# The MIT License (MIT)
#
# Copyright (c) 2017 Radomir Dopieralski for Adafruit Industries.
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
"""
``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 ESP8622 and M0-based boards:
https://github.com/adafruit/circuitpython/releases
* 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.
"""
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=bad-whitespace, bad-continuation, 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