herosdevices.hardware.raspberrypi.max318xx.max31865

MAX31865 Resistance to Digital Converter.

This module implements an interface for the MAX31865 resistance temperature detector (RTD) converter. The chip communicates over SPI. This implementation uses GPIO bit-banging on a Raspberry Pi. Using spidev would be more efficient, but initial attempts with spidev were unsuccessful.

This code is based on the work of steve71: https://github.com/steve71/MAX31865

Classes: - MAX31865: Provides methods to read resistance and temperature from PT100/PT1000 sensors.

  • Initialization allows configuration of SPI pins, reference resistance, and wiring type.

  • Methods to fetch resistance, convert resistance to temperature, and handle errors.

Usage: - Instantiate multiple sensor objects for different chip select pins. - Loop to read and print temperature readings. - Gracefully handle interruption and clean GPIO resources.

Notes: - Proper wiring and pull-down resistors are essential. - Optimized for use with a Raspberry Pi and RPi.GPIO.

Module Contents

herosdevices.hardware.raspberrypi.max318xx.max31865.A = 0.003908
herosdevices.hardware.raspberrypi.max318xx.max31865.B = -5.775e-07
herosdevices.hardware.raspberrypi.max318xx.max31865.C = -4.183e-12
herosdevices.hardware.raspberrypi.max318xx.max31865.c_v_d(r: float, r_0: float) → float

Calculate temperature using Callendar-Van Dusen equation.

This function converts resistance to temperature based on the Calendar-Van Dusen equation, which relates resistance in Ohms to temperature in Celsius. It uses numpy for polynomial root solving.

Parameters:

  • r (float) – Resistance in Ohms.

  • r_0 (float) – Resistance at 0°C in Ohms.

Returns:

Estimated temperature in degrees Celsius.

Return type:

float

Raises:

  • ImportError – If numpy is not installed.

  • ValueError – If the polynomial root calculation encounters issues (e.g., negative discriminant).

herosdevices.hardware.raspberrypi.max318xx.max31865.c_v_d_quad(r: float, r_0: float) → float

Approximate temperature using a quadratic form of the Callendar-Van Dusen equation.

This method offers a fast and reasonably accurate way to estimate temperature from resistance, suitable for class A PT100 sensors with about 0.2°C accuracy.

Parameters:

  • r (float) – Resistance in Ohms.

  • r_0 (float) – Resistance at 0°C in Ohms.

Returns:

Estimated temperature in degrees Celsius.

Return type:

float

Raises:

ValueError – If the discriminant in the quadratic formula is negative.

class herosdevices.hardware.raspberrypi.max318xx.max31865.MAX31865(cs_pin: int = 8, miso_pin: int = 9, mosi_pin: int = 10, clk_pin: int = 11, r_0: float = 1000.0, r_ref: float = 400.0, three_wire: bool = False)

Bases: herosdevices.hardware.raspberrypi.max318xx.max318xx.MAX318xx

Interface class for MAX31865 resistance temperature sensor.

Parameters:

  • cs_pin (int) – Chip select GPIO pin (default: 8).

  • miso_pin (int) – MISO GPIO pin (default: 9).

  • mosi_pin (int) – MOSI GPIO pin (default: 10).

  • clk_pin (int) – Clock GPIO pin (default: 11).

  • r_0 (float) – Resistance at 0°C, in Ohms (default: 1000 for PT1000).

  • r_ref (float) – Reference resistor resistance, in Ohms (default: 400).

  • three_wire (bool) – Use 3-wire configuration (default: False).

  • log (logging.Logger) – Optional logger for debug messages.

r_ref = 400.0
r_0 = 1000.0
config = 162
read_resistance(cs_pin: int) → float

Initiate resistance measurement and return resistance value.

Returns:

Resistance in Ohms.

Return type:

float

read_temp(convert: collections.abc.Callable[[float, float], float] = c_v_d_quad) → dict

Read resistance and convert to temperature.

Parameters:

convert (function) – Function to convert resistance to temperature. Defaults to c_v_d_quad.

Returns:

Temperature in Celsius.

Return type:

float

cs_pins
miso_pin = 9
mosi_pin = 10
clk_pin = 11
write_register(reg_num: int, data_byte: int) → None

Write a byte to a specified register.

Parameters:

  • regNum (int) – Register number to write to.

  • dataByte (int) – Data byte to write.

read_register(cs_pin: int, num_registers: int, reg_num_start: int | None = None) → list[int]

Read one or more bytes starting from a specified register.

Parameters:

  • cs_pin (int) – Chip select GPIO pin number.

  • numRegisters (int) – Number of bytes to read.

  • regNumStart (int, optional) – Starting register number for the read operation.

Returns:

List of bytes read from the device.

Return type:

list

send_byte(byte: int) → None

Send a byte via SPI by bit-banging.

Parameters:

byte (int) – Byte value to send.

recv_byte() → int

Receive a byte via SPI by bit-banging.

Returns:

Byte received.

Return type:

int

create_dict(values: list[float], observable: str, unit: str) → dict

Build dictionary of observables to be returned by the sensor.