Buffers and arrays for serial data - basic help needed to get started

Projects Programming
11

I wrote the following for a similar application.
It's part of an add-on for an IKEA PM2.5 air quality monitor

    static constexpr byte PM25_MSB {5};
    static constexpr byte PM25_LSB {6};


    enum RX_STATE {WAIT_PREAMBLE, 
                   HANDLE_PAYLOAD, 
                   CHECK_MESSAGE} rxState;

template <class Serial_T>
void IKEA_PM25<Serial_T>::resetRx ()
{
  pm25          = invalidReading;
  rxState       = WAIT_PREAMBLE;
  checksum      = 0;
  messageIndex  = 0;
}
/**************************************************** 
 *
 * Protocol details
 *   Controller: 11 02 0B 01 E1
 *   Sensor:     16 11 0B DF1-DF4 DF5-DF8 DF9-DF12 DF13 DF14 DF15 DF16 [CS]
 *   PM2.5 (ug/m^3)= DF3*256 + DF4
 *
*/ 
template <class Serial_T>
void IKEA_PM25<Serial_T>::handleSensor()
{
  uint32_t now = millis ();
  // check here for sensor timeout? ToDo
  while (serial.available()) {
    uint8_t rxChar = (uint8_t) serial.read ();  
    checksum += rxChar;

    switch (rxState) {
      case WAIT_PREAMBLE:
        if (rxChar == preamble [messageIndex++]) {
          if (messageIndex == preambleLength)
            rxState = HANDLE_PAYLOAD;
        } 
        else 
          resetRx ();
      break;

      case HANDLE_PAYLOAD: // we've had a valid preamble, all we can do is grab / ignore the rest
        if (messageIndex == PM25_MSB)
          pm25 = rxChar << 8;
        if (messageIndex == PM25_LSB)
          pm25 |= rxChar;
        messageIndex++;
        if (messageIndex == messageSize)
          rxState = CHECK_MESSAGE;
      break;

      case CHECK_MESSAGE:
        if (checksum == 0) {
          sensorOnline = true;

          rollingSum -= samples [sampleIndex];
          rollingSum += samples [sampleIndex++] = pm25;
      
          if (sampleIndex >= nSamples) {
            wrapped     = true;
            sampleIndex = 0;
          }
          int divisor = wrapped ? nSamples : sampleIndex;
          avgPM25 = rollingSum / divisor;
          lastReading = now;
        }
        resetRx ();        
      break;
      default:
      break;
    }
  }
}

Ignore the template stuff, it's just a fudge to allow soft or hard UARTs.
Here, the checksum is simply that, not an XOR, and it's over all of the message.