Accessory Development Kit 2011 Guide

The Android Open Accessory Development Kit (ADK) is a reference implementation of an Android Open Accessory, based on the Arduino open source electronics prototyping platform. The accessory's hardware design files, code that implements the accessory's firmware, and the Android application that interacts with the accessory are provided as part of the kit to help hardware builders and software developers get started building their own accessories. The hardware design files and firmware code are contained in the ADK package download.

A limited number of kits were produced and distributed at the Google I/O 2011 developer conference. However, many hardware builders have reproduced and enhanced the original design and these boards are available for purchase. The following list of distributors are currently producing Android Open Accessory compatible development boards:

  • The Arduino Store provides the Arduino Mega ADK (for EU nations or non- EU nations) that is based on the ATmega2560 and supports the ADK firmware.
  • DIY Drones provides an Arduino-compatible board geared towards RC (radio controlled) and UAV (unmanned aerial vehicle) enthusiasts.
  • mbed provides a microcontroller and a library to develop accessories that support the Android accessory protocol. For more information, see mbed with the Android ADK.
  • Microchip provides a PIC based USB microcontroller board.
  • Modern Device provides an Arduino-compatible board that supports the ADK firmware.
  • RT Corp provides an Arduino-compatible board based on the Android ADK board design.
  • Seeed Studio provides an Arduino-compatible board that supports the ADK firmware.
  • SparkFun's IOIO board now has beta support for the ADK firmware.
  • Troido has produced an Arduino-compatible version of the ADK hardware.

We expect more hardware distributers to create a variety of kits, so please stay tuned for further developments.

ADK Components

The main hardware and software components of the ADK include:

  • A USB micro-controller board that is based on the Arduino Mega2560 and Circuits@Home USB Host Shield designs (now referred to as the ADK board), which you will later implement as an Android USB accessory. The ADK board provides input and output pins that you can implement through the use of attachments called "shields." Custom firmware, written in C++, is installed on the board to define the board's functionality and interaction with the attached shield and Android-powered device. The hardware design files for the board are located in hardware/ directory.
  • An Android Demo Shield (ADK shield) that affixes atop the ADK board implements the input and output points on the board. These implementations include a joystick, LED outputs, and temperature and light sensors. You can create or buy your own shields or wire your own features to the ADK board to implement custom functionality. The hardware design files for the shield are located in hardware/.
  • A library based on the Arduino USB Host Shield library provides the logic for the USB micro-controller board to act as a USB Host. This allows the board to initiate transactions with USB devices. Describing how to use this entire library is beyond the scope of this document. Where needed, this document points out important interactions with the library. For more information, see the source code for the Arduino USB Host Shield library in the arduino_libs/USB_Host_Shield directory.
  • An Arduino sketch, arduino_libs/AndroidAccessory/examples/demokit/demokit.pde, defines the firmware that runs on the ADK board and is written in C++. The sketch calls the Android accessory protocol library to interact with the Android-powered device. It also sends data from the ADK board and shield to the Android application and receives data from the Android application and outputs it to the ADK board and shield.
  • The Android accessory protocol library, which is located in the arduino_libs/AndroidAccessory directory. This library defines how to enumerate the bus, find a connected Android-powered device that supports accessory mode, and how to setup communication with the device.
  • Other third party libraries to support the ADK board's functionality:

Getting Started with the ADK

The following sections describe how to install the Arduino software on your computer, use the Arduino IDE to install the ADK board's firmware, and install and run the accompanying Android application for the ADK board. Before you begin, download the following items to set up your development environment:

  • Arduino 1.0 or higher: contains libraries and an IDE for coding and installing firmware to the ADK board.
  • CapSense library v.04: contains the libraries to sense human capacitance. This library is needed for the capacitive button that is located on the ADK shield.
  • ADK software package: contains the firmware for the ADK board and hardware design files for the ADK board and shield.

Installing the Arduino software and necessary libraries

To install the Arduino software:

  1. Download and install the Arduino 1.0 or higher as described on the Arduino website.

    Note: If you are on a Mac, install the FTDI USB Serial Driver that is included in the Arduino package, even though the installation instructions say otherwise.

