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class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect(BLEServer* pServer) {
      deviceConnected = true;
    };

    void onDisconnect(BLEServer* pServer) {
      deviceConnected = false;


    }
};

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string rxValue = pCharacteristic->getValue();


      if (rxValue.length() > 0) {
        Serial.println("*********");
        Serial.print("Received Value: ");

        for (int i = 0; i < rxValue.length(); i++) {
          Serial.print(rxValue[i]);
        }

        Serial.println();
        Serial.println("*********");
      }

      // Do stuff based on the command received from the app
      // For some reason using rxValue.compare("A") == 0 doesn't work. Maybe
      // there are hidden characters I'm not seeing?
   

    }
};

void setup() {
  Serial.begin(115200);

  // Create the BLE Device
  BLEDevice::init("Lumos Board"); // Give it a name

  // Create the BLE Server
  BLEServer *pServer = BLEDevice::createServer();
  pServer->setCallbacks(new MyServerCallbacks());

  // Create the BLE Service
  BLEService *pService = pServer->createService(SERVICE_UUID);

  // Create a BLE Characteristic
  pCharacteristic = pService->createCharacteristic(
                      CHARACTERISTIC_UUID_TX,
                      BLECharacteristic::PROPERTY_NOTIFY
                    );

  pCharacteristic->addDescriptor(new BLE2902());

  BLECharacteristic *pCharacteristic = pService->createCharacteristic(
                                         CHARACTERISTIC_UUID_RX,
                                         BLECharacteristic::PROPERTY_WRITE
                                       );

  pCharacteristic->setCallbacks(new MyCallbacks());

  

  // Start the service
  pService->start();

  // Start advertising
  pServer->getAdvertising()->start();
  Serial.println("Waiting a client connection to notify...");
  

}

void loop() {
  EVERY_N_MILLISECONDS( 20 ) {
    gHue++;  // slowly cycle the "base color" through the rainbow
  }
   unsigned long currentMillis = millis();
  if (deviceConnected) {
  if (currentMillis - previousMillis >= interval) {
    // save the last time you sent the message
    previousMillis = currentMillis;
         
    pCharacteristic->setValue("Hi, I am trying to send a string over 20 bytes to Android"); // Sending a test message
    pCharacteristic->notify(); // Send the value to the app!
    
    
      
  }
  }


  if (!deviceConnected) {

  }

}

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BLEDevice::setMTU(uint16_t mtu)

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uint16_t mtu = 128;
  BLEDevice::setMTU(128);

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sketch\LBC.ino.cpp.o: In function `void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char const*>(char const*, char const*, std::__false_type)':

c:\users\******\documents\arduino\hardware\espressif\esp32\arduino-esp32\tools\xtensa-esp32-elf\xtensa-esp32-elf\include\c++\5.2.0\bits/basic_string.h:195: undefined reference to `BLEDevice::setMTU(unsigned short)'

collect2.exe: error: ld returned 1 exit status

exit status 1
Error compiling for board ESP32 Dev Module.

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uint16_t mtu = 128;
  BLEDevice::setMTU(128);

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#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>

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E (69195) BT: attribute value too long, to be truncated to 20