I was trying to connect my MAX7219 LED display to an MQTT server and receive a text from the MQTT subscription to display.

but I didn't get any suitable code on the internet, so I started building my own...

and the result comes quite well...

you can display any text on led display
you can adjust intensity of the display
you can set the scroll speed

Supplies

An esp8266 development board. (my case it's NODE MCU v1.0)
MAX7219 LED Matrix Display.

Software needed:

Arduino IDE.
An MQTT server. (my case Mosquitto)

Library required:

ESP8266WiFi.h
MD_MAX72xx.h
EspMQTTClient.h

Step 1: Setup Arduino IDE for Esp8266 Development

open preferences of Arduino then paste the below URL in Aditional Boards Manager URLs:

https://arduino.esp8266.com/stable/package_esp8266com_index.json

then Tools > Boards > Boards Manager and search for esp8266 and install it.

now your Arduino ide is ready for esp8266 development.

Step 2: Download External Libraries

now we need some libraries for MAX7219 and MQTT Client.

let's download and set up the libraries

navigate to Sketch > Include Library > Manage Libraries on Arduino IDE

and search for EspMQTTClient and click Install

NB: Install all dependent libraries, it's important

Again search for MD_MAX72xx and click Install

Step 3: Write Some Code Now

Now paste the below code

#include <ESP8266WiFi.h>
#include <MD_MAX72xx.h>
#include <SPI.h>
#include "EspMQTTClient.h"

#define MAX_DEVICES 4 // your device count

#define CLK_PIN   D5 // or SCK
#define DATA_PIN  D7 // or MOSI
#define CS_PIN    D4 // or SS // you can set it to any pin

#define HARDWARE_TYPE MD_MAX72XX::PAROLA_HW // change according to your display type
MD_MAX72XX mx = MD_MAX72XX(HARDWARE_TYPE, CS_PIN, MAX_DEVICES);

const uint8_t MESG_SIZE = 255;
const uint8_t CHAR_SPACING = 1;
uint8_t SCROLL_DELAY = 75; // default scroll delay
uint8_t INTENSITY = 5; // default intensity

char curMessage[MESG_SIZE];
char newMessage[MESG_SIZE];
bool newMessageAvailable = false;

void scrollDataSink(uint8_t dev, MD_MAX72XX::transformType_t t, uint8_t col) {}

uint8_t scrollDataSource(uint8_t dev, MD_MAX72XX::transformType_t t)
{
  static enum { S_IDLE, S_NEXT_CHAR, S_SHOW_CHAR, S_SHOW_SPACE } state = S_IDLE;
  static char   *p;
  static uint16_t curLen, showLen;
  static uint8_t  cBuf[8];
  uint8_t colData = 0;

  switch (state)
  {
  case S_IDLE: 
    p = curMessage;
    if (newMessageAvailable) 
    {
      strcpy(curMessage, newMessage);
      newMessageAvailable = false;
    }
    state = S_NEXT_CHAR;
    break;

  case S_NEXT_CHAR: 
    if (*p == '\0')
      state = S_IDLE;
    else
    {
      showLen = mx.getChar(*p++, sizeof(cBuf) / sizeof(cBuf[0]), cBuf);
      curLen = 0;
      state = S_SHOW_CHAR;
    }
    break;

  case S_SHOW_CHAR: 
    colData = cBuf[curLen++];
    if (curLen < showLen)
      break;

    showLen = (*p != '\0' ? CHAR_SPACING : (MAX_DEVICES*COL_SIZE)/2);
    curLen = 0;
    state = S_SHOW_SPACE;

  case S_SHOW_SPACE:  
    curLen++;
    if (curLen == showLen)
      state = S_NEXT_CHAR;
    break;

  default:
    state = S_IDLE;
  }

  return(colData);
}

EspMQTTClient client(
  "YOUR_SSID",
  "YOUR_WIFI_PASSWORD",
  "YOUR_MQTT_SERVER",  // MQTT Broker server ip
  "YOUR_MQTT_USER",   // Can be omitted if not needed
  "YOUR_MQTT_PASSWORD",   // Can be omitted if not needed
  "MAX7219_DISPLAY",     // Client name that uniquely identify your device
  1883              // The MQTT port, default to 1883. this line can be omitted
);

void scrollText(void)
{
  static uint32_t prevTime = 0;

  // Is it time to scroll the text?
  if (millis() - prevTime >= SCROLL_DELAY)
  {
    mx.transform(MD_MAX72XX::TSL);  // scroll along - the callback will load all the data
    prevTime = millis();      // starting point for next time
  }
}

void setup()
{
  Serial.begin(115200);
  mx.begin();
  mx.control(MD_MAX72XX::INTENSITY, INTENSITY);
  mx.setShiftDataInCallback(scrollDataSource);
  mx.setShiftDataOutCallback(scrollDataSink);
  curMessage[0] = newMessage[0] = '\0';
  sprintf(curMessage, "Smart Display");
}

void onConnectionEstablished()
{
// MQTT subscription topic for display text
  client.subscribe("leddisplay/text", [](const String & payload) {
    sprintf(curMessage, payload.c_str());
  });
  
<p>// MQTT subscription topic for display intensity controll</p>  client.subscribe("leddisplay/intensity", [](const String & payload) {
    mx.control(MD_MAX72XX::INTENSITY, payload.toInt());
  });

// MQTT subscription topic for display scroll speed controll
  client.subscribe("leddisplay/scroll", [](const String & payload) {
    SCROLL_DELAY = payload.toInt();
  });
}

void loop()
{
  client.loop();
  scrollText();
}

For detail information, refer to this repository

https://github.com/souravj96/max7219-mqtt-esp8266

Step 4: Circuit Diagram

connect MAX7219 display with NODE MCU

Step 5: Upload Code to Esp8266

now choose your correct board type and serial port then hit upload.

Step 6: Test Everything

if everything goes correct then your esp8266 will be connected to your MQTT server.

now, if anything will be published on leddisplay/text topic that will be displayed.

{
topic: "leddisplay/text",
payload: "your message here"
}

if you want to set the intensity of display

{
topic: "leddisplay/intensity",
payload: "2" // max is 15 and min 0
}

if you want to set the scroll speed of display

{
topic: "leddisplay/scroll",
payload: "100" // max is 255 and min 0
}

Happy coding

Introduction: MAX7219 LED Matrix MQTT Using Esp8266