  2. Download and extract the ADK package to a directory of your choice. You should have an app, arduino_libs, and hardware directories.
  3. Download and extract the CapSense package to a directory of your choice.
  4. Install the necessary libraries:

    On Windows:

    1. Copy the arduino_libs/AndroidAccessory and arduino_libs/USB_Host_Shield directories (the complete directories, not just the files within) to the <arduino_installation_root>/libraries/ directory.
    2. Copy the extracted CapSense/ library directory and its contents to the <arduino_installation_root>/libraries/ directory.

    On Mac:

    1. Create, if it does not already exist, an Arduino directory inside your user account's Documents directory, and within that, a libraries directory.
    2. Copy the arduino_libs/AndroidAccessory and arduino_libs/USB_Host_Shield directories (the complete directories, not just the files within) to your Documents/Arduino/libraries/ directory.
    3. Copy the extracted CapSense/ library directory and its contents to the Documents/Arduino/libraries/ directory.

    On Linux (Ubuntu):

    1. Copy the firmware/arduino_libs/AndroidAccessory and firmware/arduino_libs/USB_Host_Shield directories (the complete directories, not just the files within) to the <arduino_installation_root>/libraries/ directory.
    2. Copy the extracted CapSense/ library directory and its contents to the <arduino_installation_root>/libraries/ directory.
    3. Install the avr-libc library by entering sudo apt-get install avr-libc from a shell prompt.

You should now have three new directories in the Arduino libraries/ directory: AndroidAccessory, USB_Host_Shield, and CapSense.

Installing the firmware to the ADK board

To install the firmware to the ADK board:

  1. Connect the ADK board to your computer using the micro-USB port, which allows two-way communication and provides power to the ADK board.
  2. Launch the Arduino IDE.
  3. Click Tools > Board > Arduino Mega 2560 to specify the ADK board's type.
  4. Select the appropriate USB port:
    • On Windows: click Tools > Serial Port > COM# to specify the port of communication. The COM port number varies depending on your computer. COM1 is usually reserved for serial port connections. You most likely want COM2 or COM3.
    • On Mac: Click Tools > Serial Port > dev/tty.usbserial-### to specify the port of communication.
    • On Linux (Ubuntu): Click Tools > Serial Port > dev/ttyUSB# to specify the port of communication.
  5. To open the Demokit sketch (firmware code), click File > Examples > AndroidAccessory > demokit.
  6. Click Sketch > Verify/Compile to ensure that the sketch has no errors.
  7. Select File > Upload. When Arduino outputs Done uploading., the board is ready to communicate with your Android-powered device.

Running the DemoKit Android application

The DemoKit Android application runs on your Android-powered device and communicates with the ADK board. The ADK board receives commands such as lighting up the board's LEDs or sends data from the board such as joystick movement and temperature readings.

To install and run the application in Eclipse:

  1. Install the Google APIs API Level 10 add-on library, which includes the Open Accessory library for 2.3.4 devices that support accessory mode. This library is also forward compatible with Android 3.1 or newer devices that support accessory mode. If you only care about Android 3.1 or newer devices, all you need is API Level 12. For more information on deciding which API level to use, see the USB Accessory documentation.
  2. Click File > New > Project..., then select Android > Android Project
  3. In the Project name: field, type DemoKit.
  4. Choose Create project from existing source, click Browse, select the app directory, click Open to close that dialog and then click Finish.
  5. For Build Target, select Google APIs (Platform 2.3.3, API Level 10).

    Note: Even though the add-on is labeled as 2.3.3, the newest Google API add-on library for API level 10 adds USB Open Accessory API support for 2.3.4 devices.

  6. Click Finish.
  7. Install the application to your device.
  8. Connect the ADK board (USB-A) to your Android-powered device (micro-USB). Ensure that the power cable to the accessory is plugged in or that the micro-USB port on the accessory is connected to your computer for power (this also allows you to monitor the ADK board). When connected, accept the prompt that asks for whether or not to open the DemoKit application to connect to the accessory. If the prompt does not show up, connect and reconnect the accessory.

You can now interact with the ADK board by moving the color LED or servo sliders (make sure the servos are connected) or by pressing the relay buttons in the application. On the ADK shield, you can press the buttons and move the joystick to see their outputs displayed in the application.

