BLE reading com port data stream from PC on the ESP32

[j-a-n]

Hello new friends i am new and have - after 3 days of scanning the internet- a request for help.

My GOAL is to stream NMEA-sentences (positiondata) from an programm on my PC to an App on my phone using a COM port and an ESP32 with BLE. (note: NOT with BT because the app does not support BT)

I used the example of Neil Kolban. Million thanks Neil/Evandro!. In the app on my phone i can read the charter set Neil made. See below. So that works fine.

Complete code

/*
    Video: https://www.youtube.com/watch?v=oCMOYS71NIU
    Based on Neil Kolban example for IDF: https://github.com/nkolban/esp32-snippets/blob/master/cpp_utils/tests/BLE%20Tests/SampleNotify.cpp
    Ported to Arduino ESP32 by Evandro Copercini

   Create a BLE server that, once we receive a connection, will send periodic notifications.
   The service advertises itself as: 6E400001-B5A3-F393-E0A9-E50E24DCCA9E
   Has a characteristic of: 6E400002-B5A3-F393-E0A9-E50E24DCCA9E - used for receiving data with "WRITE"
   Has a characteristic of: 6E400003-B5A3-F393-E0A9-E50E24DCCA9E - used to send data with  "NOTIFY"

   The design of creating the BLE server is:
   1. Create a BLE Server
   2. Create a BLE Service
   3. Create a BLE Characteristic on the Service
   4. Create a BLE Descriptor on the characteristic
   5. Start the service.
   6. Start advertising.

   In this example rxValue is the data received (only accessible inside that function).
   And txValue is the data to be sent, in this example just a byte incremented every second.
*/
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

BLEServer *pServer = NULL;
BLECharacteristic * pTxCharacteristic;
bool deviceConnected = false;
bool oldDeviceConnected = false;
uint8_t txValue = 0;

// See the following for generating UUIDs:
// https://www.uuidgenerator.net/

#define SERVICE_UUID           "6E400001-B5A3-F393-E0A9-E50E24DCCA9E" // UART service UUID
#define CHARACTERISTIC_UUID_RX "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TX "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"


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("*********");
      }
    }
};


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

  // Create the BLE Device
  BLEDevice::init("UART Service");

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

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

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

  pTxCharacteristic->addDescriptor(new BLE2902());

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

  pRxCharacteristic->setCallbacks(new MyCallbacks());

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

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

void loop() {

  if (deviceConnected) {
    pTxCharacteristic->setValue(&txValue, 1);
    pTxCharacteristic->notify();
    txValue++;
    delay(10); // bluetooth stack will go into congestion, if too many packets are sent
  }

  // disconnecting
  if (!deviceConnected && oldDeviceConnected) {
    delay(500); // give the bluetooth stack the chance to get things ready
    pServer->startAdvertising(); // restart advertising
    Serial.println("start advertising");
    oldDeviceConnected = deviceConnected;
  }
  // connecting
  if (deviceConnected && !oldDeviceConnected) {
    // do stuff here on connecting
    oldDeviceConnected = deviceConnected;
  }
}

The ISSUE i have concerns the following part in the code above.

 if (deviceConnected) {
    pTxCharacteristic->setValue(&txValue, 1);
    pTxCharacteristic->notify();
    txValue++;
    delay(10); // bluetooth stack will go into congestion, if too many packets are sent
  }

PROBLEM: Trying to adapt the code for my goal i got stuck. I don't know how to program a read on the incoming stream of NMEA-sentences (see example below) on the com port (instead of the characterset Niel made in his example above)

So my QUESTION is does anyone have an idea on how to program that?

=====Example of incoming stream of NMEA-Sentences on the com port======
$GPGGA,130004.089,4352.1832,N,00610.0247,E,1,12,10,2266.0,M,,,,,000036*
$GPRMC,130004.089,A,4352.1832,N,00610.0247,E,48.54,258.00,,,,14
$LXWP0,Y,114.6,2266.0,4.12,,,,,,258,054,26.346
$GPGGA,130005.095,4352.1832,N,00610.0247,E,1,12,10,2266.0,M,,,,,00003A*
$GPRMC,130005.095,A,4352.1832,N,00610.0247,E,48.54,258.00,,,,18
$LXWP0,Y,114.6,2266.0,2.10,,,,,,258,054,26.342
$GPGGA,130006.101,4352.1832,N,00610.0247,E,1,12,10,2266.0,M,,,,,000035*
$GPRMC,130006.101,A,4352.1832,N,00610.0247,E,48.54,258.00,,,,17
$LXWP0,Y,114.6,2266.0,1.07,,,,,,258,054,26.347
...etc
==============================================================
Note The steam repeatedly sends 3 sentences (with updates on position and so)