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ESP32-MAX31865

A driver for the MAX31865 RTD-to-Digital converter, written using/for the esp-idf framework. Supports all documented features of the MAX31865 including fault detection and the DRDY output.

Example

See examples/simple/main/main.cpp.

Reference

Constructor

Max31856(int miso, int mosi, int sck, int cs, int drdy = -1, spi_host_device_t host = HSPI_HOST)
Parameters
  • int miso, int mosi, int sck, int cs - GPIO numbers for SPI communication to the chip
  • int drdy - GPIO number for the DRDY output of the MAX31865. Not used if equal to -1. If set, RTD reads will block until DRDY is low.
  • spi_host_device_t host - ESP32 SPI host device, useful if the application needs to communicate with other SPI devices simultaneously.

Static methods

float RTDtoTemperature(uint16_t rtd, max31865_rtd_config_t rtdConfig)
Parameters
  • uint16_t rtd - 15-bit RTD resistance value read from the MAX31865
  • max31865_rtd_config_t rtdConfig - configuration relating to the RTD itself
Returns
  • float - the temperature value in degrees Celsius represented by the RTD resistance with the given configuration. Uses the Callendar-van Dusen equation with constants taken from here.
uint16_t temperatureToRTD(float temperature, max31865_rtd_config_t rtdConfig)
Parameters
  • float temperature - a temperature in degrees Celsius.
  • max31865_rtd_config_t rtdConfig - configuration relating to the RTD itself
Returns
  • uint16_t - 15-bit RTD resistance value as would be represented in the MAX31865 registers.
const char *errorToString(Max31865Error error)
Parameters
  • Max31865Error error - an error type read from the MAX31865 fault register.
Returns
  • const char* - a string describing the error type.

Instance methods

esp_err_t begin(max31865_config_t config)

Initialises the communication with the MAX31865, and configures it. Must be called before any other instance methods!

Parameters
  • max31865_config_t config - configuration for the MAX31865
Returns
  • esp_err_t - any error encountered when communicating with the chip.
esp_err_t setConfig(max31865_config_t config)

Sets the configuration for an already initialized instance.

Parameters
  • max31865_config_t config - configuration for the MAX31865
esp_err_t getConfig(max31865_config_t *config)

Gets the current configuration from a MAX31865.

Parameters
  • max31865_config_t *config - pointer to the configuration struct to be populated with the current state.
esp_err_t setRTDThresholds(uint16_t min, uint16_t max)

Sets the threshold RTD resistances (not temperatures) below/above which the MAX31865 will declare a fault. Use with MAX31865::temperatureToRTD for setting min/max temperatures.

Parameters
  • uint16_t min - minimum RTD resistance value (15-bit)
  • uint16_t max - maximum RTD resistance value (15-bit)
esp_err_t clearFault()

Clears the fault status bit in the config register.

esp_err_t readFaultStatus(Max31865Error *fault);

Reads the specific fault type out of the fault status register.

Parameters
  • Max31865Error *fault - pointer to the fault type variable to be filled (output param).
esp_err_t getRTD(uint16_t *rtd, Max31865Error *fault = nullptr)

Gets the current RTD resistance (15-bit value) from the MAX31865 and optionally populates a fault variable. Fails with ESP_ERR_INVALID_RESPONSE if a fault is detected. Does the following in the process of reading:

  • Enables a 1-shot read and/or Vbias if they are not available, and blocks for 65ms and 10ms, respectively, after doing so in order to have valid reads.
  • If a pin for DRDY was given, blocks until receiving a negative edge interrupt on this pin.
  • Reads the fault bit, and if equal to 1 reads the full fault status using readingFaultStatus
  • Restores Vbias to 0 if it was changed for the read.
Parameters
  • uint16_t *rtd - pointer to the RTD resistance value (15-bit) to populate.
  • Max31865Error *fault - optionally, an error type to populate if a fault is detected.

Types

struct max31865_config_t
Members
  • bool vbias - Enables Vbias if true
  • bool autoConversion - Enables automatic conversion if true (otherwise 1-shot)
  • Max31865NWires nWires - 2, 3 or 4-wire RTD configuration
  • Max31865FaultDetection faultDetection - which fault detection cycle mode to enable
  • Max31865Filter filter - mains sinc filter frequenct
struct max31865_rtd_config_t
Members
  • float ref - reference resistor value (Ohms)
  • float nominal - nominal RTD resistance at 0 degrees C (eg PT100 or PT1000)
enum class Max31865NWires

RTD wiring configuration

  • Three
  • Two - default
  • Four - default
enum class Max31865FaultDetection

Fault detection cycle

  • NoAction - default
  • AutoDelay - Fault detection with automatic delay
  • ManualDelayCycle1 - Run fault detection with manual delay (cycle 1)
  • ManualDelayCycle2 - Finish fault detection with manual delay (cycle 2)
enum class Max31865Filter

Mains frequency filter

  • Hz50 - filter 50Hz mains
  • Hz60 - filter 60Hz mains (default)
enum class Max31865Error

Fault status register error type

  • NoError - default
  • Voltage - Overvoltage or undervoltage fault
  • RTDInLow - RTDIN- < 0.85*VBIAS (FORCE- open)
  • RefLow - REFIN- < 0.85*VBIAS (FORCE- open)
  • RefHigh - REFIN- > 0.85*VBIAS
  • RTDLow - RTD below low threshold
  • RTDHigh - RTD above high threshold