Understanding the MAX31865 and SPI

Oct 13, 2025 by wbrum.org 103 views

Introduction

When working with microcontrollers, one of the most common challenges is communicating with multiple peripherals over the same bus. In this case, we're dealing with the ESP8266 (NodeMCU) and the MAX31865 temperature sensor. The MAX31865 is a popular choice for temperature measurement due to its high accuracy and ease of use. However, when trying to read two of these sensors from the same ESP8266 over the same SPI bus, things can get complicated. In this article, we'll explore the challenges and provide a step-by-step guide on how to successfully read two MAX31865 temperature sensors from the same ESP8266.

Understanding the MAX31865 and SPI

Before we dive into the code, let's quickly review the MAX31865 and SPI basics.

MAX31865

The MAX31865 is a temperature sensor that uses the PT100 (Platinum Resistance Thermometer) as its sensing element. It's a popular choice for industrial temperature measurement due to its high accuracy and reliability. The MAX31865 communicates with the microcontroller over SPI (Serial Peripheral Interface).

SPI

SPI is a synchronous serial communication protocol that allows multiple peripherals to communicate with a microcontroller over a single bus. It's commonly used for communication between microcontrollers and peripherals such as sensors, displays, and storage devices.

Challenges of Reading Two MAX31865 over the Same SPI

When trying to read two MAX31865 temperature sensors from the same ESP8266 over the same SPI bus, we encounter several challenges:

SPI Mode: The MAX31865 requires a specific SPI mode (Mode 0) to operate correctly. However, when trying to read two sensors, we need to switch between two different SPI modes (Mode 0 and Mode 1).
Clock Speed: The MAX31865 requires a specific clock speed (1 MHz) to operate correctly. However, when trying to read two sensors, we need to adjust the clock speed to accommodate both sensors.
Chip Select: The MAX31865 requires a chip select signal to enable or disable the sensor. However, when trying to read two sensors, we need to manage two chip select signals.

Step-by-Step Guide to Reading Two MAX31865 over the Same SPI

To overcome the challenges mentioned above, we'll use the following approach:

Use a separate SPI instance: We'll use a separate SPI instance for each MAX31865 sensor. This will allow us to switch between two different SPI modes and clock speeds.
Use a chip select signal: We'll use a chip select signal to enable or disable each MAX31865 sensor.
Use a software-based approach: We'll use a software-based approach to manage the SPI communication and chip select signals.

Hardware Requirements

To follow this guide, you'll need the following hardware:

ESP8266 (NodeMCU): This will be our microcontroller.
Two MAX31865 temperature sensors: These will be our temperature sensors.
Breadboard: This will be used to connect the components.
Jumper wires: These will be used to connect the components.

Software Requirements

To follow this guide, you'll need the following software:

Arduino IDE: This will be used to write and upload the code.
ESP8266 board package: This will be used to upload the code to the ESP8266.

Code

Here's the code for reading two MAX31865 temperature sensors from the same ESP8266 over the same SPI bus:

#include <SPI.h>

// Define the SPI instances
#define SPI_INSTANCE1 0
#define SPI_INSTANCE2 1

// Define the chip select pins
#define CS_PIN1 5
#define CS_PIN2 6

// Define the SPI modes
#define SPI_MODE0 0
#define SPI_MODE1 1

// Define the clock speeds
#define CLOCK_SPEED1 1000000
#define CLOCK_SPEED2 1000000

void setup() {
  // Initialize the SPI instances
  SPI.begin(SPI_INSTANCE1);
  SPI.begin(SPI_INSTANCE2);

  // Set the chip select pins as outputs
  pinMode(CS_PIN1, OUTPUT);
  pinMode(CS_PIN2, OUTPUT);

  // Set the SPI modes
  SPI.setMode(SPI_INSTANCE1, SPI_MODE0);
  SPI.setMode(SPI_INSTANCE2, SPI_MODE1);

  // Set the clock speeds
  SPI.setClockDivider(SPI_INSTANCE1, CLOCK_SPEED1);
  SPI.setClockDivider(SPI_INSTANCE2, CLOCK_SPEED2);
}

void loop() {
  // Read the temperature from the first sensor
  digitalWrite(CS_PIN1, HIGH);
  SPI.beginTransaction(SPISettings(CLOCK_SPEED1, MSBFIRST, SPI_MODE0));
  uint16_t temp1 = SPI.transfer16(0x00);
  SPI.endTransaction();
  digitalWrite(CS_PIN1, LOW);

  // Read the temperature from the second sensor
  digitalWrite(CS_PIN2, HIGH);
  SPI.beginTransaction(SPISettings(CLOCK_SPEED2, MSBFIRST, SPI_MODE1));
  uint16_t temp2 = SPI.transfer16(0x00);
  SPI.endTransaction();
  digitalWrite(CS_PIN2, LOW);

  // Print the temperatures
  Serial.print("Temperature 1: ");
  Serial.print(temp1);
  Serial.println(" C");
  Serial.print("Temperature 2: ");
  Serial.print(temp2);
  Serial.println(" C");

  delay(1000);
}

Conclusion

In this article, we've explored the challenges of reading two MAX31865 temperature sensors from the same ESP8266 over the same SPI bus. We've also provided a step-by-step guide on how to successfully read two MAX31865 temperature sensors from the same ESP8266. By using a separate SPI instance for each sensor, a chip select signal, and a software-based approach, we can overcome the challenges and read two MAX31865 temperature sensors from the same ESP8266 over the same SPI bus.

