The Android 3.1 platform (also backported to Android 2.3.4) introduced 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 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 initiate the connection.
FTDI claims its support will benefit engineers using its Arduino-inspired Vinco platform to create USB 2.0 connected embedded systems. The major advantage Vinco is said to have over other development platforms is that the host controller capability is already integrated in to the board itself. It means design engineers do not need to budget for an extra USB host/controller card/IC to implement Android Open Accessories.