Android’s USB port has in the past been curiously inaccessible to programmers. Last week at Google I/O we announced the Android Open Accessory APIs for Android. These APIs allow USB accessories to connect to Android devices running Android 3.1 or Android 2.3.4 without special licensing or fees. The new “accessory mode” does not require the Android device to support USB Host mode. This post will concentrate on accessory mode, but we also announced USB Host mode APIs for devices with hardware capable of supporting it.
To understand why having a USB port is not sufficient to support accessories let’s quickly look at how USB works. USB is an asymmetric protocol in that one participant acts as a USB Host and all other participants are USB Devices.
In the PC world, a laptop or desktop acts as Host and your printer, mouse, webcam, etc., is the USB Device. The USB Host has two important tasks. The first is to be the bus master and control which device sends data at what times. The second key task is to provide power, since USB is a powered bus.
The problem with supporting accessories on Android in the traditional way is that relatively few devices support Host mode. Android’s answer is to turn the normal USB relationship on its head. In accessory mode the Android phone or tablet acts as the USB Device and the accessory acts as the USB Host. This means that the accessory is the bus master and provides power.
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