Feat: multi-sample averaging and 50Hz mains filtering

adafruit_max31856.py

@@ -49,6 +49,7 @@
 _MAX31856_CR0_CJ = const(0x08)
 _MAX31856_CR0_FAULT = const(0x04)
 _MAX31856_CR0_FAULTCLR = const(0x02)
+_MAX31856_CR0_50HZ = const(0x01)
 
 _MAX31856_CR1_REG = const(0x01)
 _MAX31856_MASK_REG = const(0x02)
@@ -76,6 +77,8 @@
 _MAX31856_FAULT_OVUV = const(0x02)
 _MAX31856_FAULT_OPEN = const(0x01)
 
+_AVGSEL_CONSTS = {1: 0x00, 2: 0x10, 4: 0x20, 8: 0x30, 16: 0x40}
+
 
 class ThermocoupleType: # pylint: disable=too-few-public-methods
  """An enum-like class representing the different types of thermocouples that the MAX31856 can
@@ -113,6 +116,8 @@ class MAX31856:
  :param ~microcontroller.Pin cs: The pin used for the CS signal.
  :param ~adafruit_max31856.ThermocoupleType thermocouple_type: The type of thermocouple.\
  Default is Type K.
+ :param ~int sampling: Number of samples to be averaged [1,2,4,8,16]
+ :param ~bool filter_50hz: Filter 50Hz mains frequency instead of 60Hz
 
  **Quickstart: Importing and using the MAX31856**
 
@@ -155,13 +160,64 @@ def __init__(self, spi, cs, thermocouple_type=ThermocoupleType.K):
  self._write_u8(_MAX31856_CR0_REG, _MAX31856_CR0_OCFAULT0)
 
  # set thermocouple type
+ self._set_thermocouple_type(thermocouple_type)
+
+ def _set_thermocouple_type(self, thermocouple_type: ThermocoupleType):
  # get current value of CR1 Reg
  conf_reg_1 = self._read_register(_MAX31856_CR1_REG, 1)[0]
  conf_reg_1 &= 0xF0 # mask off bottom 4 bits
  # add the new value for the TC type
  conf_reg_1 |= int(thermocouple_type) & 0x0F
  self._write_u8(_MAX31856_CR1_REG, conf_reg_1)
 
+ @property
+ def averaging(self) -> int:
+ """Number of samples averaged together in each result.
+ Must be 1, 2, 4, 8, or 16. Default is 1 (no averaging).
+ """
+ conf_reg_1 = self._read_register(_MAX31856_CR1_REG, 1)[0]
+ avgsel = conf_reg_1 & ~0b10001111 # clear bits other than 4-6
+ # check which byte this corresponds to
+ for key, value in _AVGSEL_CONSTS.items():
+ if value == avgsel:
+ return key
+ raise KeyError(f"AVGSEL bit pattern was not recognised ({avgsel:>08b})")
+
+ @averaging.setter
+ def averaging(self, num_samples: int):
+ # This option is set in bits 4-6 of register CR1.
+ if num_samples not in _AVGSEL_CONSTS:
+ raise ValueError("Num_samples must be one of 1,2,4,8,16")
+ avgsel = _AVGSEL_CONSTS[num_samples]
+ # get current value of CR1 Reg
+ conf_reg_1 = self._read_register(_MAX31856_CR1_REG, 1)[0]
+ conf_reg_1 &= 0b10001111 # clear bits 4-6
+ # OR the AVGSEL bits (4-6)
+ conf_reg_1 |= avgsel
+ self._write_u8(_MAX31856_CR1_REG, conf_reg_1)
+
+ @property
+ def noise_rejection(self) -> int:
+ """
+ The frequency (Hz) to be used by the noise rejection filter.
+ Must be 50 or 60. Default is 60."""
+ # this value is stored in bit 0 of register CR0.
+ conf_reg_0 = self._read_register(_MAX31856_CR0_REG, 1)[0]
+ if conf_reg_0 & _MAX31856_CR0_50HZ:
+ return 50
+ return 60
+
+ @noise_rejection.setter
+ def noise_rejection(self, frequency: int):
+ conf_reg_0 = self._read_register(_MAX31856_CR0_REG, 1)[0]
+ if frequency == 50:
+ conf_reg_0 |= _MAX31856_CR0_50HZ # set the 50hz bit
+ elif frequency == 60:
+ conf_reg_0 &= ~_MAX31856_CR0_50HZ # clear the 50hz bit
+ else:
+ raise ValueError("Frequency must be 50 or 60")
+ self._write_u8(_MAX31856_CR0_REG, conf_reg_0)
+
  @property
  def temperature(self):
  """Measure the temperature of the sensor and wait for the result.
@@ -170,7 +226,7 @@ def temperature(self):
  return self.unpack_temperature()
 
  def unpack_temperature(self) -> float:
- '''Reads the probe temperature from the register'''
+ """Reads the probe temperature from the register"""
  # unpack the 3-byte temperature as 4 bytes
  raw_temp = unpack(
  ">i", self._read_register(_MAX31856_LTCBH_REG, 3) + bytes([0])
@@ -184,16 +240,14 @@ def unpack_temperature(self) -> float:
 
  return temp_float
 
-
-
  @property
  def reference_temperature(self):
- """Wait to retreive temperature of the cold junction in degrees Celsius. (read-only)"""
+ """Wait to retrieve temperature of the cold junction in degrees Celsius. (read-only)"""
  self._perform_one_shot_measurement()
  return self.unpack_reference_temperature()
 
  def unpack_reference_temperature(self) -> float:
- '''Reads the reference temperature from the register'''
+ """Reads the reference temperature from the register"""
  raw_read = unpack(">h", self._read_register(_MAX31856_CJTH_REG, 2))[0]
 
  # effectively shift raw_read >> 8 to convert pseudo-float
@@ -277,13 +331,12 @@ def _perform_one_shot_measurement(self):
  # wait for the measurement to complete
  self._wait_for_oneshot()
 
-
  def initiate_one_shot_measurement(self):
- '''Starts a one-shot measurement and returns immediately.
+ """Starts a one-shot measurement and returns immediately.
  A measurement takes approximately 160ms.
  Check the status of the measurement with `oneshot_pending`; when it is false,
  the measurement is complete and the value can be read with `unpack_temperature`.
- '''
+ """
  # read the current value of the first config register
  conf_reg_0 = self._read_register(_MAX31856_CR0_REG, 1)[0]
 
@@ -295,20 +348,20 @@ def initiate_one_shot_measurement(self):
  # write it back with the new values, prompting the sensor to perform a measurement
  self._write_u8(_MAX31856_CR0_REG, conf_reg_0)
 
-
  @property
  def oneshot_pending(self) -> bool:
- '''A boolean indicating the status of the one-shot flag.
- A True value means the measurement is still ongoing.
- A False value means measurement is complete.'''
- oneshot_flag = self._read_register(_MAX31856_CR0_REG, 1)[0] & _MAX31856_CR0_1SHOT
+ """A boolean indicating the status of the one-shot flag.
+ A True value means the measurement is still ongoing.
+ A False value means measurement is complete."""
+ oneshot_flag = (
+ self._read_register(_MAX31856_CR0_REG, 1)[0] & _MAX31856_CR0_1SHOT
+ )
  return bool(oneshot_flag)
 
  def _wait_for_oneshot(self):
  while self.oneshot_pending:
  sleep(0.01)
 
-
  def _read_register(self, address, length):
  # pylint: disable=no-member
  # Read a 16-bit BE unsigned value from the specified 8-bit address.