Android和蓝牙低功耗（BLE）

Question

I'm struggling to read/send data from/to a custom bluetooth low energy device. I am using a Microchip RN4020 bluetooth module.

Every tutorial or example I found explained how to connect your mobile application to the device but I didn't find how to actually interact with it. As a result that is exactly what I can and can't do with my application.

for every characteristic present I did the following:

gatt.readCharacteristic(services.get(i).getCharacteristics().get(j));

this was the result when I printed the values:

getStringValue= Mobi_F934��
getStringValue= null
getStringValue= ����d���

getStringValue= 001EC030F934
getStringValue= 2.1
getStringValue= 1.10
getStringValue= 1.10
getStringValue= Microchip
getStringValue= RN4020

getStringValue= 
getStringValue=

I don't know if this is important or not...

How can I read the data send from my BLE device?

EDIT: My android code

    @TargetApi(21)
public class MainActivity extends ActionBarActivity {
    private BluetoothAdapter mBluetoothAdapter;
    private int REQUEST_ENABLE_BT = 1;
    private Handler mHandler;
    private static final long SCAN_PERIOD = 10000;
    private BluetoothLeScanner mLEScanner;
    private ScanSettings settings;
    private List<ScanFilter> filters;
    private BluetoothGatt mGatt;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mHandler = new Handler();
        if (!getPackageManager().hasSystemFeature(PackageManager.FEATURE_BLUETOOTH_LE)) {
            Toast.makeText(this, "BLE Not Supported",
                    Toast.LENGTH_SHORT).show();
            finish();
        }

        // Initializes Bluetooth adapter
        final BluetoothManager bluetoothManager =
                (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        mBluetoothAdapter = bluetoothManager.getAdapter();

    }

    @Override
    protected void onResume() {
        super.onResume();
        // Ensures Bluetooth is available on the device and it is enabled. If not,
        // displays a dialog requesting user permission to enable Bluetooth.

        if (mBluetoothAdapter == null || !mBluetoothAdapter.isEnabled()) {
            Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
        } else {
            if (Build.VERSION.SDK_INT >= 21) {
                mLEScanner = mBluetoothAdapter.getBluetoothLeScanner();
                settings = new ScanSettings.Builder()
                        .setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
                        .build();
                filters = new ArrayList<ScanFilter>();
            }
            scanLeDevice(true);
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        if (mBluetoothAdapter != null && mBluetoothAdapter.isEnabled()) {
            scanLeDevice(false);
        }
    }

    @Override
    protected void onDestroy() {
        if (mGatt == null) {
            return;
        }
        mGatt.close();
        mGatt = null;
        super.onDestroy();
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_ENABLE_BT) {
            if (resultCode == Activity.RESULT_CANCELED) {
                //Bluetooth not enabled.
                finish();
                return;
            }
        }
        super.onActivityResult(requestCode, resultCode, data);
    }

    private void scanLeDevice(final boolean enable) {
        if (enable) {
            mHandler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    if (Build.VERSION.SDK_INT < 21) {
                        mBluetoothAdapter.stopLeScan(mLeScanCallback);
                    } else {
                        mLEScanner.stopScan(mScanCallback);

                    }
                }
            }, SCAN_PERIOD);
            if (Build.VERSION.SDK_INT < 21) {
                mBluetoothAdapter.startLeScan(mLeScanCallback);
            } else {
                mLEScanner.startScan(filters, settings, mScanCallback);
            }
        } else {
            if (Build.VERSION.SDK_INT < 21) {
                mBluetoothAdapter.stopLeScan(mLeScanCallback);
            } else {
                mLEScanner.stopScan(mScanCallback);
            }
        }
    }


    private ScanCallback mScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            Log.e("callbackType", "MAX " + String.valueOf(callbackType));
            Log.e("result", "MAX " + result.toString());
            BluetoothDevice btDevice = result.getDevice();
            connectToDevice(btDevice);
        }

        @Override
        public void onBatchScanResults(List<ScanResult> results) {
            for (ScanResult sr : results) {
                Log.e("ScanResult - Results", "MAX " + sr.toString());
            }
        }

