SparkJson... Works the first time, then doesn't update

This is my udp read function, which calls a SparkJson parsing function when it works.

The first call is flawless, I am definitely receiving a good data packet over UDP (The packet is ~772 bytes). After that, the JSON object in the second function at the bottom doesn’t appear to update anymore, and the values I pull from thee JSON object don’t change. But I have verified that they are changing.

So I don’t know if the UDP is returning the same data for some reason, or if the parser is not working.

void checkUdp()
  int rxErr = 0;
  int count = udp.receivePacket(tempStatus, 1023);  //tempStatus is declared as a global char tempStatus[1024]

  if (count >= 0 && count < 1024) {
    tempStatus[count] = 0;
    rxErr = 0;
  } else if (count < -1) {
    rxErr = count;
    // need to re-initialize on error
  if (!rxErr) {
    //No Error code, but this could mean count = -1, which is no packet

void parseTempStatus(char* tempJSON)
    StaticJsonBuffer<4096> jsonBuffer;  //This is probably bigger than it needs to be, but not by much
    JsonObject& root = jsonBuffer.parseObject(tempJSON);
    displayInfo[0] = "Temp: " + String::format("%.1fF",root["Sensor"]["temp"]);
    displayInfo[1] = String(Time.local());
    displayInfo[2] = "Last: " + String::format("%d", root["Sensor"]["TS"]);

We could address the issue you are seeing but since there are better alternatives to the SparkJson library you may want to look at these first

  • the JsonParserGeneratorRK library is written and actively maintained by Rick (rickkas7) a particle employee
  • the device OS features a lightweight but slightly less convenient JSON parser
  • the original contributor of SparkJson has also uploaded that library as ArduinoJson and keeps updating that quite frequently (while the SparkJson is now dormant)

For your issue at hand: As you are using a StaticJsonBuffer you may want to clear it before parsing a new JSON string otherwise you just keep reparsing the old contents that’s “statically” stored.

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