firmware/src/rf95/RH_RF95.cpp

// RH_RF95.cpp
//
// Copyright (C) 2011 Mike McCauley
// $Id: RH_RF95.cpp,v 1.22 2020/01/05 07:02:23 mikem Exp mikem $

#include <RH_RF95.h>

// Interrupt vectors for the 3 Arduino interrupt pins
// Each interrupt can be handled by a different instance of RH_RF95, allowing you to have
// 2 or more LORAs per Arduino
RH_RF95 *RH_RF95::_deviceForInterrupt[RH_RF95_NUM_INTERRUPTS] = {0, 0, 0};
uint8_t RH_RF95::_interruptCount = 0; // Index into _deviceForInterrupt for next device

// These are indexed by the values of ModemConfigChoice
// Stored in flash (program) memory to save SRAM
PROGMEM static const RH_RF95::ModemConfig MODEM_CONFIG_TABLE[] = {
    //  1d,     1e,      26
    {0x72, 0x74, 0x04}, // Bw125Cr45Sf128 (the chip default), AGC enabled
    {0x92, 0x74, 0x04}, // Bw500Cr45Sf128, AGC enabled
    {0x48, 0x94, 0x04}, // Bw31_25Cr48Sf512, AGC enabled
    {0x78, 0xc4, 0x0c}, // Bw125Cr48Sf4096, AGC enabled

};

RH_RF95::RH_RF95(uint8_t slaveSelectPin, uint8_t interruptPin, RHGenericSPI &spi)
    : RHSPIDriver(slaveSelectPin, spi), _rxBufValid(0)
{
    _interruptPin = interruptPin;
    _myInterruptIndex = 0xff; // Not allocated yet
}

bool RH_RF95::init()
{
    if (!RHSPIDriver::init())
        return false;

    // Determine the interrupt number that corresponds to the interruptPin
    int interruptNumber = digitalPinToInterrupt(_interruptPin);
    if (interruptNumber == NOT_AN_INTERRUPT)
        return false;
#ifdef RH_ATTACHINTERRUPT_TAKES_PIN_NUMBER
    interruptNumber = _interruptPin;
#endif

    // Tell the low level SPI interface we will use SPI within this interrupt
    spiUsingInterrupt(interruptNumber);

    // No way to check the device type :-(

    // Add by Adrien van den Bossche <vandenbo@univ-tlse2.fr> for Teensy
    // ARM M4 requires the below. else pin interrupt doesn't work properly.
    // On all other platforms, its innocuous, belt and braces
    pinMode(_interruptPin, INPUT);

    bool isWakeFromDeepSleep =
        false; // true if we think we are waking from deep sleep AND the rf95 seems to have a valid configuration

    if (!isWakeFromDeepSleep) {
        // Set sleep mode, so we can also set LORA mode:
        spiWrite(RH_RF95_REG_01_OP_MODE, RH_RF95_MODE_SLEEP | RH_RF95_LONG_RANGE_MODE);
        delay(10); // Wait for sleep mode to take over from say, CAD
        // Check we are in sleep mode, with LORA set
        if (spiRead(RH_RF95_REG_01_OP_MODE) != (RH_RF95_MODE_SLEEP | RH_RF95_LONG_RANGE_MODE)) {
            //	Serial.println(spiRead(RH_RF95_REG_01_OP_MODE), HEX);
            return false; // No device present?
        }

        // Set up FIFO
        // We configure so that we can use the entire 256 byte FIFO for either receive
        // or transmit, but not both at the same time
        spiWrite(RH_RF95_REG_0E_FIFO_TX_BASE_ADDR, 0);
        spiWrite(RH_RF95_REG_0F_FIFO_RX_BASE_ADDR, 0);

        // Packet format is preamble + explicit-header + payload + crc
        // Explicit Header Mode
        // payload is TO + FROM + ID + FLAGS + message data
        // RX mode is implmented with RXCONTINUOUS
        // max message data length is 255 - 4 = 251 octets

        setModeIdle();