        @Override
        public void onScanFailed(int errorCode) {
            Log.e("Scan Failed", "MAX " + "Error Code: " + errorCode);
        }
    };

    private BluetoothAdapter.LeScanCallback mLeScanCallback =
            new BluetoothAdapter.LeScanCallback() {
                @Override
                public void onLeScan(final BluetoothDevice device, int rssi,
                                     byte[] scanRecord) {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Log.e("onLeScan", "MAX " + device.toString());
                            connectToDevice(device);
                        }
                    });
                }
            };

    public void connectToDevice(BluetoothDevice device) {
        if (mGatt == null) {
            mGatt = device.connectGatt(this, false, gattCallback);
            scanLeDevice(false);// will stop after first device detection
        }
    }

    private final BluetoothGattCallback gattCallback = new BluetoothGattCallback() {
        @Override
        public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
            Log.e("onConnectionStateChange", "MAX " + "Status: " + status);
            switch (newState) {
                case BluetoothProfile.STATE_CONNECTED:
                    Log.e("gattCallback", "MAX " + "STATE_CONNECTED");
                    gatt.discoverServices();
                    break;
                case BluetoothProfile.STATE_DISCONNECTED:
                    Log.e("gattCallback", "MAX " + "STATE_DISCONNECTED");
                    break;
                default:
                    Log.e("gattCallback", "MAX " + "STATE_OTHER");
            }

        }

        @Override
        public void onServicesDiscovered(BluetoothGatt gatt, int status) {
            List<BluetoothGattService> services = gatt.getServices();
            Log.e("onServicesDiscovered", "MAX " + services.toString());

            /*
            for(int i = 0; i < services.size(); i++){
                for (int j = 0; j < services.get(i).getCharacteristics().size(); j++){
                    Log.e("MAX", "i= " + i + " j= " + j);
                    gatt.readCharacteristic(services.get(i).getCharacteristics().get
                            (j));
                }
            }
            */

            gatt.readCharacteristic(services.get(0).getCharacteristics().get(0));
        }

        @Override
        public void onCharacteristicRead(BluetoothGatt gatt,
                                         BluetoothGattCharacteristic
                                                 characteristic, int status) {
            Log.e("onCharacteristicRead", "MAX " + characteristic.toString());
            Log.e("onCharacteristicRead", "MAX " +characteristic.getStringValue(0));
            //gatt.disconnect();
        }
    };
    }

EDIT 2: My arduino code

#include <SoftwareSerial.h>

/* Set up BT to arduino uno
BT RN4020    Arduino Uno
------------------------------------------------
Red          3.3V
Green        GND
Yellow(RX)   TX (digital pin 1)
Orange(TX)   RX (digital pin 0)
White        PIN 8 (!) This is not GND (!)
Blue         3.3V (Ioref can also be used)
*/

/* 
-----------------------------------------------------------------------------
-----EXPLANATION PROGRAM-----------------------------------------------------
----------------------------
Eerst wordt de BT module geïnitialiseerd door verschillende commando's door te
sturen. Bij elk commando wordt gecheckt of de BT module het juiste antwoord
heeft teruggestuurd.
De initialisatie moet gebeuren in CMD mode, de CMD/MLDP pin moet laag zijn. Na
de initialisatie checken we tot de BT module is geconnecteerd met het device.
Als de BT module geconnecteerd is zetten we deze uit CMD mode en in MLDP mode 
om het verzenden en ontvangen van data te ondersteunen. Daarna zenden we om de 
seconde de string 'test' door.
Dit gebeurt zolang het device geconnecteerd is met de BT module.
------------------------------------------------------------------------------
------------------------------------------------------------------------------
-----------------------------
*/

int CMD_MLD_pin = 8; // IO pin for CMD/MLDP pin (LOW --> CMD, HIGH --> MLDP)

void setup() // Initialising BT RN4020
{
  pinMode(CMD_MLD_pin, OUTPUT); 
  digitalWrite(CMD_MLD_pin, LOW); // put CMD/MLDP pin LOW for CMD mode
  delay(5000);
  /* Initialize Bluetooth */
  Serial.begin(115200); // Set up serial connection with required baud rate of 
                        // 115200
  delay(1500);
  Serial.println("R,1"); //Reboot
  while(!Serial.find("Reboot")){} // Waiting until BT RN4020 responds the 
  required string
  while(!Serial.find("CMD")){}
  //delay(2000);
  Serial.println("SF,1"); // Factory reset
  while(!Serial.find("AOK")){}
  //delay(1500);
  Serial.println("SR,32000000"); // Set device as peripheral
  while(!Serial.find("AOK")){}
  Serial.println("S-, Mobi"); // Change name of BT
  while(!Serial.find("AOK")){}
  //delay(1500);
  Serial.println("R,1"); // Reboot to make changes effective
  while(!Serial.find("Reboot")){}
  while(!Serial.find("CMD")){}
  //delay(2000);
}

