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