Monitoring the ADK Board

The ADK firmware consists of a few files that you should be looking at if you want to build your own accessory. The files in the arduino_libs/AndroidAccessory directory are the most important files and have the logic to detect and connect to Android-powered devices that support accessory mode. Feel free to add debug statements (Arduino Serial.println() statements) to the code located in the <arduino_installation_root>/libraries/AndroidAccessory directory and demokit.pde sketch and re-upload the sketch to the ADK board to discover more about how the firmware works.

You can view the debug statements in the Arduino Serial Monitor by clicking Tools > Serial Monitor and setting the baud to 115200. The following sections about how accessories communicate with Android-powered devices describe much of what you should be doing in your own accessory.

How the ADK board implements the Android Accessory protocol

If you have access to the ADK board and shield, the following sections describe the firmware code that you installed onto the ADK board. The firmware demonstrates a practical example of how to implement the Android Accessory protocol. Even if you do not have the ADK board and shield, reading through how the hardware detects and interacts with devices in accessory mode is still useful if you want to port the code over for your own accessories.

The important pieces of the firmware are the arduino_libs/AndroidAccessory/examples/demokit/demokit/demokit.pde sketch, which is the code that receives and sends data to the DemoKit application running on the Android-powered device. The code to detect and set up communication with the Android-powered device is contained in the arduino_libs/AndroidAccessory/AndroidAccessory.h and arduino_libs/AndroidAccessory/AndroidAccessory.cpp files. This code includes most of the logic that will help you implement your own accessory's firmware. It might be useful to have all three of these files open in a text editor as you read through these next sections.

The following sections describe the firmware code in the context of the algorithm described in Implementing the Android Accessory Protocol.

Wait for and detect connected devices

In the firmware code (demokit.pde), the loop() function runs repeatedly and calls AndroidAccessory::isConnected() to check for any connected devices. If there is a connected device, it continuously updates the input and output streams going to and from the board and application. If nothing is connected, it continuously checks for a device to be connected:

...

AndroidAccessory acc("Google, Inc.",
                     "DemoKit",
                     "DemoKit Arduino Board",
                     "1.0",
                     "http://www.android.com",
                     "0000000012345678");

...
void loop()
{
...
    if (acc.isConnected()) {
        //communicate with Android application
    }
    else{
        //set the accessory to its default state
    }
...
}

Determine the connected device's accessory mode support

When a device is connected to the ADK board, it can already be in accessory mode, support accessory mode and is not in that mode, or does not support accessory mode. The AndroidAccessory::isConnected() method checks for these cases and responds accordingly when the loop() function calls it. This function first checks to see if the device that is connected hasn't already been handled. If not, it gets the connected device's device descriptor to figure out if the device is already in accessory mode by calling AndroidAccessory::isAccessoryDevice(). This method checks the vendor and product ID of the device descriptor. A device in accessory mode has a vendor ID of 0x18D1 and a product ID of 0x2D00 or 0x2D01. If the device is in accessory mode, then the ADK board can establish communication with the device. If not, the board attempts to start the device in accessory mode.

bool AndroidAccessory::isConnected(void)
{
    USB_DEVICE_DESCRIPTOR *devDesc = (USB_DEVICE_DESCRIPTOR *) descBuff;
    byte err;

    max.Task();
    usb.Task();

    if (!connected &&
        usb.getUsbTaskState() >= USB_STATE_CONFIGURING &&
        usb.getUsbTaskState() != USB_STATE_RUNNING) {
        Serial.print("\nDevice addressed... ");
        Serial.print("Requesting device descriptor.");

        err = usb.getDevDescr(1, 0, 0x12, (char *) devDesc);
        if (err) {
            Serial.print("\nDevice descriptor cannot be retrieved. Program Halted\n");
            while(1);
        }

        if (isAccessoryDevice(devDesc)) {
            Serial.print("found android accessory device\n");

            connected = configureAndroid();
        } else {
            Serial.print("found possible device. switching to serial mode\n");
            switchDevice(1);
        }
    } else if (usb.getUsbTaskState() == USB_DETACHED_SUBSTATE_WAIT_FOR_DEVICE) {
        connected = false;
    }

