Rfm9xLoRaDevice NetMF Payload CRCs

To ensure I was only handling messages with valid contents I added code to appended a cyclic redundancy check(CRC) onto outbound messages and validate the CRC on inbound messages.

First step was to update the initialise method parameter list (the parameter list is huge but for most scenarios the defaults are fine)

public void Initialise(RegOpModeMode regOpModeAfterInitialise, // RegOpMode
         double frequency = FrequencyDefault, // RegFrMsb, RegFrMid, RegFrLsb
         bool rxDoneignoreIfCrcMissing = true, bool rxDoneignoreIfCrcInvalid = true,
         bool paBoost = false, byte maxPower = RegPAConfigMaxPowerDefault, byte outputPower = RegPAConfigOutputPowerDefault, // RegPaConfig
         bool ocpOn = true, byte ocpTrim = RegOcpOcpTrimDefault, // RegOcp
         RegLnaLnaGain lnaGain = LnaGainDefault, bool lnaBoost = false, // RegLna
         RegModemConfigBandwidth bandwidth = RegModemConfigBandwidthDefault, RegModemConfigCodingRate codingRate = RegModemConfigCodingRateDefault, RegModemConfigImplicitHeaderModeOn implicitHeaderModeOn = RegModemConfigImplicitHeaderModeOnDefault, //RegModemConfig1
         RegModemConfig2SpreadingFactor spreadingFactor = RegModemConfig2SpreadingFactorDefault, bool txContinuousMode = false, bool rxPayloadCrcOn = false,
         ushort symbolTimeout = SymbolTimeoutDefault,
         ushort preambleLength = PreambleLengthDefault,
         byte payloadLength = PayloadLengthDefault,
         byte payloadMaxLength = PayloadMaxLengthDefault,
         byte freqHoppingPeriod = FreqHoppingPeriodDefault,
         bool lowDataRateOptimize = false, bool agcAutoOn = false,
         byte ppmCorrection = ppmCorrectionDefault,
         RegDetectOptimizeDectionOptimize detectionOptimize = RegDetectOptimizeDectionOptimizeDefault,
         bool invertIQ = false,
         RegisterDetectionThreshold detectionThreshold = RegisterDetectionThresholdDefault,
         byte syncWord = RegSyncWordDefault)

The rxPayloadCrcOn needs to be set to True for outbound messages to have a CRC.

Then in the RxDone interrupt handler the CRC is checked (regHopChannel & regIrqFlagsMask) if this feature is enabled. Any messages with missing\invalid CRCs will currently be silently discarded and I’m not certain this is a good idea.

 // Check to see if payload has CRC
         if (RxDoneIgnoreIfCrcMissing)
         {
            byte regHopChannel = this.Rfm9XLoraModem.ReadByte((byte)Registers.RegHopChannel);
            if ((regHopChannel & (byte)RegHopChannelFlags.CrcOnPayload) != (byte)RegHopChannelFlags.CrcOnPayload)
            {
               return;
            }
         }

         // Check to see if payload CRC is valid
         if (RxDoneIgnoreIfCrcInvalid)
         {
            if (((byte)IrqFlags & (byte)RegIrqFlagsMask.PayLoadCrcErrorMask) == (byte)RegIrqFlagsMask.PayLoadCrcErrorMask)
            {
               return;
            }
         }

The conversion of the payload from an array of bytes to a string for display stopped failing with an exception. When I had a number of clients running up to 10% of the messages were getting corrupted.

3 thoughts on “Rfm9xLoRaDevice NetMF Payload CRCs

  1. Pingback: Low power LoRaWan Node Model A328 Payload Addressing Client | devMobile's blog

  2. Pingback: Maduino LoRa Radio 868MHz | devMobile's blog

  3. Pingback: Low power LoRaWan Node Model B1248 Payload Addressing Client | devMobile's blog

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s