LoRa RX-Single Mode Inconsistent RX-Timeout Interrupts

Question

I'm trying to implement LoRa communications using an SX1276 module (DRF1276G) and an ATmega328P on a custom PCB. I've got it mostly working but I'm having a strange issue with the timeout interrupt. I've set it to use RX-Single mode, with a timeout for the full 1023-symbols, but SOMETIMES it times out before that.

EDIT: Note that for the tests i've shown below, SF=7 and BW=125kHz, so the symbol period is ~1.02ms, so timeout window should be ~1043ms. The reported timeout window in the screenshots in only an approximation, as i'm dividing these values with integers. No floating-points here. But you can assume that if it's something like 643ms or 27ms, it is a shorter amount of time before the timeout flag fired.

See the image below, where 1023 symbols is indicated by it taking "~1026 ms" to time out. Anything much less than that amount of time is a shorter period for the receive-function to return, indicated the RX-timeout flag was fired before the expected 1023sym/"~1026ms" window. [EDIT: Please ignore the "TX..." text, the program doesn't actually transmit anything, i just forgot to change that in my print-out]

My test setup is simply listening for incoming packets. No packets are being transmitted.

ONE TIME ONLY, in the (i'm estimating) 4 hours of total run-time of these boards, this happened...

Here's a summary of my testing:

• It happens on all my devices, so its not faulty hardware
• Testing Dragino boards in the same environment, the Draginos work perfectly. Indicates it's not environment noise/transmissions (though the Draginos use RX-continuous mode not RX-single).
• A program to output RSSI readings shows a constant ~-100dBm output. Doesn't look like it's environment noise.
• Adding 5-second delays between receive-function calls doesn't help.
• Changing settings such as data rate, frequency, coding rate, sync word, preamble length etc. don't affect its behaviour.
• Testing multiple power configurations and CRO testing show's it's not a power issue.
• Testing showed it's not noise on the SPI bus causing wrong reads of the interrupt flag register.

Googling hasn't yielded any answers, and the only thing in the datasheet I have to go off is the following snippet...

Thing is, I'm not transmitting anything for it to pick up.

Here is the relevant code.

Main loop

// main loop
while(1) {  
    USARTSendString("RX...\n");
    rxPktNo = sx127X_RxPacket((uint8_t *)localDataBuffer);

    USARTSendString("timeout: ~");
    USARTSendString(itoa(sx127X_lastPacketDurationMilli(),string,10));
    USARTSendString("ms\n");

    // if data was received, print stuff
    if (rxPktNo) {
        printMessageContentsAndStats(rxPktNo);
        resetContentsOfDataBuffer();
    }
}

Receive Routine

/* ------------------------------------------/
    Set to single receive mode and poll for 
    the reception of a single packet of data.
    Upon reception write data from FIFO 
    buffer to the given array.
  ------------------------------------------*/
uint8_t sx127X_RxPacket(uint8_t* dataPtr) {
    uint8_t var = 0; // dual-purpose variable
    uint8_t returnValue = 0;

    // Enter standby mode
    setMode(MODE_STDBY);

    // Configure for RX if not done already
    if (rxtxConfig != CONFIGGED_FOR_RX) {
        sx127X_ConfigRX();
    }

    // Poll for RX-ongoing status, wait till stopped
    //while (sx127X_SPIReadReg(LR_RegModemStat) & 0x04);

    // Clear interrupts
    sx127X_ClearIrq();

    // Set to receive mode
    setMode(MODE_RXSINGLE);

    // Start timer
    TCNT1 = 0;

    // Poll for timeout, sx127x will go to MODE_STDBY upon timeout
    while (!(sx127X_SPIReadReg(LR_RegIrqFlags) & 0x80)) {

        // Poll for the RX-done flag
        if (sx127X_SPIReadReg(LR_RegIrqFlags) & 0x40) {

            // Read starting address of the last received packet...
            var = sx127X_SPIReadReg(LR_RegFifoRxCurrentaddr);
            // ... Write it to the address pointer for the FIFO buffer
            sx127X_SPIWriteReg(LR_RegFifoAddrPtr, var);

            // Retrieve packet size of last received packet
            var = sx127X_SPIReadReg(LR_RegRxNbBytes);

