# 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
import struct
from adafruit_bus_device.spi_device import SPIDevice
try:
import typing # pylint: disable=unused-import
from digitalio import DigitalInOut
from busio import SPI
except ImportError:
pass
__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: SPI, cs: DigitalInOut # pylint: disable=invalid-name
) -> None:
self.spi_device = SPIDevice(spi, cs)
self.data = bytearray(4)
def _read(self, internal: bool = False) -> int:
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) -> float:
"""Thermocouple temperature in degrees Celsius."""
return self._read() / 4
@property
def reference_temperature(self) -> float:
"""Internal reference temperature in degrees Celsius."""
return self._read(True) * 0.0625
@property
def temperature_NIST(self) -> float:
"""
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(f"Total thermoelectric voltage out of range:{VTOTAL}")
# compute temperature
TEMPERATURE = 0
for n, c in enumerate(DCOEF):
TEMPERATURE += c * math.pow(VTOTAL, n)
return TEMPERATURE