The MAX31865 is a resistance-to-digital converter optimised for platinum RTDs and communicates via SPI. It typically uses four SPI lines: CS (chip select), SCLK (clock), SDI (data in), and SDO (data out). If you're only using these four pins and not the DRDY (data ready) pin, the microcontroller must wait the maximum conversion time before reading data to ensure accuracy—especially in 1-shot mode, where each conversion must be manually triggered via a register write.
For temperature readings between 15°C and 50°C, you can use the simplified formula:
Temperature ≈ (ADC code / 32) - 256,
Though for higher precision, the Callendar-Van Dusen equation is recommended.
If readings appear frozen, check that SPI timing is correct, conversions are properly triggered, and the microcontroller waits long enough before reading the result.
The MAX31865 also includes overvoltage protection and uses a precision delta-sigma ADC to digitize RTD resistance. When using a PT100 with a 400Ω reference resistor, expect around 82 ADC steps per 1Ω change in resistance.