Android Accessory Protocol - FTF2014

Android Accessory ProtocolFTF2014

04/10/2014

Gary Bisson

Embedded Software Engineer

SESSION OVERVIEW

1. Introduction

2. Protocol Specifications

3. Accessory Development Kit

4. Software Implementation

5. Demonstrations

6. Conclusion

ABOUT THE PRESENTER

• Embedded Software Engineer at Adeneo Embedded

(Bellevue, WA)

I Linux / Android

♦ BSP Adaptation

♦ Driver Development

♦ System Integration

I Partners with Freescale

Introduction

Android Accessory Protocol Introduction

INTRODUCTION TO AOA

• Allows USB hardware to interact with an Android-powered

device

I No need for the Android device to act as USB Host

I Standard API

• Introduced in Android 3.1 (API level 12)

I Backported to Android 2.3.4 (API level 10)

• Version 2.0 released with Android 4.1

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Android Accessory Protocol Introduction

INTRODUCTION TO AOA

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Protocol Specifications

Android Accessory Protocol Protocol Specifications

AOA 1.0 PROTOCOL

• Android Open Accessory 1.0 is a protocol that allows an

Android device to interact with an Android USB

Accessory in a special accessory mode.

• Basically there are four steps to initiate the

communication.

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AOA 1.0 PROTOCOL

1. Wait for and detect connected devices

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AOA 1.0 PROTOCOL

2. Determine the device’s accessory mode support

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AOA 1.0 PROTOCOL

• The accessory must check the Vendor & Product ID’s of

the connected device

I Possibility to target some devices

I Detection of devices already in Accessory mode

• If it is already in accessory mode then:

I Vendor ID = 0x18D1 (Google)

I Product ID = 0x2D00 || 0x2D01

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AOA 1.0 PROTOCOL

• Accessory mode product IDs

I 0x2D00 – Supports Accessory Mode

♦ 1 interface with 2 bulk endpoints

I 0x2D01 – Supports Accessory Mode + ADB

♦ 2 interfaces with 2 bulk endpoints each

• If it is not in accessory mode then Accessory mode support

can be asked:

I Send a “Get Protocol” request (51) on endpoint 0

♦ AOA Version (1.0 or 2.0) is returned by Android

device

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AOA 1.0 PROTOCOL

3. Attempt to start the device in accessory mode

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AOA 1.0 PROTOCOL

• If the AOA Version # is okay, then send string’s identifying

our ADK to the device

I Send “Identity” request (52) for each identifier:

♦ Manufacturer

♦ Model Name

♦ Description

♦ Version

♦ URL

♦ Serial Number

I Send “Start Accessory” request (53) to request the

Android device re-introduce itself on the bus in

accessory mode

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AOA 1.0 PROTOCOL

4. Establish communications

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AOA 1.0 PROTOCOL

• The accessory must obtain the Bulk endpoints and be

prepared to initiate communication with the device

From this point the communication is defined by the ADK

Developer

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AOA 2.0 PROTOCOL

• AOA 2.0 was released at Google I/O end of June 2012

alongside Jelly Bean

• Two new features:

I Audio Output

I Accessory as HID

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AOA 2.0 PROTOCOL

• Audio output:

I From an Android device to an accessory

I Standard USB audio class interface (ISO)

I Only supports 2 Channel, 16-bit PCM @ 44100KHz

• To enable the audio support, the accessory must send a

new USB control request:

I “Audio Support” request (58)

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AOA 2.0 PROTOCOL

• HID support:

I Registers one or more USB HID with to the Android

device.

I Reverses the direction of communication for typical

USB HID (Host <-> Device).

I Uses USB control requests:

♦ ACCESSORY_REGISTER_HID

♦ ACCESSORY_UNREGISTER_HID

♦ ACCESSORY_SET_HID_REPORT_DESC

♦ ACCESSORY_SEND_HID_EVENT

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AOA 2.0 PROTOCOL

• Compatible with original AOA 1.0 protocol

• Also adds Bluetooth support

I Not the focus of this presentation...

I Example available in ADK source code

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AOA 2.0 PROTOCOL

• New Product ID’s

I 0x2D02 – Supports Audio

♦ 2 Audio interfaces (control + streaming)

I 0x2D03 – Supports Audio + ADB

♦ 3 interfaces: 2 Audio + 1 Bulk

I 0x2D04 – Supports AOA 1.0 + Audio


I 0x2D05 – Supports AOA 1.0 + Audio + ADB


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Accessory Development Kit

Android Accessory Protocol Accessory Development Kit

GOOGLE ADK

• Hardware for ADK 2011 is based on a Arduino Mega2560

• Hardware for the ADK 2012 is based on an ARM

Cortex-M3

• ADK 2012 Guide

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http://developer.android.com/tools/adk/adk2.html


WHAT PART OF AOA DO YOU CONTROL?

• ADK HW

I Schematics provided for the alarm clock

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• Other solutions?

I Any USB Host capable device that can provide power

to the Android device.

I This is what makes it easy to port the ADK SW to any

ARM-based board

♦ e.g. i.MX SoCs,Kinetis MCU

♦ Brings more capabilities (video control...)

