Why does my Arduino stop uploading to the Thingspeak server after several hours?

Question

I'm using a Maduino Zero 4G SIM7600 device to upload data from 28 sensors to a ThingSpeak IoT server. It works well for a few hours, then stops uploading the data. Sometimes it starts again after an inconsistent amount of time (an hour or two), sometimes it doesn't.

I can confirm that it's only the SIM7600 and/or Thingspeak functions that are failing, as the serial monitor demonstrates the other I2C related operations on the Arduino board are still working fine even when the server uploads fail.

Here's the relevant part of the code:

void setup(){
    SerialUSB.begin(115200);
    SerialUSB.println("Serial initialized");
    Serial1.begin(115200);
    Wire.begin();

    pinMode(LTE_PWRKEY_PIN, OUTPUT);
    digitalWrite(LTE_RESET_PIN, LOW);
    delay(100);
    digitalWrite(LTE_PWRKEY_PIN, HIGH);
    delay(2000);
    digitalWrite(LTE_PWRKEY_PIN, LOW);
    
    pinMode(LTE_FLIGHT_PIN, OUTPUT);
    digitalWrite(LTE_FLIGHT_PIN, LOW);//Normal Mode
    
    delay(5000);
     
    sendData("AT+CCID", 3000, DEBUG);
    sendData("AT+CREG?", 3000, DEBUG);
    sendData("AT+CGATT=1", 1000, DEBUG);
    sendData("AT+CGACT=1,1", 1000, DEBUG);    
    sendData("AT+CGDCONT=1,\"IP\",\"sp.mb.com\"", 1000, DEBUG);

    delay(5000);
}

void loop(){
  getData();
  
  //Sensors 0-6
  http_str = "AT+HTTPPARA=\"URL\",\"https://api.thingspeak.com/update?api_key=" + Apikey1 + "&field1=" + (String)hallData[0] + "&field2=" + (String)hallData[1] + "&field3=" + (String)hallData[2] + "&field4=" + (String)hallData[3] + "&field5=" + (String)hallData[4] + "&field6=" + (String)hallData[5] + "&field7=" + (String)hallData[6] + "\"\r\n";
  SerialUSB.print("Sent to Channel 1: ");
  SerialUSB.println(http_str);
  sendData("AT+HTTPINIT\r\n", 2000, DEBUG);
  sendData(http_str, 2000, DEBUG);
  sendData("AT+HTTPACTION=1\r\n", 3000, DEBUG);
  sendData("AT+HTTPTERM\r\n", 3000, DEBUG);

  //Sensors 7-13
  http_str = "AT+HTTPPARA=\"URL\",\"https://api.thingspeak.com/update?api_key=" + Apikey2 + "&field1=" + (String)hallData[7] + "&field2=" + (String)hallData[8] + "&field3=" + (String)hallData[9] + "&field4=" + (String)hallData[10] + "&field5=" + (String)hallData[11] + "&field6=" + (String)hallData[12] + "&field7=" + (String)hallData[13] + "\"\r\n";
  SerialUSB.print("Sent to Channel 2: ");
  SerialUSB.println(http_str);
  sendData("AT+HTTPINIT\r\n", 2000, DEBUG);
  sendData(http_str, 2000, DEBUG);
  sendData("AT+HTTPACTION=1\r\n", 3000, DEBUG);
  sendData("AT+HTTPTERM\r\n", 3000, DEBUG);

  //Sensors 14-20
  http_str = "AT+HTTPPARA=\"URL\",\"https://api.thingspeak.com/update?api_key=" + Apikey3 + "&field1=" + (String)hallData[14] + "&field2=" + (String)hallData[15] + "&field3=" + (String)hallData[16] + "&field4=" + (String)hallData[17] + "&field5=" + (String)hallData[18] + "&field6=" + (String)hallData[19] + "&field7=" + (String)hallData[20] + "\"\r\n";
  SerialUSB.print("Sent to Channel 3: ");
  SerialUSB.println(http_str);
  sendData("AT+HTTPINIT\r\n", 2000, DEBUG);
  sendData(http_str, 2000, DEBUG);
  sendData("AT+HTTPACTION=1\r\n", 3000, DEBUG);
  sendData("AT+HTTPTERM\r\n", 3000, DEBUG);

  //Sensors 21-27
  http_str = "AT+HTTPPARA=\"URL\",\"https://api.thingspeak.com/update?api_key=" + Apikey4 + "&field1=" + (String)hallData[21] + "&field2=" + (String)hallData[22] + "&field3=" + (String)hallData[23] + "&field4=" + (String)hallData[24] + "&field5=" + (String)hallData[25] + "&field6=" + (String)hallData[26] + "&field7=" + (String)hallData[27] + "\"\r\n";
  SerialUSB.print("Sent to Channel 4: ");
  SerialUSB.println(http_str);
  sendData("AT+HTTPINIT\r\n", 2000, DEBUG);
  sendData(http_str, 2000, DEBUG);
  sendData("AT+HTTPACTION=1\r\n", 3000, DEBUG);
  sendData("AT+HTTPTERM\r\n", 3000, DEBUG);
}

String sendData(String command, const int timeout, boolean debug){
  String response = "";
  Serial1.println(command);
  
  long int time = millis();
  while ( (time + timeout) > millis()){
    while (Serial1.available()){
      char c = Serial1.read();
      response += c;
    }
  }
  if (debug){
    SerialUSB.print(response);
  }
  return response;
}

All SIM7600 related operations are taken directly from the Makerfabs examples, found on this page. One thing of note is that in their example, they use the command AT+HTTPACTION=0, which, according to this page, means READ. This does work but it sometimes returns ERROR instead of OK. I've changed it to AT+HTTPACTION=1, which means POST, which seems to work a bit better - at least there are no ERROR messages. But the same problem still persists.

UPDATE #1: The ERROR messages sometimes return even with the AT+HTTPACTION=1 adjustment. Something I've noticed is that generating new API keys for each of the Thingspeak pages (there are 4 in total, 7 sensors per page) causes it to work for a while when it didn't before. No explanation for why that is.

Any advice would be greatly appreciated.

Hardware wise, I'm powering this from an AC adapter through a DC-DC 5V converter. I'm also using a 3.7 battery for backup power.

Answer 1

From the ThingSpeak License Options page:

ThingSpeak is available as a free service for non-commercial small projects (<3 million messages/year or ~8,200 messages/day).

Could you be hitting that limit?

Answer 2

You are most likely exceeding the messages-per-period-of-time limit. Give it some time and then try again, adding some delay() in your main loop.