MAX31856 STM32 Driver Implementation

MAX31856 STM32 Driver

MAX31856 is a high-precision thermocouple amplifier that supports various thermocouple types (K, J, N, etc.), with built-in cold junction compensation and open-circuit detection. Below is the driver implementation based on STM32, including SPI communication, configuration, and data reading functionalities.

1. Hardware Connections

MAX31856 Pin	STM32 Pin	Description
SCK	SPI_SCK	Serial Clock
MISO	SPI_MISO	Master In / Slave Out
MOSI	SPI_MOSI	Master Out / Slave In
CS	GPIO (e.g., PA4)	Chip Select Signal (active low)
VDD	3.3V	Power Supply
GND	GND	Ground

2. Driver Code Implementation

(1) Header File Definition (max31856.h)

#ifndef MAX31856_H
#define MAX31856_H
#include "stm32f4xx_hal.h"
// MAX31856 Register Addresses
#define MAX31856_CR0_REG    0x00  // Configuration Register 0
#define MAX31856_CR1_REG    0x01  // Configuration Register 1
#define MAX31856_MASK_REG   0x02  // Mask Register
#define MAX31856_CJHF_REG   0x03  // Cold Junction High Temperature Threshold
#define MAX31856_CJLF_REG   0x04  // Cold Junction Low Temperature Threshold
#define MAX31856_LTHFTH_REG 0x05  // Thermocouple High Temperature Threshold High Byte
#define MAX31856_LTHFTL_REG 0x06  // Thermocouple High Temperature Threshold Low Byte
#define MAX31856_LTLFTH_REG 0x07  // Thermocouple Low Temperature Threshold High Byte
#define MAX31856_LTLFTL_REG 0x08  // Thermocouple Low Temperature Threshold Low Byte
#define MAX31856_CJTO_REG   0x09  // Cold Junction Temperature Offset
#define MAX31856_CJTH_REG   0x0A  // Cold Junction Temperature High Byte
#define MAX31856_CJTL_REG   0x0B  // Cold Junction Temperature Low Byte
#define MAX31856_LTCBH_REG  0x0C  // Thermocouple Temperature High Byte
#define MAX31856_LTCBM_REG  0x0D  // Thermocouple Temperature Middle Byte
#define MAX31856_LTCBL_REG  0x0E  // Thermocouple Temperature Low Byte
#define MAX31856_SR_REG     0x0F  // Status Register

// Thermocouple Type Selection (CR1 Register)
typedef enum {
  MAX31856_TC_TYPE_B  = 0x00,
  MAX31856_TC_TYPE_E  = 0x01,
  MAX31856_TC_TYPE_J  = 0x02,
  MAX31856_TC_TYPE_K  = 0x03,
  MAX31856_TC_TYPE_N  = 0x04,
  MAX31856_TC_TYPE_R  = 0x05,
  MAX31856_TC_TYPE_S  = 0x06,
  MAX31856_TC_TYPE_T  = 0x07,
} MAX31856_TC_TypeDef;

// Device Structure
typedef struct {
  SPI_HandleTypeDef* hspi;       // SPI Handle
  GPIO_TypeDef*      cs_port;    // CS Pin Port
  uint16_t           cs_pin;     // CS Pin Number
  MAX31856_TC_TypeDef tc_type;   // Thermocouple Type
} MAX31856_HandleTypeDef;

// Function Declarations
HAL_StatusTypeDef MAX31856_Init(MAX31856_HandleTypeDef* hmax);
float MAX31856_Read_Temp(MAX31856_HandleTypeDef* hmax);
float MAX31856_Read_CJTemp(MAX31856_HandleTypeDef* hmax);
uint8_t MAX31856_Read_Status(MAX31856_HandleTypeDef* hmax);
#endif

(2) Driver Implementation (max31856.c)

#include "max31856.h"
// SPI Read/Write Functions
static void MAX31856_SPI_Write(MAX31856_HandleTypeDef* hmax, uint8_t reg, uint8_t data) {
  uint8_t tx_buf[2] = {reg & 0x7F, data}; // Write Operation: MSB 0
  HAL_GPIO_WritePin(hmax->cs_port, hmax->cs_pin, GPIO_PIN_RESET);
  HAL_SPI_Transmit(hmax->hspi, tx_buf, 2, HAL_MAX_DELAY);
  HAL_GPIO_WritePin(hmax->cs_port, hmax->cs_pin, GPIO_PIN_SET);
}

static uint8_t MAX31856_SPI_Read(MAX31856_HandleTypeDef* hmax, uint8_t reg) {
  uint8_t tx_buf = reg | 0x80; // Read Operation: MSB 1
  uint8_t rx_buf = 0;
  HAL_GPIO_WritePin(hmax->cs_port, hmax->cs_pin, GPIO_PIN_RESET);
  HAL_SPI_Transmit(hmax->hspi, &tx_buf, 1, HAL_MAX_DELAY);
  HAL_SPI_Receive(hmax->hspi, &rx_buf, 1, HAL_MAX_DELAY);
  HAL_GPIO_WritePin(hmax->cs_port, hmax->cs_pin, GPIO_PIN_SET);
  return rx_buf;
}

