Working with BLE Android 4.3 how to write characteristics?

Question

I am working on a BLE project (Android application) using Android 4.3 API, i have used sample BLE app it is only reading characteristics in DeviceControlActivity.activity, but i want to write characteristics and send it to BLE chip on clicking a button. How can I write data on chip cc2540 .. Basically i don't know the step by step procedure to write characteristics.

write i can only see the name and id of device with following piece of code in DeviceControlActivity

 private final ExpandableListView.OnChildClickListener servicesListClickListner =
        new ExpandableListView.OnChildClickListener() {
            @Override
            public boolean onChildClick(ExpandableListView parent, View v, int groupPosition,
                                        int childPosition, long id) {
                if (mGattCharacteristics != null) {
                    final BluetoothGattCharacteristic characteristic =
                            mGattCharacteristics.get(groupPosition).get(childPosition);
                    final int charaProp = characteristic.getProperties();
                    if ((charaProp | BluetoothGattCharacteristic.PROPERTY_READ) > 0) {
                        // If there is an active notification on a characteristic, clear
                        // it first so it doesn't update the data field on the user interface.
                        if (mNotifyCharacteristic != null) {
                            mBluetoothLeService.setCharacteristicNotification(
                                    mNotifyCharacteristic, false);
                            mNotifyCharacteristic = null;
                        }
                        mBluetoothLeService.readCharacteristic(characteristic);
                        showDialog("reading");
                    }

                    if ((charaProp | BluetoothGattCharacteristic.PROPERTY_WRITE) > 0) {
                        // If there is an active notification on a characteristic, clear
                        // it first so it doesn't update the data field on the user interface.
                        if (mNotifyCharacteristic != null) {
                            mBluetoothLeService.setCharacteristicNotification(
                                    mNotifyCharacteristic, false);
                            mNotifyCharacteristic = null;
                        }
                        mBluetoothLeService.writeCharacteristic(characteristic);
                        showDialog("writing");
                        //characteristic.setValue(bytes);
                        //characteristic.setValue("testing");
                        //characteristic.setWriteType(BluetoothGattCharacteristic.PERMISSION_WRITE);
                    }
                    if ((charaProp | BluetoothGattCharacteristic.PROPERTY_NOTIFY) > 0) {
                        mNotifyCharacteristic = characteristic;
                        mBluetoothLeService.setCharacteristicNotification(
                                characteristic, true);
                    }

                    byte[] value = {0x00, (byte) (0xB9) , 0x0D, (byte) (0x90), 0x2F};
                    if(!characteristic.setValue(value)) 
                    {
                        Log.w(TAG, "Couldn't set characteristic's local value"); 
                        //return;
                    }

                    characteristic.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE);
                    /*if(!writeCharacteristic.writeCharacteristic(characteristic))
                    { 
                        Log.w(TAG, "Couldn't write characteristic");
                    }*/

                    return true;
                }
                return false;
            }
};

Synod

Answer 1

The following code is write characteristic using byte[] data:

    public boolean writeCharacteristic(){

    //check mBluetoothGatt is available
    if (mBluetoothGatt == null) {
        Log.e(TAG, "lost connection");
        return false;
    }
    BluetoothGattService Service = mBluetoothGatt.getService(your Services);
    if (Service == null) {
        Log.e(TAG, "service not found!");
        return false;
    }
    BluetoothGattCharacteristic charac = Service
            .getCharacteristic(your characteristic);
    if (charac == null) {
        Log.e(TAG, "char not found!");
        return false;
    }

    byte[] value = new byte[1];
    value[0] = (byte) (21 & 0xFF);
    charac.setValue(value);
    boolean status = mBluetoothGatt.writeCharacteristic(charac);
    return status;
}

Boyette

thank you @Nam Pham kindly explain why you are doing this 21&0xFF ? – Synod

21 & 0xFF. it's just data which I want to write to BLE device. So, you should replace it by your data. Max data length you can write which is 20 bytes. If you have data length > 20 bytes, you need to fragment data into 20 byte trunk and write – Boyette

thanks but i am asking about and "&" that you are using in your code why you are using "& 0xFF" before writing.. – Synod

@Nomi: it's just my habbit. you can use "value[0] = (byte) 21;" . It's same result – Boyette

Which Service to pass on place of "your Services"? – Magallanes

"your Services" is basically the UUID of the service. in my case it is MY_SERIVCE_CONTROLLER = "0000fff0-0000-1000-8000-00805f9b34fb" – Synod

writeCharcteristics getting failed every time – Psychedelic

Sir, I have a problem with this problem " Log.e(TAG, "char not found!");" sir please guide me. Thank you so much, sir. – Bellbottoms

@chetanmahajan, Please check the service and character UUID which you want to write data. You can check them by using the LightBlue app. please ensure that service UUID is existing on your device and character UUID belongs to it – Boyette

How do you get a reference to mBluetoothGatt as the question asked doesn't have a reference to it since the connectGatt() is called from the BluetoothLeService and not from the DeviceControlActivity. @Synod Did you implement a public method in the Bluetooth service rather than the devicecontrolactivity? – Jiggle

I removed the '21 &' part to write data to rower. Basically wanted to write {7,1} but '21 &' was making it {5, 1}. – Pile

Answer 2

Please note that the logic OR in:
if ((charaProp | BluetoothGattCharacteristic.PROPERTY_READ) > 0)" in the original post should be a logic AND for the permission check to work. Same for the second charaProp comparison. Otherwise bot statements are true regardless of the actual permission flag.

Guesstimate

Answer 3

The following code is write characteristic using string data in utf-8 format:

public void writeCharacteristic(BluetoothGattCharacteristic characteristic,
            String data) {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return;
        }

        Log.i(TAG, "characteristic " + characteristic.toString());
        try {
            Log.i(TAG, "data " + URLEncoder.encode(data, "utf-8"));

            characteristic.setValue(URLEncoder.encode(data, "utf-8"));

            // TODO
            mBluetoothGatt.writeCharacteristic(characteristic);
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
    }

Hope it helps!

Caterinacatering

Answer 4

public boolean writeCharacteristic(byte value[],int type){
    //check mBluetoothGatt is available
    if (mBluetoothGatt == null) {
        Log.e(TAG, "lost connection");
        return false;
    }
    BluetoothGattService Service = mBluetoothGatt.getService(UUID_SIMPLESERVICE);
    if (Service == null) {
        Log.e(TAG, "service not found!");
        //////////NO service found...........
         return false;
    }
    BluetoothGattCharacteristic charac1 = null;
    boolean status1 = false;

    if(type==1) {
        charac1 = Service.getCharacteristic(UUID_PORT1);
        charac1.setValue(value);
        status1 = mBluetoothGatt.writeCharacteristic(charac1);
        Log.v("________BLESERVICE____", "___WRITE CHARATERISTICS STATUS:_________"+status1);
        onReliableWriteCompleted(status1);
    }
    if (charac1 == null) {
        Log.e(TAG, "char not found!");
        return false;
    }

    Log.v("___TYPE___","______________________"+type);
    return status1;
}

Warnerwarning

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