How to communicate with a USB device?

Question

I am trying to establish USB communication as host. I am following the examples here http://developer.android.com/guide/topics/usb/host.html but I cannot get this working. Here is my code: ``` private static final String ACTION_USB_PERMISSION = "com.multitools.andres.LCView"; UsbDevice device; //Pide permisos al usuario para comunicacion con el dispositivo USB private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() { @Override public void onReceive(Context context, Intent intent) { String action = intent.getAction(); if (ACTION_USB_PERMISSION.equals(action)) { synchronized (this) { UsbDevice device = (UsbDevice)intent.getParcelableExtra(UsbManager.EXTRA_DEVICE); if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) { if(device != null){ //call method to set up device communication } } else { Log.d(TAG, "permission denied for device " + device); } } } } }; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); if…

Accepted Answer

That string is just a marker so you recognize the intent that is returned when you call `registerReceiver(mUsbReceiver, filter);`. That isn't the problem. I think the problem is here: ``` UsbManager mUsbManager = (UsbManager) getSystemService(Context.USB_SERVICE); ``` This line is supposed to get you your USB Manager, but as far as I recall not all phones have to support the Android USB Accessory API. This line is probably just assigning `null` to `mUsbManager` which then causes you `NullPointerException` when you call a method on it further down in the code. Trying checking if that isn't `null` before making a call on it. For more info, checkout these links: - http://developer.android.com/guide/topics/usb/index.html - http://developer.android.com/guide/topics/usb/accessory.html EDIT: I think I see what the issue is now. You're right, it's not the USB Manager. It's the UsbDevice object (`device`). It is never initialized anywhere in your code. In this line : ``` mUsbManager.requestPermission(device, mPermissionIntent); ``` you're basically firing off an intent to ask the user if it's okay for you to work with the device represented by the UsbDevice object `device`. However when this call gets fired `device` has not yet been initialized (and therefore has the default value of `null`). So when you try to request permission you end up getting a `NullPointerException` instead of the expected result. To fix this you need to figure out which device you want to connect to and assign it to `device`. Look in here, under "Enumerating devices" to figure out different ways to do that. One way, if you know the name of the device you want to connect to, is to make the following calls after you retrieve the Usb Manager: ``` HashMap deviceList = mUsbManager.getDeviceList(); device = deviceList.get(""); ``` (Obviously, you would replace `` with the actual name of the device.

How to communicate with a USB device?

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