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Android Open Accessory Development Kit 2012 Overview

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Android's Open Accessory Development Kit 2.0 allows developers, hobbyists, and entrepreneurs to tinker with and build powerful USB accessories for use with Android devices. Shane Conder and Lauren Darcey, authors of Android Wireless Application Development Volume II: Advanced Topics, 3rd Ed, discuss what the ADK is and how it can be leveraged by developers, as well as showcase the specific features available in its latest version. They also explore the ADK hardware released at Google I/O 2012.
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The Android Open Accessory Development Kit, or ADK, was initially announced at Google I/O 2011. A number of kits, which allowed developers to dabble in building USB accessories for Android, were distributed to a small subset of conference attendees. Over the course of the coming year, several manufacturers released ADK Arduino boards much like the initial kits, and developers began to tinker and develop USB-connected gadgets like alarm clocks, weather stations, and even car-simulating baby seats.

While hardware hackers did dabble, the ADK has kicked off to a slow start. To be fair, more Android accessories have hit the market over the past few months, but to say that the Android community took the ADK and ran with it would be a bit of a stretch. Tech blogs like Engadget did feature a few ADK concept stories,  but by no means have we seen a large-scale rush to innovate in this way. Most ADK use still remains in the hands of hardware geeks and students doing school projects. On a personal note, of the two authors of this article, only one was interested enough in the ADK after Google I/O 2011 to order a reference board for himself.

But as Android app developers, we see that the ADK provides yet more ways to inject Android devices into user's daily life, and the more users become intertwined with their devices—whether it's on the treadmill or planted in front of the TV, in the kitchen or in the car—the more loyal they become to the Android platform. Loyal users download apps, which make us all successful, so of course we are in favor of developers taking the Android ADK and making awesome accessories.

Perhaps Google wanted a soft launch of the ADK to iron out the kinks, or perhaps they were disappointed in the 2011 ADK's reception. Either way, at Google I/O 2012, every ADK session attendee got a much more fully functional ADK kit. This time, the kit looked less like a tiny hobbyist kit from Radio Shack and more like a real accessory. Now we are both interested in the ADK, and have spent time messing around with the kit and discussing what cool accessories we could develop.

How the ADK Works

Starting with Android 3.1, the Android SDK provided support for connecting USB hardware to Android devices. Traditionally, mobile phones are USB clients; that is, you will connect them to a USB host like a computer. The Android Open Accessory (AOA) system does not change this. What does this mean? It means that the accessory connected to the Android device is the USB host. It can provide power to the handset. This is useful in that it means connecting an accessory will not drain the battery of your Android device. Instead, it will actually charge it, as accessories are required to provide charging power. This makes a lot of sense, as an accessory might be a radio dock, a television, a refrigerator, a treadmill, or any number of other home electronics that are always plugged in.

From there, the AOA protocol defines the method by which the Android device and the accessory communicate via USB. Once connected with the AOA protocol, the Android device will usually run an Android application to communicate with the accessory. The accessory, if based on one of the reference kits, is likely to be an Arduino board where you can write code to control the hardware for when it is connected to the Android device and when it is not. For example, an alarm clock may provide a docking station for charging. The Android application could push alarms from the phone to the clock, ringtones, the time, the date, and other information useful for the clock. When the phone is removed, the clock would retain all of this modification.

What's New in the 2012 ADK

The 2012 ADK does update the AOA protocol to version 2.0 with some compelling new features. First is the ability to directly stream audio content via USB without the need for a controller app. The new USB audio feature is very important for the ADK's future success, as it paves the way for straightforward digital audio connections to speakers, docks, receivers, cars, and other accessories you might want to use to charge your device and to listen to audio content. A second feature of AOA 2 is the ability for an accessory to act as a Human Interface Device (HID). However, instead of traditional HID USB client devices, AOA 2 HID devices are host devices like an Android Accessory device but provide the functionality of a typical HID device such as a mouse, a keyboard, game pads, and other input devices.

Accessories that use one or both of these new features can also take advantage of a new mode: the ability for the feature to be active without a controlling Android application. Users wouldn't want an app launch dialog to come up when they plugged in a keyboard. Users would also want the audio to seamlessly start playing through the new source when a USB audio accessory was plugged in. This is now possible with AOA 2.

More About the ADK 2012 "Reference" Hardware

Figure 1 ADK 2012 Showing colorful LEDs

If the ADK 2011 reference kit (Figure 1) was any indication, third-party manufacturers will probably create similar kits to the ADK 2012 (Google has released the specs to hardware manufacturers). Because the ADK 2012 kit is also a functional device out of the box, as opposed to simply a set of boards, we'll run down some of what it can do.

Figure 2 Inside the ADK 2012

First, the product itself—and it is a product with hardware and an Android app—is pretty slick. The box is held together magnetically as opposed to screws, so it's easy to expose and access the boards, which makes it inviting to play with (Figure 2). Out of the box, you can use it as an alarm clock, play music via Bluetooth, cause lights to blink and flash, and display various sensor data including temperature, pressure, and color. Yeah, there's a color sensor that will display on color LEDs the color it detects which is pretty nifty. The speaker for audio output isn't your typical speaker, either. It's a transducer, and so it vibrates the case to create perceptible audio.  


In the months since Google IO 2012, we have not seen much in the way of new ADK innovation on any of the hacker blogs. We think that part of the reason the ADK hasn't taken off in a big way is because there is so much going on with the Android platform. There are so many different ways to innovate with it that remain virtually untapped, and there is simply not enough developer bandwidth to spend time innovate in this area, especially when there's not a really clear way for the dabbler to monetize such products.

Instead, app developers are still focusing on the race to develop the core Android apps that users are demanding. The good news is that the ADK isn't going to disappear. We expect that, over time, developers will continue to improve their existing Android apps and look for ways to extend their reach outside the traditional device boundaries, at which point the ADK is there and ready to help developers take their apps to the next level.

Until that time, happy hacking!

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