Android USB accessory

How to leverage the accessory mode to communicate between 2 Android devices. You connect 2 devices via one USB-cable + one USB-OTG adapter and chat between devices ( or exchange any other data in a real-world application ).

The App comes in 2 flavors — one host flavor and one device flavor. You need to install one of these flavors on each device.

Install

for the host:

$> gradle installHostDebug

for the device:

$> gradle installDeviceDebug

Notes

Be aware that the direction of the cable matters even though there is micro-USB on both ends. The OTG adapter has to be on the device with the host flavor installed.

A core problem was to figure out how to get a device to switch to accessory mode — the Idea was taken from some C code inside the Android Compatibility Test Suite — this is how it looks in java here:

private boolean initAccessory(final UsbDevice device) {
        final UsbDeviceConnection connection = mUsbManager.openDevice(device);
        if (connection == null) {
            return false;
        }
        
        initStringControlTransfer(connection, 0, "quandoo"); // MANUFACTURER
        initStringControlTransfer(connection, 1, "Android2AndroidAccessory"); // MODEL
        initStringControlTransfer(connection, 2, "showcasing android2android USB communication"); // DESCRIPTION
        initStringControlTransfer(connection, 3, "0.1"); // VERSION
        initStringControlTransfer(connection, 4, "http://quandoo.de"); // URI
        initStringControlTransfer(connection, 5, "42"); // SERIAL
        connection.controlTransfer(0x40, 53, 0, 0, new byte[]{}, 0, Constants.USB_TIMEOUT_IN_MS);
        connection.close();
        return true;
    }
    private void initStringControlTransfer(final UsbDeviceConnection deviceConnection,
                                           final int index,
                                           final String string) {
        deviceConnection.controlTransfer(0x40, 52, 0, index, string.getBytes(), string.length(), Constants.USB_TIMEOUT_IN_MS);
    }

Source: https://github.com/VNAPNIC/USB-accessory-sample