        // Set up default configuration
        // No Sync Words in LORA mode.
        setModemConfig(Bw125Cr45Sf128); // Radio default
                                        //    setModemConfig(Bw125Cr48Sf4096); // slow and reliable?
        setPreambleLength(8);           // Default is 8
        // An innocuous ISM frequency, same as RF22's
        setFrequency(434.0);
        // Lowish power
        setTxPower(13);

        Serial.printf("IRQ flag mask 0x%x\n", spiRead(RH_RF95_REG_11_IRQ_FLAGS_MASK));
    } else {
        // FIXME
        // restore mode base off reading RS95 registers

        // Only let CPU enter deep sleep if RF95 is sitting waiting on a receive or is in idle or sleep.
    }

    // geeksville: we do this last, because if there is an interrupt pending from during the deep sleep, this attach will cause it
    // to be taken.

    // Set up interrupt handler
    // Since there are a limited number of interrupt glue functions isr*() available,
    // we can only support a limited number of devices simultaneously
    // ON some devices, notably most Arduinos, the interrupt pin passed in is actuallt the
    // interrupt number. You have to figure out the interruptnumber-to-interruptpin mapping
    // yourself based on knwledge of what Arduino board you are running on.
    if (_myInterruptIndex == 0xff) {
        // First run, no interrupt allocated yet
        if (_interruptCount <= RH_RF95_NUM_INTERRUPTS)
            _myInterruptIndex = _interruptCount++;
        else
            return false; // Too many devices, not enough interrupt vectors
    }
    _deviceForInterrupt[_myInterruptIndex] = this;
    if (_myInterruptIndex == 0)
        attachInterrupt(interruptNumber, isr0, RISING);
    else if (_myInterruptIndex == 1)
        attachInterrupt(interruptNumber, isr1, RISING);
    else if (_myInterruptIndex == 2)
        attachInterrupt(interruptNumber, isr2, RISING);
    else
        return false; // Too many devices, not enough interrupt vectors

    return true;
}

void RH_RF95::prepareDeepSleep()
{
    // Determine the interrupt number that corresponds to the interruptPin
    int interruptNumber = digitalPinToInterrupt(_interruptPin);

    detachInterrupt(interruptNumber);
}

bool RH_RF95::isReceiving()
{
    // 0x0b == Look for header info valid, signal synchronized or signal detected
    uint8_t reg = spiRead(RH_RF95_REG_18_MODEM_STAT) & 0x1f;
    // Serial.printf("reg %x\n", reg);
    return _mode == RHModeRx && (reg & (RH_RF95_MODEM_STATUS_SIGNAL_DETECTED | RH_RF95_MODEM_STATUS_SIGNAL_SYNCHRONIZED |
                                        RH_RF95_MODEM_STATUS_HEADER_INFO_VALID)) != 0;
}

// C++ level interrupt handler for this instance
// LORA is unusual in that it has several interrupt lines, and not a single, combined one.
// On MiniWirelessLoRa, only one of the several interrupt lines (DI0) from the RFM95 is usefuly
// connnected to the processor.
// We use this to get RxDone and TxDone interrupts
void RH_RF95::handleInterrupt()
{
    // Read the interrupt register
    uint8_t irq_flags = spiRead(RH_RF95_REG_12_IRQ_FLAGS);

    // Note: there can be substantial latency between ISR assertion and this function being run, therefore
    // multiple flags might be set.  Handle them all