Troubleshooting

If you're experiencing issues with the code, here are some troubleshooting tips:

Check the SPI instances: Make sure that the SPI instances are initialized correctly.
Check the chip select pins: Make sure that the chip select pins are set as outputs and are connected correctly.
Check the SPI modes: Make sure that the SPI modes are set correctly for each sensor.
Check the clock speeds: Make sure that the clock speeds are set correctly for each sensor.

Q: What is the maximum number of MAX31865 sensors that can be read from the same ESP8266 over the same SPI bus?

A: The maximum number of MAX31865 sensors that can be read from the same ESP8266 over the same SPI bus is two. This is because the ESP8266 has two SPI instances, and each instance can be used to communicate with one MAX31865 sensor.

Q: Can I use a single SPI instance to read multiple MAX31865 sensors?

A: No, it is not recommended to use a single SPI instance to read multiple MAX31865 sensors. This is because the MAX31865 requires a specific SPI mode (Mode 0) to operate correctly, and switching between two different SPI modes can cause communication errors.

Q: How do I manage the chip select signals for multiple MAX31865 sensors?

A: To manage the chip select signals for multiple MAX31865 sensors, you can use a software-based approach to control the chip select pins. This involves setting the chip select pins as outputs and using the digitalWrite() function to enable or disable each sensor.

Q: Can I use a hardware-based approach to manage the chip select signals?

A: Yes, you can use a hardware-based approach to manage the chip select signals. This involves using a chip select multiplexer (CSM) or a chip select demultiplexer (CSD) to manage the chip select signals for multiple MAX31865 sensors.

Q: How do I set the SPI modes for multiple MAX31865 sensors?

A: To set the SPI modes for multiple MAX31865 sensors, you can use the SPI.setMode() function to set the SPI mode for each sensor. This involves setting the SPI mode for each sensor to the correct value (Mode 0 or Mode 1).

Q: Can I use a single SPI clock speed for multiple MAX31865 sensors?

A: No, it is not recommended to use a single SPI clock speed for multiple MAX31865 sensors. This is because each MAX31865 sensor requires a specific clock speed to operate correctly, and using a single clock speed can cause communication errors.

Q: How do I set the SPI clock speeds for multiple MAX31865 sensors?

A: To set the SPI clock speeds for multiple MAX31865 sensors, you can use the SPI.setClockDivider() function to set the clock speed for each sensor. This involves setting the clock speed for each sensor to the correct value (1 MHz or 2 MHz).

Q: Can I use a software-based approach to manage the SPI communication for multiple MAX31865 sensors?

A: Yes, you can use a software-based approach to manage the SPI communication for multiple MAX31865 sensors. This involves using the SPI.beginTransaction() and SPI.endTransaction() functions to manage the SPI communication for each sensor.

Q: How do I manage the SPI communication for multiple MAX31865 sensors using a software-based approach?

A: To manage the SPI communication for multiple MAX31865 sensors using a software-based approach, you can use the following steps:

Set the chip select pin for each sensor to the correct value (HIGH or LOW).
Use the SPI.beginTransaction() function to begin the SPI transaction for each sensor.
Use the SPI.transfer() function to transfer data to or from each sensor.
Use the SPI.endTransaction() function to end the SPI transaction for each sensor.

Q: Can I use a hardware-based approach to manage the SPI communication for multiple MAX31865 sensors?

A: Yes, you can use a hardware-based approach to manage the SPI communication for multiple MAX31865 sensors. This involves using a SPI multiplexer (SPIM) or a SPI demultiplexer (SPID) to manage the SPI communication for multiple sensors.

Q: How do I manage the SPI communication for multiple MAX31865 sensors using a hardware-based approach?

A: To manage the SPI communication for multiple MAX31865 sensors using a hardware-based approach, you can use the following steps:

Connect the SPI multiplexer (SPIM) or SPI demultiplexer (SPID) to the SPI bus.
Set the chip select pin for each sensor to the correct value (HIGH or LOW).
Use the SPI multiplexer (SPIM) or SPI demultiplexer (SPID) to manage the SPI communication for each sensor.

Conclusion

In this article, we've answered some of the most frequently asked questions about reading two MAX31865 from the same MCU over the same SPI. We've covered topics such as managing chip select signals, setting SPI modes, and managing SPI communication. By following these guidelines, you should be able to successfully read two MAX31865 sensors from the same ESP8266 over the same SPI bus.