// Initialize MAX31856
HAL_StatusTypeDef MAX31856_Init(MAX31856_HandleTypeDef* hmax) {
  // Configure CR0: Enable Cold Junction Compensation, Disable Open-Circuit Detection
  MAX31856_SPI_Write(hmax, MAX31856_CR0_REG, 0x80);
  // Configure CR1: Select Thermocouple Type
  MAX31856_SPI_Write(hmax, MAX31856_CR1_REG, hmax->tc_type &lt;&lt; 4);
  // Read Status Register for Verification
  if (MAX31856_Read_Status(hmax) != 0) {
    return HAL_ERROR;
  }
  return HAL_OK;
}

// Read Thermocouple Temperature (°C)
float MAX31856_Read_Temp(MAX31856_HandleTypeDef* hmax) {
  uint8_t temp_h = MAX31856_SPI_Read(hmax, MAX31856_LTCBH_REG);
  uint8_t temp_m = MAX31856_SPI_Read(hmax, MAX31856_LTCBM_REG);
  uint8_t temp_l = MAX31856_SPI_Read(hmax, MAX31856_LTCBL_REG);
  // Combine 24-bit Data (Sign Extension)
  int32_t temp = ((int32_t)temp_h &lt;&lt; 16) | ((uint16_t)temp_m &lt;&lt; 8) | temp_l;
  temp &gt;&gt;= 5; // Low 5 bits are invalid
  // Convert to Temperature Value (1 LSB = 0.0078125 °C)
  return temp * 0.0078125f;
}

// Read Cold Junction Temperature (°C)
float MAX31856_Read_CJTemp(MAX31856_HandleTypeDef* hmax) {
  uint8_t cj_h = MAX31856_SPI_Read(hmax, MAX31856_CJTH_REG);
  uint8_t cj_l = MAX31856_SPI_Read(hmax, MAX31856_CJTL_REG);
  // Combine 16-bit Data (Sign Extension)
  int16_t cj_temp = ((int16_t)cj_h &lt;&lt; 8) | cj_l;
  cj_temp &gt;&gt;= 4; // Low 4 bits are invalid
  // Convert to Temperature Value (1 LSB = 0.0625 °C)
  return cj_temp * 0.0625f;
}

// Read Status Register
uint8_t MAX31856_Read_Status(MAX31856_HandleTypeDef* hmax) {
  return MAX31856_SPI_Read(hmax, MAX31856_SR_REG);
}

(3) Usage Example (main.c)

#include "stm32f4xx_hal.h"
#include "max31856.h"
// SPI Handle (modify according to actual configuration)
SPI_HandleTypeDef hspi1;
// MAX31856 Handle
MAX31856_HandleTypeDef hmax31856 = {
  .hspi = &hspi1,
  .cs_port = GPIOA,
  .cs_pin = GPIO_PIN_4,
  .tc_type = MAX31856_TC_TYPE_K, // K-type thermocouple
};

int main(void) {
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();
  MX_SPI1_Init();
  // Initialize MAX31856
  if (MAX31856_Init(&hmax31856) != HAL_OK) {
    Error_Handler();
  }
  while (1) {
    float temp = MAX31856_Read_Temp(&hmax31856);
    float cj_temp = MAX31856_Read_CJTemp(&hmax31856);
    uint8_t status = MAX31856_Read_Status(&hmax31856);
    // Print Temperature Data (implement serial output)
    printf("Thermocouple Temperature: %.2f°C\r\n", temp);
    printf("Cold Junction Temperature: %.2f°C\r\n", cj_temp);
    printf("Status Register: 0x%02X\r\n", status);
    HAL_Delay(1000);
  }
}

3. Key Notes

SPI Configuration: STM32 SPI should be configured to Mode 1 (CPOL=0, CPHA=1), 8-bit data, MSB first.
Open-Circuit Detection: Can be enabled by configuringMASK_REG andCR0_REG, the status register will indicate the type of fault.
Precision Calibration: Can be adjusted usingCJTO_REG to improve measurement accuracy.
Thermocouple Type: Modify thetc_type parameter according to the actual thermocouple type used.

4. Troubleshooting

Abnormal Readings: Check if SPI communication is normal and if the CS pin level is correct.
Status Register Non-Zero:

Bit0: Thermocouple Open Circuit
Bit1: Thermocouple Shorted to VDD
Bit2: Thermocouple Shorted to GND
Bit3: Cold Junction High/Low Temperature Alarm

This driver can be directly adapted to STM32F4/F1 series; other series may require adjustments to GPIO and SPI initialization code.

I am using STM32F103RCT6,