    // Note: we are running the chip in continuous receive mode (currently, so RX_TIMEOUT shouldn't ever occur)
    bool haveRxError = irq_flags & (RH_RF95_RX_TIMEOUT | RH_RF95_PAYLOAD_CRC_ERROR);
    if (haveRxError)
    //    if (_mode == RHModeRx && irq_flags & (RH_RF95_RX_TIMEOUT | RH_RF95_PAYLOAD_CRC_ERROR))
    {
        _rxBad++;
        clearRxBuf();
    }

    if ((irq_flags & RH_RF95_RX_DONE) && !haveRxError) {
        // Read the RegHopChannel register to check if CRC presence is signalled
        // in the header. If not it might be a stray (noise) packet.*
        uint8_t crc_present = spiRead(RH_RF95_REG_1C_HOP_CHANNEL) & RH_RF95_RX_PAYLOAD_CRC_IS_ON;

        spiWrite(RH_RF95_REG_12_IRQ_FLAGS, 0xff); // Clear all IRQ flags, required before reading fifo (according to datasheet)

        if (!crc_present) {
            _rxBad++;
            clearRxBuf();
        } else {
            // Have received a packet
            uint8_t len = spiRead(RH_RF95_REG_13_RX_NB_BYTES);

            // Reset the fifo read ptr to the beginning of the packet
            spiWrite(RH_RF95_REG_0D_FIFO_ADDR_PTR, spiRead(RH_RF95_REG_10_FIFO_RX_CURRENT_ADDR));
            spiBurstRead(RH_RF95_REG_00_FIFO, _buf, len);
            _bufLen = len;

            // Remember the last signal to noise ratio, LORA mode
            // Per page 111, SX1276/77/78/79 datasheet
            _lastSNR = (int8_t)spiRead(RH_RF95_REG_19_PKT_SNR_VALUE) / 4;

            // Remember the RSSI of this packet, LORA mode
            // this is according to the doc, but is it really correct?
            // weakest receiveable signals are reported RSSI at about -66
            _lastRssi = spiRead(RH_RF95_REG_1A_PKT_RSSI_VALUE);
            // Adjust the RSSI, datasheet page 87
            if (_lastSNR < 0)
                _lastRssi = _lastRssi + _lastSNR;
            else
                _lastRssi = (int)_lastRssi * 16 / 15;
            if (_usingHFport)
                _lastRssi -= 157;
            else
                _lastRssi -= 164;

            // We have received a message.
            validateRxBuf();
            if (_rxBufValid)
                setModeIdle(); // Got one
        }
    }

    if (irq_flags & RH_RF95_TX_DONE) {
        _txGood++;
        setModeIdle();
    }

    if (_mode == RHModeCad && (irq_flags & RH_RF95_CAD_DONE)) {
        _cad = irq_flags & RH_RF95_CAD_DETECTED;
        setModeIdle();
    }

    // ack all interrupts, note - we did this already in the RX_DONE case above, and we don't want to do it twice
    if (!(irq_flags & RH_RF95_RX_DONE)) {
        // Sigh: on some processors, for some unknown reason, doing this only once does not actually
        // clear the radio's interrupt flag. So we do it twice. Why?
        // kevinh: turn this off until root cause is known, because it can cause missed interrupts!
        // spiWrite(RH_RF95_REG_12_IRQ_FLAGS, 0xff); // Clear all IRQ flags
        spiWrite(RH_RF95_REG_12_IRQ_FLAGS, 0xff); // Clear all IRQ flags
    }
}

// These are low level functions that call the interrupt handler for the correct
// instance of RH_RF95.
// 3 interrupts allows us to have 3 different devices
void RH_INTERRUPT_ATTR RH_RF95::isr0()
{
    if (_deviceForInterrupt[0])
        _deviceForInterrupt[0]->handleInterrupt();
}
void RH_INTERRUPT_ATTR RH_RF95::isr1()
{
    if (_deviceForInterrupt[1])
        _deviceForInterrupt[1]->handleInterrupt();
}
void RH_INTERRUPT_ATTR RH_RF95::isr2()
{
    if (_deviceForInterrupt[2])
        _deviceForInterrupt[2]->handleInterrupt();
}

