SoftwareSerial data being corrupted - looking for suggestions

Question

I'm having problems running a serial communication between a NodeMCU ESP8266 and an Arduino Uno. I'm using the "standard" SUART setup, i.e. pins D1&D2 on the ESP are hooked up with GPIOs 2&3 on the UNO; the tx from UNO to ESP is fed through a voltage divider to step it down to 3.3V.

The setup used to work fine; suddently - yeah I've read it before, my setup suddently doesn't work, but honestly that's what happened - data is being corrupted. The devices exchange a json formatted string:

{"action":"position","lat":123,"lon":123}

and it just worked. Now what I receive is something like:

{"⸮⸮⸮⸮⸮⸮":"pos⸮⸮⸮onb,"l⸮tF:0,"lo⸮":0,"⸮⸮s⸮⸮":0⸮

Both SUART are on 115200 baudrate, physical connections have been checked and are fine, both devices are grounded on the same reference level.

I've tried swapping the ESP with another one and changing TX/RX pins on the UNO - no results.

So I'm looking for suggestions what to do next to debug the issue, what to check and what could be wrong. Thank you in advance.

Below are UNO and ESP code snippets.

ESP8266

SoftwareSerial SUART(4, 5);

int16_t last_fetch = 0;
int16_t last_serial = 0;

double azimut, elevation;

#define DATA_FETCH_TIME_INT 3
#define SERIAL_TRASMISSION_TIME_INT 5

void setup() {
  Serial.begin(115200);  //enable Serial Monitor
  SUART.begin(115200);   //enable SUART Port

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  Serial.println();
  Serial.println(WiFi.localIP());
}

void loop() {
  if (secs() - last_fetch >= DATA_FETCH_TIME_INT) {
    last_fetch = secs();
    fetchData();
  }

  if (secs() - last_serial >= SERIAL_TRASMISSION_TIME_INT) {
    last_serial = secs();
    sendDataViaSerial();
    }
}

void fetchData() {
  //connects to server and fetches data
}

void sendDataViaSerial() {
  int lat_i = (int) elevation;
  int lon_i = (int) azimut;
  int checksum = lat_i + lon_i;

  const int capacity = 200;
  StaticJsonDocument<capacity> doc;

  doc["action"] = "position";
  doc["lat"] = lat_i;
  doc["lon"] = lon_i;
  doc["cksum"] = checksum;

  Serial.println("Sending data via serial: ");
  serializeJson(doc, Serial);

  //SUART.flush();
  //serializeJson(doc, SUART);
  // works the same

  String outstring;
  serializeJson(doc, outstring);
  SUART.println(outstring);
}

ARDUINO UNO

SoftwareSerial SUART(2, 3);
#define arraySize 150
char inData[arraySize];

void setup() {
  pinMode(STATUS_LED, OUTPUT);
  digitalWrite(STATUS_LED, HIGH);

  Serial.begin(115200);
  SUART.begin(115200);

  Serial.println("Ready to go.\n");
}

void loop() {
  readSerial();
}

void readSerial() {
  short index = 0;
  char inChar;

  if (SUART.available() > 0) {
    while (SUART.available() > 0 && index < arraySize - 1) {
      inChar = SUART.read();    
      inData[index] = inChar;

      index++;
      inData[index] = '\0';  // Add a null at the end
    }

    SUART.flush();
    String myData = (String)inData;
    Serial.println(myData);

    // consume data
    //parseIncomingData(inData);
  }
}

Answer 1

As @Juraj mentions in the comments, don't use 115200 baud with SoftwareSerial.

The problem is that the AVR isn't fast enough to handle incoming data at that speed reliably. Yes, it can receive a byte fine at that speed, but what it can't necessarily do is store that byte away in the receive buffer in time to be ready to receive the next byte - so it misses the first few bits and corruption ensues.

Running at a lower baud rate (such as 9600 baud) simply gives it more time to process the data as it arrives.