            // Use packet size and FIFO to read RX packet
            sx127X_SPIBurstRead(LR_RegFifo, dataPtr, var);

            // If successfully retrieved a packet, return true with number of bytes
            returnValue = var;
            break;
        }
    }
    // give timer value to global variable
    lastTimeoutDuration = TCNT1;

    // If no packet has been retrieved this call return false
    return returnValue;
}

Settings

/* -------------------------------------------------------- /
  Configuration settings common to both the receiver and 
  transmitter modes of operation.
  ------------------------------------------------------- */ 
void sx127X_ConfigCommon(void) {
    /*------------ Hardware Settings ------------*/
    // Set the access-shared-registers mode
    sx127X_setASRMode(ASR_LORA);
    // Set low or high frequency mode
    sx127X_setFrequencyMode(LFM_HF);
    // Set either PA_RFO (+14dB) or PA_BOOST (+20dB)
    sx127X_setPaSelect(PA_BOOST);
    // Set the output power in dBm (PA_BOOST 2 to 17)
    sx127x_SetPowerDBm(14);
    // Set the PA ramp-up time
    sx127X_setPARampTime(RAMP_40US);
    // Set overload current protection
    sx127X_setOCP(OCP_ON);
    // Set overload current protection limit
    sx127X_setOCPLimit(OCP_100MA);
    // Set the low noise amp gain
    sx127X_setLNAGain(LNA_GAIN_G1);
    // set LNA boost for low frequency
    sx127X_setLNABoostLF(LNA_BOOST_LF_DEFAULT);
    // set LNA boost for high frequency
    sx127X_setLNABoostHF(LNA_BOOST_HF_DEFAULT);
    // Set LNA gain manual or auto
    sx127X_setLNAGainAuto(LNA_GAIN_AUTO_ON);

    /*----------- Communication Settings -----------*/
    // Set base frequency (channel) of the LoRa module
    sx127X_setChannel(channel);
    // Set the data rate (bandwidth and spreading factor) of the LoRa module
    sx127X_setDataRate(dataRate);
    // Set the coding rate of the LoRa module
    sx127X_setCodeRate(CR_4OF8);
    // Set the toggle for implicit header
    sx127X_setImplHeaderMode(IHM_OFF); // Explicit
    // Set TX continuous mode on or off(normal/packet mode)
    sx127X_setTXContMode(TXCM_NORM);
    // Set RX payload CRC mode on or off
    sx127X_setRXPayloadCRC(RXPL_CRC_ON);
    // Set the RX symbol timeout in symbols
    sx127X_setRXSymbolTimeout(SYMB_TIMEOUT_1023SYM);
    // Set preamble length in symbols
    sx127X_setPreambleLength(8);
    // Set the payload length in bytes
    sx127X_setPayloadLength(0x01); // default
    // Set payload maximum length in bytes
    sx127X_setPayloadMaxLength(0x80);
    // Set the frequency hopping period (0=disable)
    sx127X_setFreqHopPeriod(0);
    // Set Sync Word
    sx127X_setSyncWord(0xF3);
    // Select inversion of I and Q signals
    sx127X_invertIQ(INVERT_IQ_OFF);
}

/* -------------------------------------------------------- /
  Configure the sx127x module for receiver operation.
  Use entire FIFO.
  ------------------------------------------------------- */
void sx127X_ConfigRX(void) {
    uint8_t addr;
    // Set DIO0 for the RX done flag
    sx127X_SPIWriteReg(REG_LR_DIOMAPPING1, 0x00);
    // Set interrupt mask for RX timeout and RX done
    sx127X_SPIWriteReg(LR_RegIrqFlagsMask, 0x3F);

    // Set the RX base address to bottom of FIFO
    sx127X_SPIWriteReg(LR_RegFifoRxBaseAddr, 0x00);
    // Set address pointer to bottom of FIFO as well
    sx127X_SPIWriteReg(LR_RegFifoAddrPtr, 0x00);

    // locally store RX mode config flag
    rxtxConfig = CONFIGGED_FOR_RX;
}

If anyone knows what the issue might be, or can give me more ideas to test, it would be much appreciated.