// Check whether the latest received message is complete and uncorrupted
void RH_RF95::validateRxBuf()
{
    if (_bufLen < 4)
        return; // Too short to be a real message
    // Extract the 4 headers
    _rxHeaderTo = _buf[0];
    _rxHeaderFrom = _buf[1];
    _rxHeaderId = _buf[2];
    _rxHeaderFlags = _buf[3];
    if (_promiscuous || _rxHeaderTo == _thisAddress || _rxHeaderTo == RH_BROADCAST_ADDRESS) {
        _rxGood++;
        _rxBufValid = true;
    }
}

bool RH_RF95::available()
{
    if (_mode == RHModeTx)
        return false;
    setModeRx();
    return _rxBufValid; // Will be set by the interrupt handler when a good message is received
}

void RH_RF95::clearRxBuf()
{
    ATOMIC_BLOCK_START;
    _rxBufValid = false;
    _bufLen = 0;
    ATOMIC_BLOCK_END;
}

bool RH_RF95::send(const uint8_t *data, uint8_t len)
{
    if (len > RH_RF95_MAX_MESSAGE_LEN)
        return false;

    waitPacketSent(); // Make sure we dont interrupt an outgoing message
    setModeIdle();

    if (!waitCAD())
        return false; // Check channel activity

    // Position at the beginning of the FIFO
    spiWrite(RH_RF95_REG_0D_FIFO_ADDR_PTR, 0);
    // The headers
    spiWrite(RH_RF95_REG_00_FIFO, _txHeaderTo);
    spiWrite(RH_RF95_REG_00_FIFO, _txHeaderFrom);
    spiWrite(RH_RF95_REG_00_FIFO, _txHeaderId);
    spiWrite(RH_RF95_REG_00_FIFO, _txHeaderFlags);
    // The message data
    spiBurstWrite(RH_RF95_REG_00_FIFO, data, len);
    spiWrite(RH_RF95_REG_22_PAYLOAD_LENGTH, len + RH_RF95_HEADER_LEN);

    setModeTx(); // Start the transmitter
    // when Tx is done, interruptHandler will fire and radio mode will return to STANDBY
    return true;
}

bool RH_RF95::printRegisters()
{
#ifdef RH_HAVE_SERIAL
    uint8_t registers[] = {0x01, 0x06, 0x07, 0x08,  0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10,
                           0x11, 0x12, 0x13, 0x014, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c,
                           0x1d, 0x1e, 0x1f, 0x20,  0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27};

    uint8_t i;
    for (i = 0; i < sizeof(registers); i++) {
        Serial.print(registers[i], HEX);
        Serial.print(": ");
        Serial.println(spiRead(registers[i]), HEX);
    }
#endif
    return true;
}

uint8_t RH_RF95::maxMessageLength()
{
    return RH_RF95_MAX_MESSAGE_LEN;
}

bool RH_RF95::setFrequency(float centre)
{
    // Frf = FRF / FSTEP
    uint32_t frf = (centre * 1000000.0) / RH_RF95_FSTEP;
    spiWrite(RH_RF95_REG_06_FRF_MSB, (frf >> 16) & 0xff);
    spiWrite(RH_RF95_REG_07_FRF_MID, (frf >> 8) & 0xff);
    spiWrite(RH_RF95_REG_08_FRF_LSB, frf & 0xff);
    _usingHFport = (centre >= 779.0);

    return true;
}

void RH_RF95::setModeIdle()
{
    if (_mode != RHModeIdle) {
        spiWrite(RH_RF95_REG_01_OP_MODE, RH_RF95_MODE_STDBY);
        _mode = RHModeIdle;
    }
}

bool RH_RF95::sleep()
{
    if (_mode != RHModeSleep) {
        spiWrite(RH_RF95_REG_01_OP_MODE, RH_RF95_MODE_SLEEP);
        _mode = RHModeSleep;
    }
    return true;
}

void RH_RF95::setModeRx()
{
    if (_mode != RHModeRx) {
        spiWrite(RH_RF95_REG_01_OP_MODE, RH_RF95_MODE_RXCONTINUOUS);
        spiWrite(RH_RF95_REG_40_DIO_MAPPING1, 0x00); // Interrupt on RxDone
        _mode = RHModeRx;
    }
}