void loop()
{
  // Wait until the device responds it is connected
  // If the device is connected we will send every second the string 'test'
  while(!Serial.find("Connected")){} // Wait until BT RN4020 is connected to a 
                                     // device
  digitalWrite(CMD_MLD_pin, HIGH); // Go to MLDP to support data stream
  while(!Serial.find("MLDP")){} // Wait until BT RN4020 is effectively in MLDP 
                                // mode
  delay(500);
  while (!Serial.find("Connection End")) // Send string to BTRN4020 while the 
                                         // BT RN4020 is conected to a device
  {
    Serial.println("test");
    delay(1000);
  }  
}

Answer 1

I managed to solve my problem a few months after asking the question. I forgot to share my solution back then. Anyhow, better late than never!

This is some code I wrote while testing the android - TI sensortag communication over BLE.

A full example is available at https://github.com/maxhelskens/SmergyBicycle

package com.example.max.smergy;

import android.annotation.TargetApi;
import android.app.Activity;
import android.app.ProgressDialog;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothGatt;
import android.bluetooth.BluetoothGattCallback;
import android.bluetooth.BluetoothGattCharacteristic;
import android.bluetooth.BluetoothGattDescriptor;
import android.bluetooth.BluetoothGattService;
import android.bluetooth.BluetoothManager;
import android.bluetooth.BluetoothProfile;
import android.bluetooth.le.BluetoothLeScanner;
import android.bluetooth.le.ScanCallback;
import android.bluetooth.le.ScanFilter;
import android.bluetooth.le.ScanResult;
import android.bluetooth.le.ScanSettings;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.util.SparseArray;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

@TargetApi(21)
public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private static final String DEVICE_NAME = "SensorTag";

    /* Humidity Service */
    private static final UUID HUMIDITY_SERVICE = UUID.fromString("F000AA20-0451-4000-B000-000000000000");
    private static final UUID HUMIDITY_DATA_CHAR = UUID.fromString("f000aa21-0451-4000-b000-000000000000");
    private static final UUID HUMIDITY_CONFIG_CHAR = UUID.fromString("f000aa22-0451-4000-b000-000000000000");
    /* magneto Service */
    private static final UUID MAGNETO_SERVICE = UUID.fromString("F000AA80-0451-4000-B000-000000000000");
    private static final UUID MAGNETO_DATA_CHAR = UUID.fromString("f000aa81-0451-4000-b000-000000000000");
    private static final UUID MAGNETO_CONFIG_CHAR = UUID.fromString("f000aa82-0451-4000-b000-000000000000");
    /* Barometric Pressure Service */
    private static final UUID PRESSURE_SERVICE = UUID.fromString("f000aa40-0451-4000-b000-000000000000");
    private static final UUID PRESSURE_DATA_CHAR = UUID.fromString("f000aa41-0451-4000-b000-000000000000");
    private static final UUID PRESSURE_CONFIG_CHAR = UUID.fromString("f000aa42-0451-4000-b000-000000000000");
    private static final UUID PRESSURE_CAL_CHAR = UUID.fromString("f000aa43-0451-4000-b000-000000000000");
    /* Client Configuration Descriptor */
    private static final UUID CONFIG_DESCRIPTOR = UUID.fromString("00002902-0000-1000-8000-00805f9b34fb");


    private BluetoothAdapter mBluetoothAdapter;
    private SparseArray<BluetoothDevice> mDevices;

    private BluetoothGatt mGatt;

    private TextView mTemperature, mHumidity, mPressure;

    private ProgressDialog mProgress;

    private int REQUEST_ENABLE_BT = 1;
    private BluetoothLeScanner mLEScanner;
    private ScanSettings settings;
    private List<ScanFilter> filters;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        setProgressBarIndeterminate(true);

        // Find the toolbar view inside the activity layout
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        // Sets the Toolbar to act as the ActionBar for this Activity window.
        // Make sure the toolbar exists in the activity and is not null
        setSupportActionBar(toolbar);


