wait for and detect connected 37 devices</h2> 38 39 <p>Accessories should continuously check for connected Android-powered devices. 40 When a device is connected, the accessory should determine if the device 41 supports accessory mode.</p> 42 43 <h2 id="determine-accessory-mode-support">Determine accessory mode support</h2> 44 45 <p>When an Android-powered device connects, it can be in one of three states: 46 </p> 47 48 <ul> 49 <li>Supports Android accessory mode and is already in accessory mode.</li> 50 <li>Supports Android accessory mode but it is not in accessory mode.</li> 51 <li>Does not support Android accessory mode.</li> 52 </ul> 53 54 <p>During the initial connection, the accessory should check the vendor ID and 55 product ID of the connected device's USB device descriptor. The vendor ID 56 should match Google's ID (<code>0x18D1</code>). If the device is already in 57 accessory mode, the product ID should be <code>0x2D00</code> or 58 <code>0x2D01</code> and the accessory can 59 <a href="#establish-communication-with-the-device">establish communication with 60 the device</a> through bulk transfer endpoints using its own communication 61 protocol (the device does not need to be started in accessory mode).</p> 62 63 <p class="note"><strong>Note:</strong> <code>0x2D00</code> is reserved for 64 Android-powered devices that support accessory mode. <code>0x2D01</code> is 65 reserved for devices that support accessory mode as well as the Android Debug 66 Bridge (ADB) protocol, which exposes a second interface with two bulk endpoints 67 for ADB. You can use these endpoints for debugging the accessory application if 68 you are simulating the accessory on a computer. In general, do not use this 69 interface unless the accessory implements a passthrough to ADB on the device. 70 </p> 71 72 <p>If the vendor ID or the product ID found in USB device descriptor do not 73 match expected values, the accessory cannot determine if the device supports 74 Android accessory mode. The accessory should attempt to start the device in 75 accessory mode (detailed below) to determine device support.</p> 76 77 <h2 id="attempt-to-start-in-accessory-mode">Attempt to start in accessory 78 mode</h2> 79 80 <p>If the vendor and product IDs do not correspond to an Android-powered device 81 in accessory mode, the accessory cannot discern whether the device supports (but 82 is not in) accessory mode or if the device does not support accessory mode. This 83 can occur because devices that support accessory mode (but are not in accessory 84 mode) initially report the <em>device</em> manufacturer vendor and product IDs 85 instead of the <em>AOA</em> vendor and product IDs.</p> 86 87 <p>The accessory should try to start the device in accessory mode to determine 88 if the device supports that mode:</p> 89 90 <ol> 91 <li>Send a 51 control request ("Get Protocol") to determine if the device 92 supports the Android accessory protocol. If the device supports the protocol, 93 it returns a non-zero number that represents the supported protocol version. 94 The control request is on endpoint 0 with the following characteristics: 95 96 <pre> 97 requestType: USB_DIR_IN | USB_TYPE_VENDOR 98 request: 51 99 value: 0 100 index: 0 101 data: protocol version number (16 bits little endian sent from the 102 device to the accessory) 103 </pre> 104 </li> 105 106 <li>If the device returns a supported protocol version, send a control request 107 with identifying string information to the device. This information allows the 108 device to determine an appropriate application for the accessory (or present a 109 URL to the user if an appropriate application does not exist). The control 110 request is on endpoint 0 (for each string ID) with the following 111 characteristics: 112 113 <pre> 114 requestType: USB_DIR_OUT | USB_TYPE_VENDOR 115 request: 52 116 value: 0 117 index: string ID 118 data zero terminated UTF8 string sent from accessory to device 119 </pre> 120 121 <p>The following string IDs are supported, with a maximum size of 256 bytes 122 for each string (must be zero-terminated with <code>\0</code>).</p> 123 124 <pre> 125 manufacturer name: 0 126 model name: 1 127 description: 2 128 version: 3 129 URI: 4 130 serial number: 5 131 </pre> 132 </li> 133 134 <li>Send a control request to ask the device to start in accessory mode. The 135 control request is on endpoint 0 with the following characteristics: 136 137 <pre> 138 requestType: USB_DIR_OUT | USB_TYPE_VENDOR 139 request: 53 140 value: 0 141 index: 0 142 data: none 143 </pre> 144 </li> 145 </ol> 146 147 <p>After completing these steps, the accessory should wait for the connected USB 148 device to re-introduce itself on the bus in accessory mode, then re-enumerate 149 connected devices. The algorithm 150 <a href="#determine-accessory-mode-support">determines accessory mode support</a> 151 by checking the vendor and product IDs, which should be correct (e.g. correspond 152 to Google's vendor and product IDs instead of the device manufacturer's IDs) if 153 the device successfully switched to accessory mode. If IDs are correct, the 154 accessory moves to <a href="#establish-communication-with-the-device">establish 155 communication with the device</a>.</p> 156 157 <p class="note"><strong>Note:</strong> AOA does not currently support 158 simultaneous AOA and MTP connections. To switch from AOA to MTP, the accessory 159 must first disconnect the USB device (either physically or in an electrically 160 equivalent way) then reconnect using MTP.</p> 161 162 <p>If any step fails, the accessory determines the device does not support 163 Android accessory mode and waits for the next device to connect.</p> 164 165 166 <h2 id="establish-communication-with-the-device">Establish communication with 167 the device</h2> 168 169 <p>If the accessory detects an Android-powered device in accessory mode, the 170 accessory can query the device interface and endpoint descriptors to obtain the 171 bulk endpoints for communicating with the device.</p> 172 173 <p>The number of interfaces and bulk endpoints depends on the product ID. An 174 Android-powered device with a product ID of:</p> 175 176 <ul> 177 <li><code>0x2D00</code> has one interface with two bulk endpoints for input and 178 output communication.</li> 179 <li><code>0x2D01</code> has two interfaces with two bulk endpoints each for 180 input and output communication. The first interface handles standard 181 communication and the second interface handles ADB communication. To use an 182 interface, locate the first bulk input and output endpoints, set the 183 device configuration to a value of 1 with a <code>SET_CONFIGURATION</code> 184 (<code>0x09</code>) device request, then communicate using the endpoints.</li> 185 </ul> 186