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• ADK SW

I Developer controls what runs on the ADK HW and

what runs on the Android device

• Google provides sample software:

I Source code repo:

♦ android.googlesource.com/accessories/manifest

I Android device application can apply on any device

with API level > 10

♦ Android Application presents UI for control, and

communication

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http://android.googlesource.com/accessories/manifest


ADK SOFTWARE

• Accessory side:

I Full-featured:

♦ Accessory

I Basic controls (LEDs, GPIOs)

♦ Audio

♦ Bluetooth

I Code is Arduino-specific

♦ For other solutions, need for a “portable”

Accessory sample code

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ADK SOFTWARE

• Android device side:

I Hardware control

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Software Implementation

Android Accessory Protocol Software Implementation

SOFTWARE IMPLEMENTATION

1. Software on the ADK HW

I Handles communication with Android device

I Controls sensors, displays, etc. on the ADK HW

2. Software on the Android device

I Can be installed automatically via pop-up URL on

connection

I Handles communication with ADK HW

I Presents a GUI for control, data input/output from ADK

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SOFTWARE INSTALLATION

In case the software is not already installed:

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FIRST USE

Otherwise the user must authorize the execution of the

Accessory app:

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ANDROID APPLICATION

• Must discover the accessory when it is attached

• Communicate with the accessory via ADK Developer

defined commands over USB

• Utilizes the USB API in Android

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http://developer.android.com/guide/topics/connectivity/usb/accessory.html


ANDROID APPLICATION

• Android contains two different packages to support AOA

protocol:

I android.hardware.usb

♦ Works with no add-on library for Android 3.1 or

higher

I com.android.future.usb♦ From Google API add-on library for Android 2.3.4

or higher

♦ Wrapper around android.hardware.usb

♦ Better choice to support the widest range of

devices

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http://developer.android.com/reference/android/hardware/usb/package-summary.html


ANDROID APPLICATION

• First add an intent-filter to the application’s manifest

(manifest.xml)

1 <intent-filter >2 <action android:name="android.intent.action.MAIN" />3 <category android:name="android.intent.category.LAUNCHER" />4 <action

android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"/>5 </intent-filter>6 <meta-data

android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"7 android:resource="@xml/device_filter" />

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ANDROID APPLICATION

• Then define a resource file that details which USB

Accessory this Application communicates with

(accessory_filter.xml)

1 <?xml version="1.0" encoding="utf-8"?>2 <resources>3 <usb-accessory4 manufacturer="Google, Inc."5 model="DemoKit"6 version="2.0" />7 </resources>

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COMMUNICATION WITH THE ACCESSORY

• Grab the UsbAccessory handle from the Intent received

• Open up input/output file streams with the Accessory

1 if (UsbManager.ACTION_USB_ACCESSORY_ATTACHED.equals(action)) {2 UsbAccessory accessory = UsbManager.getAccessory(intent);3 if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED

, false)) {4 mAccessory = accessory;5 fd = UsbManager.getInstance(getApplicationContext()).

openAccessory(accessory).getFileDescriptor();6 mFout = new FileOutputStream(fd);7 mFin = new FileInputStream(fd);8 }9 }

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ACCESSORY APPLICATION

• Control transfers for initialization

• Bulk transfers for custom com

MCU sample code:

1 res = usbh_check_transfer(outpipe);2 if (res != PIPE_RESULT_NOT_QUEUED)3 return -1;4 usbh_queue_transfer(outpipe, buf, len);

Linux sample code:

1 ret = libusb_bulk_transfer(acc->handle,2 AOA_ACCESSORY_EP_IN, acc_buf,3 sizeof(acc_buf), &transferred,4 200);

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• Audio streaming

I Arduino ADK or MCU:

♦ DMA to DAC

♦ Simple solution

I Linux implementation:

♦ ALSA sound card

♦ Custom ISO implementation

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• HID support:

I 4-step process

♦ Register HID device

♦ Set HID report (defines capabilities)

♦ Send HID reports

♦ Unregister HID device

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• Tool to create your HID report structure:

I HID Descriptor Tool

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http://www.usb.org/developers/hidpage

Demonstrations

Android Accessory Protocol Demonstrations

HARDWARE SELECTION

• Android Device

I Stock Nexus 7 Tablet running Android KitKat (v4.4.2)

• Custom ARM-based accessory

I i.MX6 SabreLite

I GNU/Linux

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HARDWARE SELECTION

• Why were these particular platforms chosen?

I i.MX6: powerful & famous nowadays

I Show ease of using libusb-based code

I Nexus 7: stock device that needs no modification

♦ Any Android device would work the same

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DEMONSTRATION #1

• Create an accessory device that:

I Displays pictures

I Allows control of the slideshow from Android

• Shows AOA v1.0 capabilities

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DEMONSTRATION #2


I Get ADK app data

I Plays audio


I Audio streaming

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DEMONSTRATION #3


I Requires no app

I Plays audio

I Send HID events


I HID device

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MUSIC

• Moby - Innocents

I Open-source tracks

I Available on SoundCloud

I More on mobygratis

• Blend.io

I GitHub for Music

I Pull entire project

I Ableton / Logic /

GarageBand

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https://soundcloud.com/moby/sets/innocents/

http://www.mobygratis.com/

http://blend.io/

Conclusion

Android Accessory Protocol Conclusion

CONCLUSION

• Interesting standard

I Great flexibility

I Can overcome HW access issues

I Compatibility v2.0 / v1.0

• Limitations

• Code source of Linux ADK available:

https://github.com/gibsson/linux-adk

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https://github.com/gibsson/linux-adk


QUESTIONS?

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REFERENCES

• Di Cerbo, Manuel: “Turn your Linux computer into a huge

Android USB Accessory”

http://android.serverbox.ch/?p=262

• libusb: http://www.libusb.org/

• Android Developers:

http://developer.android.com/index.html

• Jesse Dannenbring: AOA presentation

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http://android.serverbox.ch/?p=262

http://www.libusb.org/

http://developer.android.com/index.html

https://speakerdeck.com/jdannenbring/ad-2002slides-dannenbring

Android Accessory Protocol - FTF2014

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