With Android 3.1, the platform introduces Android Open Accessory support, which allows external USB hardware (an Android USB accessory) to interact with an Android-powered device in a special accessory mode. When an Android-powered powered device is in accessory mode, the connected accessory acts as the USB host (powers the bus and enumerates devices) and the Android-powered device acts as the USB device. Android USB accessories are specifically designed to attach to Android-powered devices and adhere to a simple protocol (Android accessory protocol) that allows them to detect Android-powered devices that support accessory mode. Accessories must also provide 500mA at 5V for charging power. Many previously released Android-powered devices are only capable of acting as a USB device and cannot initiate connections with external USB devices. Android Open Accessory support overcomes this limitation and allows you to build accessories that can interact with an assortment of Android-powered devices by allowing the accessory to initiate the connection.
Note: Accessory mode is ultimately dependent on the device's
hardware and not all devices support accessory mode. Devices that support accessory mode can
be filtered using a
<uses-feature> element in your corresponding application's
Android manifest. For more information, see the USB Accessory developer
Implementing the Android Accessory Protocol
An Android USB accessory must adhere to Android Accessory Protocol, which defines how an accessory detects and sets up communication with an Android-powered device. In general, an accessory should carry out the following steps:
- Wait for and detect connected devices
- Determine the device's accessory mode support
- Attempt to start the device in accessory mode if needed
- Establish communication with the device if it supports the Android accessory protocol
The following sections go into depth about how to implement these steps.
Wait for and detect connected devices
Your accessory should have logic to continuously check for connected Android-powered devices. When a device is connected, your accessory should determine if the device supports accessory mode.
Determine the device's accessory mode support
When an Android-powered device is connected, it can be in one of three states:
- The attached device supports Android accessory mode and is already in accessory mode.
- The attached device supports Android accessory mode, but it is not in accessory mode.
- The attached device does not support Android accessory mode.
During the initial connection, the accessory should check the vendor and product IDs of the connected device's USB device descriptor. The vendor ID should match Google's ID (0x18D1) and the product ID should be 0x2D00 or 0x2D01 if the device is already in accessory mode (case A). If so, the accessory can now establish communication with the device through bulk transfer endpoints with its own communication protocol. There is no need to start the device in accessory mode.
Note: 0x2D00 is reserved for Android-powered devices that support accessory mode. 0x2D01 is reserved for devices that support accessory mode as well as the ADB (Android Debug Bridge) protocol, which exposes a second interface with two bulk endpoints for ADB. You can use these endpoints for debugging the accessory application if you are simulating the accessory on a computer. In general, do not use this interface unless your accessory is implementing a passthrough to ADB on the device.
If the vendor and product ID do not match, there is no way to distinguish between states b and c, so the accessory attempts to start the device in accessory mode to figure out if the device is supported.
Attempt to start the device in accessory mode
If the vendor and product IDs do not correspond to an Android-powered device in accessory mode, the accessory cannot discern whether the device supports accessory mode and is not in that state, or if the device does not support accessory mode at all. This is because devices that support accessory mode but aren't in it initially report the device's manufacturer vendor ID and product ID, and not the special Android Open Accessory ones. In either case, the accessory should try to start the device into accessory mode to figure out if the device supports it. The following steps explain how to do this:
- Send a 51 control request ("Get Protocol") to figure out if the device supports the Android
accessory protocol. A non-zero number is returned if the protocol is supported, which
represents the version of the protocol that the device supports (currently, only version 1
exists). This request is a control request on endpoint 0 with the following characteristics:
requestType: USB_DIR_IN | USB_TYPE_VENDOR request: 51 value: 0 index: 0 data: protocol version number (16 bits little endian sent from the device to the accessory)
- If the device returns a proper protocol version, send identifying string information to the
device. This information allows the device to figure out an appropriate application for this
accessory and also present the user with a URL if an appropriate application does not exist.
These requests are control requests on endpoint 0 (for each string ID) with the following
requestType: USB_DIR_OUT | USB_TYPE_VENDOR request: 52 value: 0 index: string ID data zero terminated UTF8 string sent from accessory to device
The following string IDs are supported, with a maximum size of 256 bytes for each string (must be zero terminated with \0).
manufacturer name: 0 model name: 1 description: 2 version: 3 URI: 4 serial number: 5
- When the identifying strings are sent, request the device start up in accessory mode. This
request is a control request on endpoint 0 with the following characteristics:
requestType: USB_DIR_OUT | USB_TYPE_VENDOR request: 53 value: 0 index: 0 data: none
After sending the final control request, the connected USB device should re-introduce itself on the bus in accessory mode and the accessory can re-enumerate the connected devices. The algorithm jumps back to determining the device's accessory mode support to check for the vendor and product ID. The vendor ID and product ID of the device will be different if the device successfully switched to accessory mode and will now correspond to Google's vendor and product IDs instead of the device manufacturer's IDs. The accessory can now establish communication with the device.
If at any point these steps fail, the device does not support Android accessory mode and the accessory should wait for the next device to be connected.
Establish communication with the device
If an Android-powered device in accessory mode is detected, the accessory can query the device's interface and endpoint descriptors to obtain the bulk endpoints to communicate with the device. An Android-powered device that has a product ID of 0x2D00 has one interface with two bulk endpoints for input and output communication. A device with product ID of 0x2D01 has two interfaces with two bulk endpoints each for input and output communication. The first interface is for standard communication while the second interface is for ADB communication. To communicate on an interface, all you need to do is find the first bulk input and output endpoints, set the device's configuration to a value of 1 with a SET_CONFIGURATION (0x09) device request, then communicate using the endpoints.