    return connected;
}

Attempt to start the device in accessory mode

If the device is not already in accessory mode, then the ADK board must determine whether or not it supports it by sending control request 51 to check the version of the USB accessory protocol that the device supports (see AndroidAccessory::getProtocol()). Protocol version 1 is supported by Android 2.3.4 (API Level 10) and higher. Protocol version 2 is supported by Android 4.1 (API Level 16) and higher. Versions greater than 2 may supported in the future. If the appropriate protocol version is returned, the board sends control request 52 (one for each string with AndroidAcessory:sendString()) to send it's identifying information, and tries to start the device in accessory mode with control request 53. The AndroidAccessory::switchDevice() method takes care of this:

bool AndroidAccessory::switchDevice(byte addr)
{
    int protocol = getProtocol(addr);
    if (protocol >= 1) {
        Serial.print("device supports protocol 1\n");
    } else {
        Serial.print("could not read device protocol version\n");
        return false;
    }

    sendString(addr, ACCESSORY_STRING_MANUFACTURER, manufacturer);
    sendString(addr, ACCESSORY_STRING_MODEL, model);
    sendString(addr, ACCESSORY_STRING_DESCRIPTION, description);
    sendString(addr, ACCESSORY_STRING_VERSION, version);
    sendString(addr, ACCESSORY_STRING_URI, uri);
    sendString(addr, ACCESSORY_STRING_SERIAL, serial);

    usb.ctrlReq(addr, 0, USB_SETUP_HOST_TO_DEVICE | USB_SETUP_TYPE_VENDOR | USB_SETUP_RECIPIENT_DEVICE,
                ACCESSORY_START, 0, 0, 0, 0, NULL);
    return true;
}
If this method returns false, the board waits until a new device is connected. If it is successful, the device displays itself on the USB bus as being in accessory mode when the ADK board re-enumerates the bus. When the device is in accessory mode, the accessory then establishes communication with the device.

Establish communication with the device

If a device is detected as being in accessory mode, the accessory must find the proper bulk endpoints and set up communication with the device. When the ADK board detects an Android-powered device in accessory mode, it calls the AndroidAccessory::configureAndroid() function:

...
if (isAccessoryDevice(devDesc)) {
            Serial.print("found android acessory device\n");

            connected = configureAndroid();
        }
...

which in turn calls the findEndpoints() function:

...
bool AndroidAccessory::configureAndroid(void)
{
    byte err;
    EP_RECORD inEp, outEp;

    if (!findEndpoints(1, &inEp, &outEp))
        return false;
...

The AndroidAccessory::findEndpoints() function queries the Android-powered device's configuration descriptor and finds the bulk data endpoints in which to communicate with the USB device. To do this, it first gets the device's first four bytes of the configuration descriptor (only need descBuff[2] and descBuff[3]), which contains the information about the total length of data returned by getting the descriptor. This data is used to determine whether or not the descriptor can fit in the descriptor buffer. This descriptor also contains information about all the interfaces and endpoint descriptors. If the descriptor is of appropriate size, the method reads the entire configuration descriptor and fills the entire descriptor buffer with this device's configuration descriptor. If for some reason the descriptor is no longer attainable, an error is returned.

...

bool AndroidAccessory::findEndpoints(byte addr, EP_RECORD *inEp, EP_RECORD *outEp)
{
    int len;
    byte err;
    uint8_t *p;

    err = usb.getConfDescr(addr, 0, 4, 0, (char *)descBuff);
    if (err) {
        Serial.print("Can't get config descriptor length\n");
        return false;
    }


    len = descBuff[2] | ((int)descBuff[3] << 8);
    if (len > sizeof(descBuff)) {
        Serial.print("config descriptor too large\n");
            /* might want to truncate here */
        return false;
    }

    err = usb.getConfDescr(addr, 0, len, 0, (char *)descBuff);
    if (err) {
        Serial.print("Can't get config descriptor\n");
        return false;
    }

...

Once the descriptor is in memory, a pointer is assigned to the first position of the buffer and is used to index the buffer for reading. There are two endpoint pointers (input and output) that are passed into AndroidAccessory::findEndpoints() and their addresses are set to 0, because the code hasn't found any suitable bulk endpoints yet. A loop reads the buffer, parsing each configuration, interface, or endpoint descriptor. For each descriptor, Position 0 always contains the size of the descriptor in bytes and position 1 always contains the descriptor type. Using these two values, the loop skips any configuration and interface descriptors and increments the buffer with the descLen variable to get to the next descriptor.

