// https://arduinojson.org/v6/how-to/do-serial-communication-between-two-boards/

#include <ArduinoJson.h>

void setup() {
  // Initialize "debug" serial port
  // The data rate must be much higher than the "link" serial port
  Serial.begin(115200);
  while (!Serial) continue;

  // Initialize the "link" serial port
  // Use a low data rate to reduce the error ratio
  Serial1.begin(9600);


  // Check if the other Arduino is transmitting
  if (Serial1.available()) 
  {
    // Allocate the JSON document
    // This one must be bigger than the sender's because it must store the strings
    StaticJsonDocument<300> doc;

    // Read the JSON document from the "link" serial port
    DeserializationError err = deserializeJson(doc, Serial1);

    if (err == DeserializationError::Ok) 
    {
      // Print the values
    
String cmd =		doc["cmd"];
Serial.print("cmd = ");
Serial.println(cmd);

    } 
    else 
    {
      // Print error to the "debug" serial port
      Serial.print("deserializeJson() returned ");
      Serial.println(err.c_str());
  
      // Flush all bytes in the "link" serial port buffer
      while (Serial1.available() > 0)
        Serial1.read();
    }
  }
}
 
void loop() {}