        /*
        * We are going to display the results in some text fields
        */
        mTemperature = (TextView) findViewById(R.id.text_temperature);
        mHumidity = (TextView) findViewById(R.id.text_humidity);
        mPressure = (TextView) findViewById(R.id.text_pressure);

        //Check if BLE is supported
        if (!getPackageManager().hasSystemFeature(PackageManager.FEATURE_BLUETOOTH_LE)) {
            Toast.makeText(this, "BLE Not Supported",
                    Toast.LENGTH_SHORT).show();
            finish();
        }
        final BluetoothManager bluetoothManager =
                (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        mBluetoothAdapter = bluetoothManager.getAdapter();

        mDevices = new SparseArray<BluetoothDevice>();


        /*
        * A progress dialog will be needed while the connection process is
        * taking place
        */
        mProgress = new ProgressDialog(this);
        mProgress.setIndeterminate(true);
        mProgress.setCancelable(false);

    }

    @Override
    protected void onResume() {
        super.onResume();
        if (mBluetoothAdapter == null || !mBluetoothAdapter.isEnabled()) {
            Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
        } else {
            if (Build.VERSION.SDK_INT >= 21) {
                mLEScanner = mBluetoothAdapter.getBluetoothLeScanner();
                settings = new ScanSettings.Builder()
                        .setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
                        .build();
                filters = new ArrayList<ScanFilter>();
            }
        }

        clearDisplayValues();
    }

    @Override
    protected void onPause() {
        super.onPause();

        //Make sure dialog is hidden
        mProgress.dismiss();
        //Cancel any scans in progress
        mHandler.removeCallbacks(mStopRunnable);
        mHandler.removeCallbacks(mStartRunnable);

        if (Build.VERSION.SDK_INT < 21) {
            mBluetoothAdapter.stopLeScan(mLeScanCallback);
        } else {
            mLEScanner.stopScan(mScanCallback);

        }
    }

    @Override
    protected void onStop() {
        super.onStop();
        //Disconnect from any active tag connection
        if (mGatt != null) {
            mGatt.close();
            mGatt = null;
        }
    }

    @Override
    protected void onDestroy() {
        if (mGatt == null) {
            return;
        }
        mGatt.close();
        mGatt = null;
        super.onDestroy();
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_ENABLE_BT) {
            if (resultCode == Activity.RESULT_CANCELED) {
                //Bluetooth not enabled.
                finish();
                return;
            }
        }
        super.onActivityResult(requestCode, resultCode, data);
    }

    // Menu icons are inflated just as they were with actionbar
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);

        //Add any device elements we've discovered to the overflow menu
        for (int i=0; i < mDevices.size(); i++) {
            BluetoothDevice device = mDevices.valueAt(i);
            menu.add(0, mDevices.keyAt(i), 0, device.getName());
        }

        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.bluetoothSearch:
                mDevices.clear();
                startScan();
                //scanLeDevice(true);

                return true;
            default:
                //Obtain the discovered device to connect with
                BluetoothDevice device = mDevices.get(item.getItemId());
                Log.i(TAG, "Connecting to " + device.getName());
                /*
                * Make a connection with the device
                */

                connectToDevice(device);
                //Display progress UI
                mHandler.sendMessage(Message.obtain(null, MSG_PROGRESS, "Connecting to " + device.getName() + "..."));
                return super.onOptionsItemSelected(item);
        }
    }

    private void clearDisplayValues() {
        mTemperature.setText("---");
        mHumidity.setText("---");
        mPressure.setText("---");
    }

    private Runnable mStopRunnable = new Runnable() {
        @Override
        public void run() {
            stopScan();
        }
    };
    private Runnable mStartRunnable = new Runnable() {
        @Override
        public void run() {
            startScan();
        }
    };

    private void startScan() {
        if (Build.VERSION.SDK_INT < 21) {
            mBluetoothAdapter.startLeScan(mLeScanCallback);
        } else {
            mLEScanner.startScan(filters, settings, mScanCallback);
        }
        setProgressBarIndeterminateVisibility(true);

        mHandler.postDelayed(mStopRunnable, 2500);
    }

    private void stopScan() {
        if (Build.VERSION.SDK_INT < 21) {
            mBluetoothAdapter.stopLeScan(mLeScanCallback);
        } else {
            mLEScanner.stopScan(mScanCallback);

