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Basic library for the MAX31865

 
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Ttelmah



Joined: 11 Mar 2010
Posts: 17476

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Basic library for the MAX31865
PostPosted: Thu Aug 26, 2021 11:58 am     Reply with quote

This is based on some code posted by Jody, fixed by me, and adapted to
use spi_xfer. Gives a basis for anyone wanting to use this chip.

//main31865.c
Code:

#include <18F87J50.h>
#device ADC=10

#FUSES NOWDT                    //No Watch Dog Timer
#FUSES NOXINST
#FUSES NOIESO

#use delay(crystal=8000000)

#use SPI(SPI1, baud=4000000, MODE=1, bits=32, STREAM=MAX)

#include "math.h"
#include "stdint.h"
#include "MAX31865.h"

void main()
{
   float   Rref    = 470.0;
   uint16_t   result    = 0;
   float   Temperature_vriezer;
   uint16_t   vriezer;
   float   Resistance;
   
   output_high(MAX31865_CS);
   delay_ms(100); //ensure chip has time to wake up
   MAX31865_init(); //wake up chip
   
   while(1)
   {
      result = MAX31865_read_data();
      if (result & 1)
      {
         //error bit is set, read the error status
         result=MAX31865_read8(MAX31856_FAULTSTAT_REG);
         //here add fault diagmostics
      }
      else
      {
         result>>=1; //remove low bit
         if(result == 0x7fff) result = 0;
         Resistance = ((double)result*Rref)/32768;
         Temperature_vriezer = CallendarVanDusen(Resistance);
         vriezer = (int16) Temperature_vriezer;
         delay_ms(17); //minimum 16.5mSec between readings;
      }
   }
}


//max31865.h
Code:

/***************************************************************************************/
// SPI chip select pin
#define MAX31865_CS  PIN_D1

#define MAX31856_CONFIG_REG_READ       0x00
#define MAX31856_CONFIG_REG_WRITE      0x80
#define MAX31856_CONFIG_BIAS           0x80
#define MAX31856_CONFIG_MODEAUTO       0x40
#define MAX31856_CONFIG_MODEOFF        0x00
#define MAX31856_CONFIG_1SHOT          0x20
#define MAX31856_CONFIG_3WIRE          0x10
#define MAX31856_CONFIG_24WIRE         0x00
#define MAX31856_CONFIG_FAULTSTAT      0x02
#define MAX31856_CONFIG_FILT50HZ       0x01
#define MAX31856_CONFIG_FILT60HZ       0x00

#define MAX31856_RTDMSB_REG           0x01
#define MAX31856_RTDLSB_REG           0x02
#define MAX31856_HFAULTMSB_REG        0x03
#define MAX31856_HFAULTLSB_REG        0x04
#define MAX31856_LFAULTMSB_REG        0x05
#define MAX31856_LFAULTLSB_REG        0x06
#define MAX31856_FAULTSTAT_REG        0x07

#define MAX31865_FAULT_HIGHTHRESH     0x80
#define MAX31865_FAULT_LOWTHRESH      0x40
#define MAX31865_FAULT_REFINLOW       0x20
#define MAX31865_FAULT_REFINHIGH      0x10
#define MAX31865_FAULT_RTDINLOW       0x08
#define MAX31865_FAULT_OVUV           0x04
/***************************************************************************************/

//---------------------------------------
// Call this function to read a 16bit value from a register pair
uint16_t MAX31865_read16(int regno)
{
   uint16_t rtd;

   output_low(MAX31865_CS);
   delay_us(1);
   spi_xfer(MAX,regno,8); //send the register number
   rtd = spi_xfer(MAX,0L,16);  // clock out 16 dummy bits and read the reply
   delay_us(1);
   output_high(MAX31865_CS);
   return(rtd);
}

// Call this function to read an 8bit value from a register
uint8_t MAX31865_read8(int regno)
{
   uint8_t rtd;

   output_low(MAX31865_CS);
   delay_us(1);
   spi_xfer(MAX,regno,8); //send the register number
   rtd = spi_xfer(MAX,0,8);  // clock out 8 dummy bits and read the reply
   delay_us(1);
   output_high(MAX31865_CS);
   return(rtd);
}

//specific version for data register
#define MAX31865_read_data() MAX31865_read16(MAX31856_RTDMSB_REG)

//---------------------------------------
// Call this function to write a 8-bit value
// to a MAX31865 register.
void MAX31865_write_register(uint8_t reg_address,uint8_t value)
{
   output_low(MAX31865_CS);
   delay_us(1);
   spi_xfer(MAX,reg_address,8);
   spi_xfer(MAX,value,8);
   delay_us(1);
   output_high(MAX31865_CS);
}

//Resistance to degrees celcius - your PIC does not support a double type
float CallendarVanDusen(float r)
{
   
   float a     = 3.9083E-03;
   float b     = -5.7750E-07;
   float R0    = 100;//bij 0C
   float deel1 = -R0*a;
   float deel2 = ((R0*a)*(R0*a));
   float deel3 = 4*(R0*b*(R0-r));
   float deel4 = 2*R0*b;
   float deel5 = sqrt(deel2 - deel3);   

   float temp = (deel1+deel5)/deel4;
   if(r == 0) temp =0;
delay_us(1);
   return (temp);
}

void MAX31865_init(void)
{
     MAX31865_write_register(MAX31856_CONFIG_REG_WRITE,0xC3);//BIAS, AUTO, 50Hz
   delay_ms(63); //minimum 62.5mSec before a result can be read
}


Remember needs to be a 3.3v PIC.
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