void RH_RF95::setModeTx()
{
    if (_mode != RHModeTx) {
        spiWrite(RH_RF95_REG_01_OP_MODE, RH_RF95_MODE_TX);
        spiWrite(RH_RF95_REG_40_DIO_MAPPING1, 0x40); // Interrupt on TxDone
        _mode = RHModeTx;
    }
}

void RH_RF95::setTxPower(int8_t power, bool useRFO)
{
    // Sigh, different behaviours depending on whther the module use PA_BOOST or the RFO pin
    // for the transmitter output
    if (useRFO) {
        if (power > 14)
            power = 14;
        if (power < -1)
            power = -1;
        spiWrite(RH_RF95_REG_09_PA_CONFIG, RH_RF95_MAX_POWER | (power + 1));
    } else {
        if (power > 23)
            power = 23;
        if (power < 5)
            power = 5;

        // For RH_RF95_PA_DAC_ENABLE, manual says '+20dBm on PA_BOOST when OutputPower=0xf'
        // RH_RF95_PA_DAC_ENABLE actually adds about 3dBm to all power levels. We will us it
        // for 21, 22 and 23dBm
        if (power > 20) {
            spiWrite(RH_RF95_REG_4D_PA_DAC, RH_RF95_PA_DAC_ENABLE);
            power -= 3;
        } else {
            spiWrite(RH_RF95_REG_4D_PA_DAC, RH_RF95_PA_DAC_DISABLE);
        }

        // RFM95/96/97/98 does not have RFO pins connected to anything. Only PA_BOOST
        // pin is connected, so must use PA_BOOST
        // Pout = 2 + OutputPower.
        // The documentation is pretty confusing on this topic: PaSelect says the max power is 20dBm,
        // but OutputPower claims it would be 17dBm.
        // My measurements show 20dBm is correct
        spiWrite(RH_RF95_REG_09_PA_CONFIG, RH_RF95_PA_SELECT | (power - 5));
    }
}

// Sets registers from a canned modem configuration structure
void RH_RF95::setModemRegisters(const ModemConfig *config)
{
    spiWrite(RH_RF95_REG_1D_MODEM_CONFIG1, config->reg_1d);
    spiWrite(RH_RF95_REG_1E_MODEM_CONFIG2, config->reg_1e);
    spiWrite(RH_RF95_REG_26_MODEM_CONFIG3, config->reg_26);
}

// Set one of the canned FSK Modem configs
// Returns true if its a valid choice
bool RH_RF95::setModemConfig(ModemConfigChoice index)
{
    if (index > (signed int)(sizeof(MODEM_CONFIG_TABLE) / sizeof(ModemConfig)))
        return false;

    ModemConfig cfg;
    memcpy_P(&cfg, &MODEM_CONFIG_TABLE[index], sizeof(RH_RF95::ModemConfig));
    setModemRegisters(&cfg);

    return true;
}

void RH_RF95::setPreambleLength(uint16_t bytes)
{
    spiWrite(RH_RF95_REG_20_PREAMBLE_MSB, bytes >> 8);
    spiWrite(RH_RF95_REG_21_PREAMBLE_LSB, bytes & 0xff);
}

bool RH_RF95::isChannelActive()
{
    // Set mode RHModeCad
    if (_mode != RHModeCad) {
        spiWrite(RH_RF95_REG_01_OP_MODE, RH_RF95_MODE_CAD);
        spiWrite(RH_RF95_REG_40_DIO_MAPPING1, 0x80); // Interrupt on CadDone
        _mode = RHModeCad;
    }

    while (_mode == RHModeCad)
        YIELD;

    return _cad;
}

void RH_RF95::enableTCXO()
{
    while ((spiRead(RH_RF95_REG_4B_TCXO) & RH_RF95_TCXO_TCXO_INPUT_ON) != RH_RF95_TCXO_TCXO_INPUT_ON) {
        sleep();
        spiWrite(RH_RF95_REG_4B_TCXO, (spiRead(RH_RF95_REG_4B_TCXO) | RH_RF95_TCXO_TCXO_INPUT_ON));
    }
}

