1 page.title=Android Open Accessory Protocol 1.0 2 @jd:body 3 4 <!-- 5 Copyright 2013 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>An Android USB accessory must adhere to Android Accessory Protocol, which defines how 20 an accessory detects and sets up communication with an Android-powered device. In general, an 21 accessory should carry out the following steps:</p> 22 <ul> 23 <li>Wait for and detect connected devices</li> 24 <li>Determine the device's accessory mode support</li> 25 <li>Attempt to start the device in accessory mode if needed</li> 26 <li>Establish communication with the device if it supports the Android accessory protocol</li> 27 </ul> 28 <p>The following sections explain how to implement these steps.</p> 29 <h2 id="wait-for-and-detect-connected-devices">Wait for and Detect Connected Devices</h2> 30 <p>Your accessory should have logic to continuously check for connected Android-powered devices. 31 When a device is connected, your accessory should determine if the device supports accessory mode.</p> 32 <h2 id="determine-accessory-mode-support">Determine Accessory Mode Support</h2> 33 <p>When an Android-powered device is connected, it can be in one of three states:</p> 34 <ul> 35 <li>The attached device supports Android accessory mode and is already in accessory mode.</li> 36 <li>The attached device supports Android accessory mode, but it is not in accessory mode.</li> 37 <li>The attached device does not support Android accessory mode.</li> 38 </ul> 39 <p>During the initial connection, the accessory should check the vendor and product IDs of the 40 connected device's USB device descriptor. The vendor ID should match Google's ID (<code>0x18D1</code>) and the 41 product ID should be <code>0x2D00</code> or <code>0x2D01</code> if the device is already in accessory mode (case A). If 42 so, the accessory can now 43 <a href="#establish-communication-with-the-device">establish communication with the device</a> through 44 bulk transfer endpoints with its own communication protocol. There is no need to start the device 45 in accessory mode.</p> 46 <p><strong>Note:</strong> <code>0x2D00</code> is reserved for Android-powered devices that 47 support accessory mode. <code>0x2D01</code> is reserved for devices that support accessory mode as well as the 48 ADB (Android Debug Bridge) protocol, which exposes a second interface with two bulk endpoints for 49 ADB. You can use these endpoints for debugging the accessory application if you are simulating 50 the accessory on a computer. In general, do not use this interface unless your accessory is 51 implementing a passthrough to ADB on the device.</p> 52 <p>If the vendor and product ID do not match, there is no way to distinguish between states b and c, so 53 the accessory <a href="#attempt-to-start-in-accessory-mode">attempts to start the device in accessory mode</a> 54 to determine if the device is supported.</p> 55 <h2 id="attempt-to-start-in-accessory-mode">Attempt to Start in Accessory Mode</h2> 56 <p>If the vendor and product IDs do not correspond to an Android-powered device in accessory 57 mode, the accessory cannot discern whether the device supports accessory mode and is not in that 58 state, or if the device does not support accessory mode at all. This is because devices that 59 support accessory mode but aren't in it initially report the device's manufacturer vendor ID and 60 product ID, and not the special Android Open Accessory ones. In either case, the accessory should 61 try to start the device into accessory mode to figure out if the device supports it. The following 62 steps explain how to do this:</p> 63 <ul> 64 <li>Send a 51 control request ("Get Protocol") to figure out if the device supports the Android 65 accessory protocol. A non-zero number is returned if the protocol is supported, which 66 represents the version of the protocol that the device supports (currently, only version 1 67 exists). This request is a control request on endpoint 0 with the following characteristics: 68 69 <pre> 70 requestType: USB_DIR_IN | USB_TYPE_VENDOR 71 request: 51 72 value: 0 73 index: 0 74 data: protocol version number (16 bits little endian sent from the 75 device to the accessory) 76 </pre> 77 </li> 78 <li>If the device returns a proper protocol version, send identifying string information to the 79 device. This information allows the device to figure out an appropriate application for this 80 accessory and also present the user with a URL if an appropriate application does not exist. 81 These requests are control requests on endpoint 0 (for each string ID) with the following 82 characteristics: 83 84 <pre> 85 requestType: USB_DIR_OUT | USB_TYPE_VENDOR 86 request: 52 87 value: 0 88 index: string ID 89 data zero terminated UTF8 string sent from accessory to device 90 </pre> 91 92 <p>The following string IDs are supported, with a maximum size of 256 bytes for each string 93 (must be zero terminated with `\0`).</p> 94 95 <pre> 96 manufacturer name: 0 97 model name: 1 98 description: 2 99 version: 3 100 URI: 4 101 serial number: 5 102 </pre> 103 </li> 104 <li>When the identifying strings are sent, request the device start up in accessory mode. This 105 request is a control request on endpoint 0 with the following characteristics: 106 107 <pre> 108 requestType: USB_DIR_OUT | USB_TYPE_VENDOR 109 request: 53 110 value: 0 111 index: 0 112 data: none 113 </pre> 114 </li> 115 </ul> 116 117 <p>After sending the final control request, the connected USB device should re-introduce itself 118 on the bus in accessory mode and the accessory can re-enumerate the connected devices. The 119 algorithm jumps back to 120 <a href="#determine-accessory-mode-support">determining the device's accessory mode support</a> 121 to check for the vendor and product ID. The vendor ID and product ID of the device will be 122 different if the device successfully switched to accessory mode and will now correspond to 123 Google's vendor and product IDs instead of the device manufacturer's IDs. The accessory can now 124 <a href="#establish-communication-with-the-device">establish communication with the device</a>.</p> 125 <p>If at any point these steps fail, the device does not support Android accessory mode and the 126 accessory should wait for the next device to be connected.</p> 127 <h2 id="establish-communication-with-the-device">Establish Communication with the Device</h2> 128 <p>If an Android-powered device in accessory mode is detected, the accessory can query the 129 device's interface and endpoint descriptors to obtain the bulk endpoints to communicate with the 130 device. An Android-powered device that has a product ID of <code>0x2D00</code> has one interface with two bulk 131 endpoints for input and output communication. A device with product ID of <code>0x2D01</code> has two 132 interfaces with two bulk endpoints each for input and output communication. The first interface 133 is for standard communication while the second interface is for ADB communication. To communicate 134 on an interface, all you need to do is find the first bulk input and output endpoints, set the 135 device's configuration to a value of 1 with a <code>SET_CONFIGURATION</code> (<code>0x09</code>) device request, then 136 communicate using the endpoints.</p> 137 138 139
