Maxim Integrated MAX31865 RTD to digital converter

Hardware driver for mbedOS

Required hardware

For prototyping the following hardware is recommended:

e.g. NUCLEO_L476RG - a bare-metal STM32 development board
e.g. Adafruit MAX31865 - MAX31865 breakout board (there are cheaper versions available)
e.g. AT102 PT100 - 4-wire temperature sensor

The library will work with any board supported by mbedOS. Enable RTOS for the best experience

How to connect an RTD to MAX31865 device

For wiring refer to RTD Wiring & Config

Library features

Covers the entire functionality of the device
Non-blocking timeouts when RTOS is enabled

System design (typical application)

Interfacing with 1.8V MCU

Maximum difference between MCU and MAX DVDD should be 1.5V. When using

Hardware

Provide a capacitor between RTDIN+ and RTDIN- as a filter in noisy environments
Do not solder any jumpers on the breakout board for the 4-wire setup
It is recommended to use an external pullup resistor for the DRDY-pin (indicating a finished conversion)
Do not use any pull-ups on SDI/SDO or SCLK line, they do not have any influcence on the data communication, SPI is internally switched with push-pull logic
A finished conversion is indicated by a falling edge

Software

The device has only 8 bit registers

General

Faults

Faults are only supported in one-shot conversion mode (NORMALLY OFF bit 6 in config register must be set)
The device supports automatic fault detection and manual fault detection; the bits 2 and 3 must be set in the configuration register
Faults are encoded in and read from the Fault Status Register 0x07 (each bit of the register represents one fault)

Characteristics

the device does not have an on-board EEPROM; the configuration has to be set after every power cycle; use power-cylce for hard-resetting the device

Examples

General

Configure the pins (MOSI, MISO, CLK, CS, RDY) in the mbed_lib.json file
Also, configure the nominal resistance of the RTD (typically 100Ω or 1000Ω) and matching resistance in the mbed_lib.json file

max31865_oneshot.cpp

This example shows how to set up the device in one-shot mode

Known issues

the time constant in data sheet depends on the external RTD interface circuitry which is not accounted for. Suggestions to implement that are appreciated

References

Datasheet Maxim Integrated MAX31865 RTD-to-Digital Converter [1]