// From section 4.1.5 of SX1276/77/78/79
// Ferror = FreqError * 2**24 * BW / Fxtal / 500
int RH_RF95::frequencyError()
{
    int32_t freqerror = 0;

    // Convert 2.5 bytes (5 nibbles, 20 bits) to 32 bit signed int
    // Caution: some C compilers make errors with eg:
    // freqerror = spiRead(RH_RF95_REG_28_FEI_MSB) << 16
    // so we go more carefully.
    freqerror = spiRead(RH_RF95_REG_28_FEI_MSB);
    freqerror <<= 8;
    freqerror |= spiRead(RH_RF95_REG_29_FEI_MID);
    freqerror <<= 8;
    freqerror |= spiRead(RH_RF95_REG_2A_FEI_LSB);
    // Sign extension into top 3 nibbles
    if (freqerror & 0x80000)
        freqerror |= 0xfff00000;

    int error = 0; // In hertz
    float bw_tab[] = {7.8, 10.4, 15.6, 20.8, 31.25, 41.7, 62.5, 125, 250, 500};
    uint8_t bwindex = spiRead(RH_RF95_REG_1D_MODEM_CONFIG1) >> 4;
    if (bwindex < (sizeof(bw_tab) / sizeof(float)))
        error = (float)freqerror * bw_tab[bwindex] * ((float)(1L << 24) / (float)RH_RF95_FXOSC / 500.0);
    // else not defined

    return error;
}

int RH_RF95::lastSNR()
{
    return _lastSNR;
}

///////////////////////////////////////////////////
//
// additions below by Brian Norman 9th Nov 2018
// brian.n.norman@gmail.com
//
// Routines intended to make changing BW, SF and CR
// a bit more intuitive
//
///////////////////////////////////////////////////

void RH_RF95::setSpreadingFactor(uint8_t sf)
{
    if (sf <= 6)
        sf = RH_RF95_SPREADING_FACTOR_64CPS;
    else if (sf == 7)
        sf = RH_RF95_SPREADING_FACTOR_128CPS;
    else if (sf == 8)
        sf = RH_RF95_SPREADING_FACTOR_256CPS;
    else if (sf == 9)
        sf = RH_RF95_SPREADING_FACTOR_512CPS;
    else if (sf == 10)
        sf = RH_RF95_SPREADING_FACTOR_1024CPS;
    else if (sf == 11)
        sf = RH_RF95_SPREADING_FACTOR_2048CPS;
    else if (sf >= 12)
        sf = RH_RF95_SPREADING_FACTOR_4096CPS;

    // set the new spreading factor
    spiWrite(RH_RF95_REG_1E_MODEM_CONFIG2, (spiRead(RH_RF95_REG_1E_MODEM_CONFIG2) & ~RH_RF95_SPREADING_FACTOR) | sf);
    // check if Low data Rate bit should be set or cleared
    setLowDatarate();
}

void RH_RF95::setSignalBandwidth(long sbw)
{
    uint8_t bw; // register bit pattern

    if (sbw <= 7800)
        bw = RH_RF95_BW_7_8KHZ;
    else if (sbw <= 10400)
        bw = RH_RF95_BW_10_4KHZ;
    else if (sbw <= 15600)
        bw = RH_RF95_BW_15_6KHZ;
    else if (sbw <= 20800)
        bw = RH_RF95_BW_20_8KHZ;
    else if (sbw <= 31250)
        bw = RH_RF95_BW_31_25KHZ;
    else if (sbw <= 41700)
        bw = RH_RF95_BW_41_7KHZ;
    else if (sbw <= 62500)
        bw = RH_RF95_BW_62_5KHZ;
    else if (sbw <= 125000)
        bw = RH_RF95_BW_125KHZ;
    else if (sbw <= 250000)
        bw = RH_RF95_BW_250KHZ;
    else
        bw = RH_RF95_BW_500KHZ;

