Source code for adafruit_max31855

# SPDX-FileCopyrightText: 2017 Radomir Dopieralski for Adafruit Industries
#
# SPDX-License-Identifier: MIT

"""
`adafruit_max31855`
===========================

This is a CircuitPython driver for the Maxim Integrated MAX31855 thermocouple
amplifier module.

* Author(s): Radomir Dopieralski

Implementation Notes
--------------------

**Hardware:**

* Adafruit `MAX31855 Thermocouple Amplifier Breakout
  <https://www.adafruit.com/product/269>`_ (Product ID: 269)

**Software and Dependencies:**

* Adafruit CircuitPython firmware for the supported boards:
  https://circuitpython.org/downloads

* Adafruit's Bus Device library: https://github.com/adafruit/Adafruit_CircuitPython_BusDevice

"""
import math

try:
    import struct
except ImportError:
    import ustruct as struct

from adafruit_bus_device.spi_device import SPIDevice

__version__ = "0.0.0-auto.0"
__repo__ = "https://github.com/adafruit/Adafruit_CircuitPython_MAX31855.git"


[docs]class MAX31855: """ Driver for the MAX31855 thermocouple amplifier. :param ~busio.SPI spi: The SPI bus the MAX31856 is connected to. :param ~microcontroller.Pin cs: The pin used for the CS signal. **Quickstart: Importing and using the MAX31855** Here is an example of using the :class:`MAX31855` class. First you will need to import the libraries to use the sensor .. code-block:: python import board from digitalio import DigitalInOut, Direction import adafruit_max31855 Once this is done you can define your `board.SPI` object and define your sensor object .. code-block:: python spi = board.SPI() cs = digitalio.DigitalInOut(board.D5) # Chip select of the MAX31855 board. sensor = adafruit_max31856.MAX31855(spi, cs) Now you have access to the :attr:`temperature` attribute .. code-block:: python temperature = sensor.temperature """ def __init__(self, spi, cs): self.spi_device = SPIDevice(spi, cs) self.data = bytearray(4) def _read(self, internal=False): with self.spi_device as spi: spi.readinto(self.data) # pylint: disable=no-member if self.data[3] & 0x01: raise RuntimeError("thermocouple not connected") if self.data[3] & 0x02: raise RuntimeError("short circuit to ground") if self.data[3] & 0x04: raise RuntimeError("short circuit to power") if self.data[1] & 0x01: raise RuntimeError("faulty reading") temp, refer = struct.unpack(">hh", self.data) refer >>= 4 temp >>= 2 if internal: return refer return temp @property def temperature(self): """Thermocouple temperature in degrees Celsius.""" return self._read() / 4 @property def reference_temperature(self): """Internal reference temperature in degrees Celsius.""" return self._read(True) * 0.0625 @property def temperature_NIST(self): """ Thermocouple temperature in degrees Celsius, computed using raw voltages and NIST approximation for Type K, see: https://srdata.nist.gov/its90/download/type_k.tab """ # pylint: disable=invalid-name # temperature of remote thermocouple junction TR = self.temperature # temperature of device (cold junction) TAMB = self.reference_temperature # thermocouple voltage based on MAX31855's uV/degC for type K (table 1) VOUT = 0.041276 * (TR - TAMB) # cold junction equivalent thermocouple voltage if TAMB >= 0: VREF = ( -0.176004136860e-01 + 0.389212049750e-01 * TAMB + 0.185587700320e-04 * math.pow(TAMB, 2) + -0.994575928740e-07 * math.pow(TAMB, 3) + 0.318409457190e-09 * math.pow(TAMB, 4) + -0.560728448890e-12 * math.pow(TAMB, 5) + 0.560750590590e-15 * math.pow(TAMB, 6) + -0.320207200030e-18 * math.pow(TAMB, 7) + 0.971511471520e-22 * math.pow(TAMB, 8) + -0.121047212750e-25 * math.pow(TAMB, 9) + 0.1185976 * math.exp(-0.1183432e-03 * math.pow(TAMB - 0.1269686e03, 2)) ) else: VREF = ( 0.394501280250e-01 * TAMB + 0.236223735980e-04 * math.pow(TAMB, 2) + -0.328589067840e-06 * math.pow(TAMB, 3) + -0.499048287770e-08 * math.pow(TAMB, 4) + -0.675090591730e-10 * math.pow(TAMB, 5) + -0.574103274280e-12 * math.pow(TAMB, 6) + -0.310888728940e-14 * math.pow(TAMB, 7) + -0.104516093650e-16 * math.pow(TAMB, 8) + -0.198892668780e-19 * math.pow(TAMB, 9) + -0.163226974860e-22 * math.pow(TAMB, 10) ) # total thermoelectric voltage VTOTAL = VOUT + VREF # determine coefficients # https://srdata.nist.gov/its90/type_k/kcoefficients_inverse.html if -5.891 <= VTOTAL <= 0: DCOEF = ( 0.0000000e00, 2.5173462e01, -1.1662878e00, -1.0833638e00, -8.9773540e-01, -3.7342377e-01, -8.6632643e-02, -1.0450598e-02, -5.1920577e-04, ) elif 0 < VTOTAL <= 20.644: DCOEF = ( 0.000000e00, 2.508355e01, 7.860106e-02, -2.503131e-01, 8.315270e-02, -1.228034e-02, 9.804036e-04, -4.413030e-05, 1.057734e-06, -1.052755e-08, ) elif 20.644 < VTOTAL <= 54.886: DCOEF = ( -1.318058e02, 4.830222e01, -1.646031e00, 5.464731e-02, -9.650715e-04, 8.802193e-06, -3.110810e-08, ) else: raise RuntimeError( "Total thermoelectric voltage out of range:{}".format(VTOTAL) ) # compute temperature TEMPERATURE = 0 for n, c in enumerate(DCOEF): TEMPERATURE += c * math.pow(VTOTAL, n) return TEMPERATURE