        }
        setProgressBarIndeterminateVisibility(false);
    }

    private BluetoothAdapter.LeScanCallback mLeScanCallback =
            new BluetoothAdapter.LeScanCallback() {
                @Override
                public void onLeScan(final BluetoothDevice device, int rssi, byte[] scanRecord) {
                    Log.i("onLeScan", device.toString());

                    mDevices.put(device.hashCode(), device);
                    //Update the overflow menu
                    invalidateOptionsMenu();
                }
            };

/*

    private void scanLeDevice(final boolean enable) {
        if (enable) {
            mHandler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    if (Build.VERSION.SDK_INT < 21) {
                        mBluetoothAdapter.stopLeScan(mLeScanCallback);
                    } else {
                        mLEScanner.stopScan(mScanCallback);

                    }
                }
            }, SCAN_PERIOD);
            if (Build.VERSION.SDK_INT < 21) {
                mBluetoothAdapter.startLeScan(mLeScanCallback);
            } else {
                mLEScanner.startScan(filters, settings, mScanCallback);
            }
        } else {
            if (Build.VERSION.SDK_INT < 21) {
                mBluetoothAdapter.stopLeScan(mLeScanCallback);
            } else {
                mLEScanner.stopScan(mScanCallback);
            }
        }
    }

*/

    private ScanCallback mScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            Log.i("callbackType", String.valueOf(callbackType));
            Log.i("result", result.toString());
            BluetoothDevice btDevice = result.getDevice();

            mDevices.put(btDevice.hashCode(), btDevice);
            //Update the overflow menu
            invalidateOptionsMenu();
        }

        @Override
        public void onBatchScanResults(List<ScanResult> results) {
            for (ScanResult sr : results) {
                Log.i("ScanResult - Results", sr.toString());
            }
        }

        @Override
        public void onScanFailed(int errorCode) {
            Log.e("Scan Failed", "Error Code: " + errorCode);
        }
    };


    public void connectToDevice(BluetoothDevice device) {
        if (mGatt == null) {
            mGatt = device.connectGatt(this, false, gattCallback);
            stopScan();
        }
    }


    /*
    * In this callback, we've created a bit of a state machine to enforce that only
    * one characteristic be read or written at a time until all of our sensors
    * are enabled and we are registered to get notifications.
    */
    private final BluetoothGattCallback gattCallback = new BluetoothGattCallback() {
        /* State Machine Tracking */
        private int mState = 0;

        private void reset() { mState = 0; }

        private void advance() { mState++; }

        /*
        * Send an enable command to each sensor by writing a configuration
        * characteristic.  This is specific to the SensorTag to keep power
        * low by disabling sensors you aren't using.
        */
        private void enableNextSensor(BluetoothGatt gatt) {
            BluetoothGattCharacteristic characteristic;
            switch (mState) {
                /*
                case 0:
                    Log.d(TAG, "Enabling pressure cal");
                    characteristic = gatt.getService(PRESSURE_SERVICE)
                            .getCharacteristic(PRESSURE_CONFIG_CHAR);
                    characteristic.setValue(new byte[] {0x02});
                    break;
                case 1:
                    Log.d(TAG, "Enabling pressure");
                    characteristic = gatt.getService(PRESSURE_SERVICE)
                            .getCharacteristic(PRESSURE_CONFIG_CHAR);
                    characteristic.setValue(new byte[] {0x01});
                    break;  */
                case 0:
                    Log.d(TAG, "Enabling humidity");
                    characteristic = gatt.getService(HUMIDITY_SERVICE)
                            .getCharacteristic(HUMIDITY_CONFIG_CHAR);
                    characteristic.setValue(new byte[] {0x01});
                    break;
                case 1:
                    Log.d(TAG, "Enabling magneto");
                    characteristic = gatt.getService(MAGNETO_SERVICE)
                            .getCharacteristic(MAGNETO_CONFIG_CHAR);
                    characteristic.setValue(new byte[] {(byte)0x7F, (byte)0x00});
                    break;
                default:
                    mHandler.sendEmptyMessage(MSG_DISMISS);
                    Log.i(TAG, "All Sensors Enabled");
                    return;
            }