    // top 4 bits of reg 1D control bandwidth
    spiWrite(RH_RF95_REG_1D_MODEM_CONFIG1, (spiRead(RH_RF95_REG_1D_MODEM_CONFIG1) & ~RH_RF95_BW) | bw);
    // check if low data rate bit should be set or cleared
    setLowDatarate();
}

void RH_RF95::setCodingRate4(uint8_t denominator)
{
    int cr = RH_RF95_CODING_RATE_4_5;

    //    if (denominator <= 5)
    //	cr = RH_RF95_CODING_RATE_4_5;
    if (denominator == 6)
        cr = RH_RF95_CODING_RATE_4_6;
    else if (denominator == 7)
        cr = RH_RF95_CODING_RATE_4_7;
    else if (denominator >= 8)
        cr = RH_RF95_CODING_RATE_4_8;

    // CR is bits 3..1 of RH_RF95_REG_1D_MODEM_CONFIG1
    spiWrite(RH_RF95_REG_1D_MODEM_CONFIG1, (spiRead(RH_RF95_REG_1D_MODEM_CONFIG1) & ~RH_RF95_CODING_RATE) | cr);
}

void RH_RF95::setLowDatarate()
{
    // called after changing bandwidth and/or spreading factor
    //  Semtech modem design guide AN1200.13 says
    // "To avoid issues surrounding  drift  of  the  crystal  reference  oscillator  due  to  either  temperature  change
    // or  motion,the  low  data  rate optimization  bit  is  used. Specifically for 125  kHz  bandwidth  and  SF  =  11  and  12,
    // this  adds  a  small  overhead  to increase robustness to reference frequency variations over the timescale of the LoRa
    // packet."

    // read current value for BW and SF
    uint8_t BW = spiRead(RH_RF95_REG_1D_MODEM_CONFIG1) >> 4; // bw is in bits 7..4
    uint8_t SF = spiRead(RH_RF95_REG_1E_MODEM_CONFIG2) >> 4; // sf is in bits 7..4

    // calculate symbol time (see Semtech AN1200.22 section 4)
    float bw_tab[] = {7800, 10400, 15600, 20800, 31250, 41700, 62500, 125000, 250000, 500000};

    float bandwidth = bw_tab[BW];

    float symbolTime = 1000.0 * pow(2, SF) / bandwidth; // ms

    // the symbolTime for SF 11 BW 125 is 16.384ms.
    // and, according to this :-
    // https://www.thethingsnetwork.org/forum/t/a-point-to-note-lora-low-data-rate-optimisation-flag/12007
    // the LDR bit should be set if the Symbol Time is > 16ms
    // So the threshold used here is 16.0ms

    // the LDR is bit 3 of RH_RF95_REG_26_MODEM_CONFIG3
    uint8_t current = spiRead(RH_RF95_REG_26_MODEM_CONFIG3) & ~RH_RF95_LOW_DATA_RATE_OPTIMIZE; // mask off the LDR bit
    if (symbolTime > 16.0)
        spiWrite(RH_RF95_REG_26_MODEM_CONFIG3, current | RH_RF95_LOW_DATA_RATE_OPTIMIZE);
    else
        spiWrite(RH_RF95_REG_26_MODEM_CONFIG3, current);
}

void RH_RF95::setPayloadCRC(bool on)
{
    // Payload CRC is bit 2 of register 1E
    uint8_t current = spiRead(RH_RF95_REG_1E_MODEM_CONFIG2) & ~RH_RF95_PAYLOAD_CRC_ON; // mask off the CRC

    if (on)
        spiWrite(RH_RF95_REG_1E_MODEM_CONFIG2, current | RH_RF95_PAYLOAD_CRC_ON);
    else
        spiWrite(RH_RF95_REG_1E_MODEM_CONFIG2, current);
}