HEX data from BLE characteristic to Byte Array, how to convert to String

2
votes

My BLE android app can currently connect to my BLE hardware and connect to the GATT server. I can also enable notifications and read the characteristic. However the characteristic advertised is of HEX format. On my Service I tried receiving the data on String or Byte Array format, tried a couple of conversion procedures but still i get nonsensical data (ie. ??x etc)

Any ideas on how to receive/convert the hex data? Service:

private void broadcastUpdate(final String action,
        final BluetoothGattCharacteristic characteristic) {
    final Intent intent = new Intent(action);

    if (UUID_BLE_SHIELD_RX.equals(characteristic.getUuid())) {

        //final byte[] rx = characteristic.getValue();

        final String rx=characteristic.getStringValue(0);
        //final     char rx=raw(char.value)


        intent.putExtra(EXTRA_DATA, rx);
    }

Main Activity:

   private final BroadcastReceiver mGattUpdateReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        final String action = intent.getAction();

        if (Service.ACTION_GATT_DISCONNECTED.equals(action)) {
        } else if (Service.ACTION_GATT_SERVICES_DISCOVERED
                .equals(action)) {
                getGattService(mBluetoothLeService.getSupportedGattService());
        } else if (Service.ACTION_DATA_AVAILABLE.equals(action)) {
           displayData(intent.getExtras().getString(Service.EXTRA_DATA));
        }

Data from BLE module as seen in NRF connect: (0x)04-01-19-00-BE

2
androidhexbluetooth-lowenergybluetooth-gatt

2 Answers

0
votes

For the characteristics property, it is commonly used:

String yourHexData = theValueOfYourHexData;
yourHexData.getBytes();

This is the method I use to properly convert a hex string to a byte []:

public static byte[] hexToByteData(String hex)
{
    byte[] convertedByteArray = new byte[hex.length()/2];
    int count  = 0;

    for( int i = 0; i < hex.length() -1; i += 2 )
    {
        String output;
        output = hex.substring(i, (i + 2));
        int decimal = (int)(Integer.parseInt(output, 16));
        convertedByteArray[count] =  (byte)(decimal & 0xFF);
        count ++;
    }
    return convertedByteArray;
}

I don't know your intentions on it, but a byte array is also "no human readable". For this, it would be necessary to use a proper hex to String conversion. But, if you need a Hex to Byte Array conversion, try my method.

Hope it helps.

0
votes

In your broadcastUpdate function change it as below

if (UUID_BLE_SHIELD_RX.equals(characteristic.getUuid())) {
    final byte[] rx = characteristic.getValue();
    intent.putExtra(EXTRA_DATA, rx);
}

In MainActivity 

} else if (Service.ACTION_DATA_AVAILABLE.equals(action)) {
       displayData(intent.getByteArrayExtra(Service.EXTRA_DATA));
}

In displayData call the function to convert byteArray to hexString

public String bytesToHex(byte[] bytes) {
    try {
        String hex="";
        for (byte b : bytes) {
            String st = String.format("%02X", b);
            hex+=st;
        }
        return hex;
    } catch (Exception ex) {
        // Handler for exception
    }
}