            gatt.writeCharacteristic(characteristic);
        }

        /*
        * Read the data characteristic's value for each sensor explicitly
        */
        private void readNextSensor(BluetoothGatt gatt) {
            BluetoothGattCharacteristic characteristic;
            switch (mState) {
                /*
                case 0:
                    Log.d(TAG, "Reading pressure cal");
                    characteristic = gatt.getService(PRESSURE_SERVICE)
                            .getCharacteristic(PRESSURE_CAL_CHAR);
                    break;
                case 1:
                    Log.d(TAG, "Reading pressure");
                    characteristic = gatt.getService(PRESSURE_SERVICE)
                            .getCharacteristic(PRESSURE_DATA_CHAR);
                    break;*/
                case 0:
                    Log.d(TAG, "Reading humidity");
                    characteristic = gatt.getService(HUMIDITY_SERVICE)
                            .getCharacteristic(HUMIDITY_DATA_CHAR);
                    break;
                case 1:
                    Log.d(TAG, "Reading magneto");
                    characteristic = gatt.getService(MAGNETO_SERVICE)
                            .getCharacteristic(MAGNETO_DATA_CHAR);
                    break;
                default:
                    mHandler.sendEmptyMessage(MSG_DISMISS);
                    Log.i(TAG, "All Sensors Enabled");
                    return;
            }

            gatt.readCharacteristic(characteristic);
        }


        /*
        * Enable notification of changes on the data characteristic for each sensor
        * by writing the ENABLE_NOTIFICATION_VALUE flag to that characteristic's
        * configuration descriptor.
        */
        private void setNotifyNextSensor(BluetoothGatt gatt) {
            BluetoothGattCharacteristic characteristic;
            switch (mState) {
                /*
                case 0:
                    Log.d(TAG, "Set notify pressure cal");
                    characteristic = gatt.getService(PRESSURE_SERVICE)
                            .getCharacteristic(PRESSURE_CAL_CHAR);
                    break;
                case 1:
                    Log.d(TAG, "Set notify pressure");
                    characteristic = gatt.getService(PRESSURE_SERVICE)
                            .getCharacteristic(PRESSURE_DATA_CHAR);
                    break;*/
                case 0:
                    Log.d(TAG, "Set notify humidity");
                    characteristic = gatt.getService(HUMIDITY_SERVICE)
                            .getCharacteristic(HUMIDITY_DATA_CHAR);
                    break;
                case 1:
                    Log.d(TAG, "Set notify magneto");
                    characteristic = gatt.getService(MAGNETO_SERVICE)
                            .getCharacteristic(MAGNETO_DATA_CHAR);
                    break;
                default:
                    mHandler.sendEmptyMessage(MSG_DISMISS);
                    Log.i(TAG, "All Sensors Enabled");
                    return;
            }

            //Enable local notifications
            gatt.setCharacteristicNotification(characteristic, true);
            //Enabled remote notifications
            BluetoothGattDescriptor desc = characteristic.getDescriptor(CONFIG_DESCRIPTOR);
            desc.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
            gatt.writeDescriptor(desc);
        }


        @Override
        public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
            Log.i("onConnectionStateChange", "Status: " + status);
            if (status == BluetoothGatt.GATT_SUCCESS && newState == BluetoothProfile.STATE_CONNECTED) {
                /*
                * Once successfully connected, we must next discover all the services on the
                * device before we can read and write their characteristics.
                */
                gatt.discoverServices();
                mHandler.sendMessage(Message.obtain(null, MSG_PROGRESS, "Discovering Services..."));
            } else if (status == BluetoothGatt.GATT_SUCCESS && newState == BluetoothProfile.STATE_DISCONNECTED) {
                /*
                * If at any point we disconnect, send a message to clear the weather values
                * out of the UI
                */
                mHandler.sendEmptyMessage(MSG_CLEAR);
            } else if (status != BluetoothGatt.GATT_SUCCESS) {
                /*
                * If there is a failure at any stage, simply disconnect
                */
                gatt.close();
            }

        }

        @Override
        public void onServicesDiscovered(BluetoothGatt gatt, int status) {
            List<BluetoothGattService> services = gatt.getServices();
            Log.i("onServicesDiscovered", services.toString());

            mHandler.sendMessage(Message.obtain(null, MSG_PROGRESS, "Enabling Sensors..."));
            /*
            * With services discovered, we are going to reset our state machine and start
            * working through the sensors we need to enable
            */
            reset();
            enableNextSensor(gatt);
        }

