Android Open Accessory support 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 in the USB accessory role. Android USB accessories are specifically designed to attach to Android-powered devices and adhere to the Android Open 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
element in your corresponding application's Android manifest. For more information, see the
Android Open Accessory support is included in Android 3.1 (API Level 12) and higher, and supported through an Add-On Library in Android 2.3.4 (API Level 10) and higher.