Boiler-Controller/MAX31855.py at master · PCipolle/Boiler-Controller · GitHub

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import math
import spidev

spi = spidev.SpiDev()
spi.open(0,0)
spi.max_speed_hz = 5000

def readInternalC():
    """Return internal temperature value in degrees celsius."""
    v = _read32()
    # Ignore bottom 4 bits of thermocouple data.
    v >>= 4
    # Grab bottom 11 bits as internal temperature data.
    internal = v & 0x7FF
    if v & 0x800:
        # Negative value, take 2's compliment. Compute this with subtraction
        # because python is a little odd about handling signed/unsigned.
        internal -= 4096
    # Scale by 0.0625 degrees C per bit and return value.
    return internal * 0.0625

def readTempC():
    """Return the thermocouple temperature value in degrees celsius."""
    v = _read32()
    # Check for error reading value.
    if v & 0x7:
        return float('NaN')
    # Check if signed bit is set.
    if v & 0x80000000:
        # Negative value, take 2's compliment. Compute this with subtraction
        # because python is a little odd about handling signed/unsigned.
        v >>= 18
        v -= 16384
    else:
        # Positive value, just shift the bits to get the value.
        v >>= 18
    # Scale by 0.25 degrees C per bit and return value.
    return v * 0.25


def readLinearizedTempC():
    # MAX31855 thermocouple voltage reading in mV
    thermocoupleVoltage = (readTempC() - readInternalC()) * 0.041276
    # MAX31855 cold junction voltage reading in mV
    coldJunctionTemperature = readInternalC()
    coldJunctionVoltage = (-0.176004136860E-01 +
        0.389212049750E-01  * coldJunctionTemperature +
        0.185587700320E-04  * math.pow(coldJunctionTemperature, 2.0) +
        -0.994575928740E-07 * math.pow(coldJunctionTemperature, 3.0) +
        0.318409457190E-09  * math.pow(coldJunctionTemperature, 4.0) +
        -0.560728448890E-12 * math.pow(coldJunctionTemperature, 5.0) +
        0.560750590590E-15  * math.pow(coldJunctionTemperature, 6.0) +
        -0.320207200030E-18 * math.pow(coldJunctionTemperature, 7.0) +
        0.971511471520E-22  * math.pow(coldJunctionTemperature, 8.0) +
        -0.121047212750E-25 * math.pow(coldJunctionTemperature, 9.0) +
        0.118597600000E+00  * math.exp(-0.118343200000E-03 * math.pow((coldJunctionTemperature-0.126968600000E+03), 2.0)))
    # cold junction voltage + thermocouple voltage
    voltageSum = thermocoupleVoltage + coldJunctionVoltage
    # calculate corrected temperature reading based on coefficients for 3 different ranges
    # float b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, b10;
    if thermocoupleVoltage < 0:
        b0 = 0.0000000E+00
        b1 = 2.5173462E+01
        b2 = -1.1662878E+00
        b3 = -1.0833638E+00
        b4 = -8.9773540E-01
        b5 = -3.7342377E-01
        b6 = -8.6632643E-02
        b7 = -1.0450598E-02
        b8 = -5.1920577E-04
        b9 = 0.0000000E+00
    elif thermocoupleVoltage < 20.644:
        b0 = 0.000000E+00
        b1 = 2.508355E+01
        b2 = 7.860106E-02
        b3 = -2.503131E-01
        b4 = 8.315270E-02
        b5 = -1.228034E-02
        b6 = 9.804036E-04
        b7 = -4.413030E-05
        b8 = 1.057734E-06
        b9 = -1.052755E-08
    elif thermocoupleVoltage < 54.886:
        b0 = -1.318058E+02
        b1 = 4.830222E+01
        b2 = -1.646031E+00
        b3 = 5.464731E-02
        b4 = -9.650715E-04
        b5 = 8.802193E-06
        b6 = -3.110810E-08
        b7 = 0.000000E+00
        b8 = 0.000000E+00
        b9 = 0.000000E+00
    else:
        # TODO: handle error - out of range
        return 0
    return (b0 +
        b1 * voltageSum +
        b2 * pow(voltageSum, 2.0) +
        b3 * pow(voltageSum, 3.0) +
        b4 * pow(voltageSum, 4.0) +
        b5 * pow(voltageSum, 5.0) +
        b6 * pow(voltageSum, 6.0) +
        b7 * pow(voltageSum, 7.0) +
        b8 * pow(voltageSum, 8.0) +
        b9 * pow(voltageSum, 9.0))


def _read32():
    # Read 32 bits from the SPI bus.
    raw = spi.readbytes(4)
    if raw is None or len(raw) != 4:
        raise RuntimeError('Did not read expected number of bytes from device!')
    value = raw[0] << 24 | raw[1] << 16 | raw[2] << 8 | raw[3]
    return value