Library to interface with the MAX31855 Cold Junction Compensated Thermocouple-to-Digital Converter

Dependents:   max31855Sample Reflow_Oven_Controller test_draft Soldering_Tips_Thermometer

Interface library for the MAX31855 Cold Junction Compendated Thermocouple-to-Digital Converter. This part is the reaplacement for the now discontinued MAX6675. The library is based on the original MAX6675 library found on this site altered to fit the serial specification of the new chip as well as adding a few new features.

The main new feature of note is that the library itself deals with the 0.25 second delay required to complete a conversion. Polling the 'ready()' function will return a '1' when the chip is read to submit a new reading.

Sample Program

#include "mbed.h"
#include "max31855.h"

DigitalOut myled(LED1);

//SPI Interfaces
SPI testSPI(p11,p12,p13);

max31855 max1(testSPI,p21);

int main() {
    //Initialise chip (starts internal timer)
    //Float value to hold temperature returned
    float fvalue = 0;
    while(1) {
        //Check if the chip is ready for a reading to be taken
        if (max1.ready()==1){
            //Get the reading
            fvalue = max1.read_temp();
            if (fvalue > 2000){
                    printf("No TC");
                }else if(fvalue==2002){
                    printf("Short to GND");
                }else if(fvalue==2004){
                    printf("Short to VCC");
                printf("Temperature is: %f\n\r", fvalue);
        //Heartbeat signal (not necessary)
        myled = !myled;
        //Delay is not required, here simply for test program

Fault Codes:

  • 1 (return value of 2001) - No Thermocouple detected
  • 2 (return value of 2002) - Thermocouple short to GND
  • 4 (return value of 2004) - Thermocouple short to VCC



File content as of revision 1:5eeee89cb281:

#include <mbed.h>
#include "max31855.h"

max31855::max31855(SPI& _spi, PinName _ncs) : spi(_spi), ncs(_ncs) {


float max31855::read_temp() {
    short value = 0;
    float temp = 0;
    //Variables to hold probe temperature
    uint8_t tempProbeHigh=0;
    uint8_t tempProbeLow=0;
    //Variables to hold chip temperature and device status
    uint8_t tempChipHigh=0;
    uint8_t tempChipLow=0;
    if (pollTimer.read_ms() > 250){
        //Set CS to initiate transfer and stop conversion
        //Read in Probe tempeature
        tempProbeHigh = spi.write(0);
        tempProbeLow = spi.write(0);
        //Get the chip temperature and the fault data
        tempChipHigh = spi.write(0);
        tempChipLow = spi.write(0);
        //Set the chip temperature    
        chipTemp = (tempChipHigh<<4 | tempChipLow>>4)*0.25;
        //Set CS to stop transfer and restart conversion
        //Check for a fault (last bit of transfer is fault bit)
        if ((tempProbeLow & 1)==1){
            //Chip reports a fault, extract fault from Chip Temperature data
            int faultType = (tempChipLow & 7);
            return 2000+faultType;
            /*if (faultType==1){
                //Open circuit (no TC)
                return 2000 + faultType;
            }else if (faultType==2){
                //Short to GND
                return 2000 + faultType;
            }else if (faultType==4){
                //Short to VCC               
                return 0.4;
                return 0.5;
            //Integer value of temperature
            value = (tempProbeHigh<< 6 | tempProbeLow>>2);
            //Get actual temperature (last 2 bits of integer are decimal 0.5 and 0.25)
            temp = (value*0.25); // Multiply the value by 0.25 to get temp in C or
                             //  * (9.0/5.0)) + 32.0;   // Convert value to F (ensure proper floats!)
            return temp;
        //Chip not ready for reading
        return -1;

void max31855::select() {
    //Set CS low to start transmission (interrupts conversion)
    ncs = 0;

void max31855::deselect() {
    //Set CS high to stop transmission (restarts conversion)
    ncs = 1;
    //Reset conversion timer

void max31855::initialise(int setType) {
    //Start the conversion timer

int max31855::ready() {
    //Check to see if conversion is complete
    if (pollTimer.read_ms() > 250) {
        //Conversion complete
        return 1;
        //Conversion incomplete
        return 0;