        @Override
        public void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
            Log.i("onCharacteristicRead", characteristic.toString());
            //For each read, pass the data up to the UI thread to update the display
            if (HUMIDITY_DATA_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_HUMIDITY, characteristic));
            }
            if (MAGNETO_DATA_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_MAGNETO, characteristic));
            }
            if (PRESSURE_DATA_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_PRESSURE, characteristic));
            }
            if (PRESSURE_CAL_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_PRESSURE_CAL, characteristic));
            }

            //After reading the initial value, next we enable notifications
            setNotifyNextSensor(gatt);
        }

        @Override
        public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
            //After writing the enable flag, next we read the initial value
            readNextSensor(gatt);
        }

        @Override
        public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {
            /*
            * After notifications are enabled, all updates from the device on characteristic
            * value changes will be posted here.  Similar to read, we hand these up to the
            * UI thread to update the display.
            */
            if (HUMIDITY_DATA_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_HUMIDITY, characteristic));
            }
            if (MAGNETO_DATA_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_MAGNETO, characteristic));
            }
            if (PRESSURE_DATA_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_PRESSURE, characteristic));
            }
            if (PRESSURE_CAL_CHAR.equals(characteristic.getUuid())) {
                mHandler.sendMessage(Message.obtain(null, MSG_PRESSURE_CAL, characteristic));
            }
        }

        @Override
        public void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
            //Once notifications are enabled, we move to the next sensor and start over with enable
            advance();
            enableNextSensor(gatt);
        }

        @Override
        public void onReadRemoteRssi(BluetoothGatt gatt, int rssi, int status) {
            Log.d(TAG, "Remote RSSI: " + rssi);
        }

        private String connectionState(int status) {
            switch (status) {
                case BluetoothProfile.STATE_CONNECTED:
                    return "Connected";
                case BluetoothProfile.STATE_DISCONNECTED:
                    return "Disconnected";
                case BluetoothProfile.STATE_CONNECTING:
                    return "Connecting";
                case BluetoothProfile.STATE_DISCONNECTING:
                    return "Disconnecting";
                default:
                    return String.valueOf(status);
            }
        }
    };


    /*
    * We have a Handler to process event results on the main thread
    */
    private static final int MSG_HUMIDITY = 101;
    private static final int MSG_PRESSURE = 102;
    private static final int MSG_PRESSURE_CAL = 103;
    private static final int MSG_MAGNETO = 104;
    private static final int MSG_PROGRESS = 201;
    private static final int MSG_DISMISS = 202;
    private static final int MSG_CLEAR = 301;
    private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            BluetoothGattCharacteristic characteristic;
            switch (msg.what) {
                case MSG_HUMIDITY:
                    characteristic = (BluetoothGattCharacteristic) msg.obj;
                    if (characteristic.getValue() == null) {
                        Log.w(TAG, "Error obtaining humidity value");
                        return;
                    }
                    updateHumidityValues(characteristic);
                    break;
                case MSG_MAGNETO:
                    characteristic = (BluetoothGattCharacteristic) msg.obj;
                    if (characteristic.getValue() == null) {
                        Log.w(TAG, "Error obtaining magneto value");
                        return;
                    }
                    updateMagnetoValues(characteristic);
                    break;
                case MSG_PRESSURE:
                    characteristic = (BluetoothGattCharacteristic) msg.obj;
                    if (characteristic.getValue() == null) {
                        Log.w(TAG, "Error obtaining pressure value");
                        return;
                    }
                    updatePressureValue(characteristic);
                    break;
                case MSG_PRESSURE_CAL:
                    characteristic = (BluetoothGattCharacteristic) msg.obj;
                    if (characteristic.getValue() == null) {
                        Log.w(TAG, "Error obtaining cal value");
                        return;
                    }
                    updatePressureCals(characteristic);
                    break;
                case MSG_PROGRESS:
                    mProgress.setMessage((String) msg.obj);
                    if (!mProgress.isShowing()) {
                        mProgress.show();
                    }
                    break;
                case MSG_DISMISS:
                    mProgress.hide();
                    break;
                case MSG_CLEAR:
                    clearDisplayValues();
                    break;
            }
        }
    };

