How to Use MAX31856: Examples, Pinouts, and Specs

The MAX31856, manufactured by Maxim Integrated, is a high-precision thermocouple-to-digital converter designed for accurate temperature measurement applications. It supports a wide range of thermocouple types, including K, J, N, R, S, T, E, and B, making it versatile for various industrial and scientific applications. The device features integrated cold-junction compensation, digital filtering, and communicates via an SPI interface, ensuring reliable and precise temperature readings.

• Industrial temperature monitoring and control
• Scientific research and laboratory equipment
• HVAC systems
• Food processing and storage
• Automotive and aerospace temperature sensing

The MAX31856 is available in a 10-pin TDFN package. Below is the pinout and description:

1. Power Supply: Connect the VDD pin to a 3.3V power source and the GND pin to ground.
2. Thermocouple Connection: Attach the positive lead of the thermocouple to the T+ pin and the negative lead to the T- pin.
3. SPI Communication: Connect the CS, SCK, SDI, and SDO pins to the corresponding SPI pins on your microcontroller.
4. Bypass Capacitor: Place a 0.1µF ceramic capacitor close to the VDD and GND pins for power supply decoupling.
5. Pull-Up Resistors: Use pull-up resistors on the SPI lines if required by your microcontroller.

• Ensure the thermocouple leads are properly connected to avoid polarity issues.
• Use shielded cables for the thermocouple to minimize noise interference.
• Avoid placing the MAX31856 near high-frequency switching components to reduce EMI.
• Calibrate the system if high accuracy is required for your application.

Below is an example of how to interface the MAX31856 with an Arduino UNO using the SPI library:

#include <SPI.h>

// Define MAX31856 SPI pins
#define CS_PIN 10  // Chip Select pin

// MAX31856 Registers
#define REG_CR0 0x00  // Configuration Register 0
#define REG_CR1 0x01  // Configuration Register 1
#define REG_LTCBH 0x0C // High byte of temperature data

void setup() {
  // Initialize Serial Monitor
  Serial.begin(9600);
  
  // Initialize SPI
  SPI.begin();
  pinMode(CS_PIN, OUTPUT);
  digitalWrite(CS_PIN, HIGH); // Set CS high to start
  
  // Configure MAX31856
  writeRegister(REG_CR0, 0x80); // Enable conversion mode
  writeRegister(REG_CR1, 0x03); // Set thermocouple type (e.g., Type K)
}

void loop() {
  // Read temperature data
  int16_t tempData = readTemperature();
  float temperature = tempData * 0.0078125; // Convert to Celsius
  
  // Print temperature to Serial Monitor
  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.println(" °C");
  
  delay(1000); // Wait 1 second before next reading
}

// Function to write to a MAX31856 register
void writeRegister(uint8_t reg, uint8_t value) {
  digitalWrite(CS_PIN, LOW); // Select the MAX31856
  SPI.transfer(reg | 0x80);  // Set MSB to 1 for write operation
  SPI.transfer(value);       // Write the value
  digitalWrite(CS_PIN, HIGH); // Deselect the MAX31856
}

// Function to read temperature data
int16_t readTemperature() {
  digitalWrite(CS_PIN, LOW); // Select the MAX31856
  SPI.transfer(REG_LTCBH & 0x7F); // Set MSB to 0 for read operation
  uint8_t msb = SPI.transfer(0x00); // Read high byte
  uint8_t lsb = SPI.transfer(0x00); // Read low byte
  digitalWrite(CS_PIN, HIGH); // Deselect the MAX31856
  
  return (msb << 8) | lsb; // Combine high and low bytes
}

1. No Temperature Reading:

   • Ensure the thermocouple is properly connected to the T+ and T- pins.
   • Verify that the SPI connections are correct and secure.
   • Check the power supply voltage (3.3V) and ensure it is stable.

2. Incorrect Temperature Values:

   • Confirm the thermocouple type is correctly configured in the MAX31856 registers.
   • Check for noise or interference on the thermocouple leads.
   • Verify the cold-junction compensation is functioning properly.

3. SPI Communication Fails:

   • Ensure the CS, SCK, SDI, and SDO pins are correctly connected to the microcontroller.
   • Verify the SPI clock speed is within the MAX31856's supported range.

Q: Can the MAX31856 measure multiple thermocouples simultaneously?
A: No, the MAX31856 is designed to interface with a single thermocouple at a time. For multiple thermocouples, you will need multiple MAX31856 devices or a multiplexer.

Q: What is the accuracy of the MAX31856?
A: The accuracy depends on the thermocouple type and temperature range. For example, with a Type K thermocouple, the accuracy is typically ±0.15% of the reading.

Q: Can I use the MAX31856 with a 5V microcontroller?
A: Yes, but you will need level shifters to interface the 3.3V MAX31856 with the 5V logic of the microcontroller.