Note: An Android-powered device in accessory mode can potentially have two interfaces, one for the default communication to the device and the other for ADB communication. The default communication interface is always indexed first, so finding the first input and output bulk endpoints will return the default communication endpoints, which is what the demokit.pde sketch does. If you are writing your own firmware, the logic to find the appropriate endpoints for your accessory might be different.

When it finds the first input and output endpoint descriptors, it sets the endpoint pointers to those addresses. If the findEndpoints() function finds both an input and output endpoint, it returns true. It ignores any other endpoints that it finds (the endpoints for the ADB interface, if present).

...
    p = descBuff;
    inEp->epAddr = 0;
    outEp->epAddr = 0;
    while (p < (descBuff + len)){
        uint8_t descLen = p[0];
        uint8_t descType = p[1];
        USB_ENDPOINT_DESCRIPTOR *epDesc;
        EP_RECORD *ep;

        switch (descType) {
        case USB_DESCRIPTOR_CONFIGURATION:
            Serial.print("config desc\n");
            break;

        case USB_DESCRIPTOR_INTERFACE:
            Serial.print("interface desc\n");
            break;

        case USB_DESCRIPTOR_ENDPOINT:
            epDesc = (USB_ENDPOINT_DESCRIPTOR *)p;
            if (!inEp->epAddr && (epDesc->bEndpointAddress & 0x80))
                ep = inEp;
            else if (!outEp->epAddr)
                ep = outEp;
            else
                ep = NULL;

            if (ep) {
                ep->epAddr = epDesc->bEndpointAddress & 0x7f;
                ep->Attr = epDesc->bmAttributes;
                ep->MaxPktSize = epDesc->wMaxPacketSize;
                ep->sndToggle = bmSNDTOG0;
                ep->rcvToggle = bmRCVTOG0;
            }
            break;

        default:
            Serial.print("unkown desc type ");
            Serial.println( descType, HEX);
            break;
        }

        p += descLen;
    }

    if (!(inEp->epAddr && outEp->epAddr))
        Serial.println("can't find accessory endpoints");

    return inEp->epAddr && outEp->epAddr;
}

...

Back in the configureAndroid() function, if there were endpoints found, they are appropriately set up for communication. The device's configuration is set to 1 and the state of the device is set to "running", which signifies that the device is properly set up to communicate with your USB accessory. Setting this status prevents the device from being re-detected and re-configured in the AndroidAccessory::isConnected() function.

bool AndroidAccessory::configureAndroid(void)
{
    byte err;
    EP_RECORD inEp, outEp;

    if (!findEndpoints(1, &inEp, &outEp))
        return false;

    memset(&epRecord, 0x0, sizeof(epRecord));

    epRecord[inEp.epAddr] = inEp;
    if (outEp.epAddr != inEp.epAddr)
        epRecord[outEp.epAddr] = outEp;

    in = inEp.epAddr;
    out = outEp.epAddr;

    Serial.print("inEp: ");
    Serial.println(inEp.epAddr, HEX);
    Serial.print("outEp: ");
    Serial.println(outEp.epAddr, HEX);

    epRecord[0] = *(usb.getDevTableEntry(0,0));
    usb.setDevTableEntry(1, epRecord);

    err = usb.setConf( 1, 0, 1 );
    if (err) {
        Serial.print("Can't set config to 1\n");
        return false;
    }

    usb.setUsbTaskState( USB_STATE_RUNNING );

    return true;
}

Lastly, methods to read and write to the appropriate endpoints are needed. The demokit.pde sketch calls these methods depending on the data that is read from the Android-powered device or sent by the ADK board. For instance, moving the joystick on the ADK shield writes data that is read by the DemoKit application running on the Android-powered device. Moving sliders on the DemoKit application is read by the demokit.pde sketch and changes the state of the accessory, such as lighting up or changing the color of the LED lights.

int AndroidAccessory::read(void *buff, int len, unsigned int nakLimit) {
  return usb.newInTransfer(1, in, len, (char *)buff, nakLimit); }

int AndroidAccessory::write(void *buff, int len) {
  usb.outTransfer(1, out, len, (char *)buff);
  return len; }

See the demokit.pde sketch for information about how the ADK board reads and writes data.