    /* Methods to extract sensor data and update the UI */

    private void updateHumidityValues(BluetoothGattCharacteristic characteristic) {
        double humidity = SensorTagData.extractHumidity(characteristic);

        mHumidity.setText(String.format("%.0f%%", humidity));
    }

    ArrayList<Double> magnets = new ArrayList<>();
    double average = 0;
    int count = -1;
    int wait5 = 0;

    private void updateMagnetoValues(BluetoothGattCharacteristic characteristic) {
        double magnet = SensorTagData.extractMagnetoX(characteristic);
        Log.e("MAGNETO", "" + magnet);
        magnet = Math.abs(magnet);

        if (wait5 == 0) {

            if (magnet > average + 300 ||  magnet < average - 300) {
                wait5 = 6;

                count ++;
                mPressure.setText("" + count);
            }
        }
        else {
            wait5 --;
        }

        if (magnets.size() >= 5) {
            magnets.remove(0);
        }
        magnets.add(magnet);
        double sum = 0;
        for (int i = 0; i < magnets.size(); i++) {
            sum += magnets.get(i);
        }
        average = sum/magnets.size();
        average = magnets.get(2);

        //mHumidity.setText(String.format("%.0f%%", humidity));

    }

    private int[] mPressureCals;
    private void updatePressureCals(BluetoothGattCharacteristic characteristic) {
        mPressureCals = SensorTagData.extractCalibrationCoefficients(characteristic);
    }

    private void updatePressureValue(BluetoothGattCharacteristic characteristic) {
        if (mPressureCals == null) return;
        double pressure = SensorTagData.extractBarometer(characteristic, mPressureCals);
        double temp = SensorTagData.extractBarTemperature(characteristic, mPressureCals);

        mTemperature.setText(String.format("%.1f\u00B0C", temp));
        mPressure.setText(String.format("%.2f", pressure));
    }
}

Answer 2

Well I think you should do a bit more reading on how to use the characteristics. Your chip has a bunch of characteristics by default which contain information from the chip mostly. In order to send and receive data, you have to program your chip first to send data through a specific characteristic and receive data through another specific characteristic.

Some chips have default Rx and Tx characteristics which you should use to send and receive data on. These characteristics act as pipelines between BLE and the Tx and Rx pins on your chip and will simply echo all the data through.

Alternatively, you can create your own characteristics on the BLE service and program your own send and receive protocols.

Let me give you a simple example:

Let's say that you are using the default characteristic of your chip to send data. This data has to be sent in small chunks, for example 20 bytes. To read this data on Android, you have to first get the descriptor for that characteristic and enable notification. This will tell Android to listen to that characteristic and monitor changes.

    characteristic = gatt.getService(UUID.fromString(SERVICE_UUID)).getCharacteristic(UUID.fromString(CHARACTERISTIC_UUID)); //Find you characteristic
    mGatt.setCharacteristicNotification(characteristic, true); //Tell you gatt client that you want to listen to that characteristic
    List<BluetoothGattDescriptor> descriptors = characteristic.getDescriptors(); //find the descriptors on the characteristic
    BluetoothGattDescriptor descriptor = descriptors.get(1); //get the right descriptor for setting notifications
    descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
    mGatt.writeDescriptor(descriptor); //apply these changes to the ble chip to tell it we are ready for the data

After doing this, you will be able to use the onCharacteristicChanged method to read the data:

@Override
public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {
    super.onCharacteristicChanged(gatt, characteristic);
    String data = characteristic.getStringValue(0);
}

Sending data would be a bit simpler. Once you find the characteristic, you just have to write your data in chunks to that characteristic:

byte[] value = new byte[1];
value[0] = (byte) (21 & 0xFF); //your data
characteristic.setValue(value); //assign the new data value to the characteristic
boolean success = mBluetoothGatt.writeCharacteristic(charac); //write to the chip

Once written, the chip will flush the characteristic and send the data to the chip's Tx pin.

EDIT:

Going through the user manual for RN4020 , you can see that the chip has a default battery service and some characteristics for the model, software version, etc, but no default Rx and Tx characteristics.

In this case, the characteristics that you found in your code were just device information and practically useless. You have to define your own private service and characteristics on the chip first. Look at Example 3-1 in the manual . Once you define the private service and characteristic using unique UUIDs, you can write your data to that, and read it on android using the same UUID.

Answer 3

I got the RN4020 working in processing.org in combination with the arduino platform. If you're still interesting let me know.

Do you have the SW-wake and HW wake pin connected to vcc 3.3v? You don't have to go to CMD everythime you start.

I just connectecd CDM directly to VCC. Once you give the reboot command it goes directly to MLDP.