Connect with ESP32

This article mainly introduces how to use PubSubClient in the ESP32 project, including implementing the connection, subscription, messaging, and other functions between the client and MQTT broker.

As an upgraded version of ESP8266, ESP32 is an ideal choice for IoT projects. In addition to the Wi-Fi module, this module also includes a Bluetooth 4.0 module. The dual-core CPU operates at a frequency of 80 to 240 MHz. It contains two Wi-Fi and Bluetooth modules and various input and output pins.

This article demonstrates how to connect an ESP32 client to MQTT broker via the TCP port and TLS/SSL port respectively. For Serverless deployments, see the demonstration on TLS/SSL port connection. Settings for connections over TCP port are different from those for connections over TLS/SSL port, but the code used in the publishing and subscribing functions is the same.

Prerequisites

Before the connection, you need to get the broker and client ready.

Get MQTT Broker

You can use the free public MQTT broker provided by EMQX. This service was created based on the EMQX Platform. The information about broker access is as follows:

Broker: broker.emqx.io
TCP Port: 1883
SSL/TLS Port: 8883

You can create a deployment as well. Find connection information in deployment overview. Make sure the deployment is running. Use the TCP port or TLS/SSL port to test the connection to the MQTT server.

If you are creating your own deployment, check Authentication and set the username and password in Access Control -> Authentication for verification.

Arduino IDE

This article uses the Arduino IDE as the code editor and uploader. The open-source Arduino Software (IDE) makes it easy to write code and upload it to the board. This software can be used with any Arduino board.

Installation Dependencies

In Arduino IDE, complete the following installations:

Install ESP32 development board. Click Tools -> Development Board -> Development Board Management. Search ESP32 and click Install.
Install PubSub client. Click Project -> Load library -> Library manager.... Search PubSubClient and Install PubSubClient by Nick O’Leary.

Connect over TCP Port

This section describes how to connect an ESP32 client to MQTT broker over TCP port in Arduino IDE.

Import the WiFi and PubSubClient libraries.

#include <WiFi.h>
#include <PubSubClient.h>

Set the Wi-Fi name and password, as well as the MQTT server connection address and port, and set the topic to esp32/test.

The sample code will use a public MQTT server to connect, and the public MQTT server does not require a username and password. If you create a deployment, please refer to Default Authentication to set username and password.

// WiFi Credentials
const char *ssid = "WIFI_SSID";            // Replace with your WiFi name
const char *password = "WIFI_PASSWORD";  // Replace with your WiFi password

// MQTT Broker Settings
const char *mqtt_broker = "broker.emqx.io";
const char *mqtt_topic = "emqx/esp32";
const char *mqtt_username = "emqx";
const char *mqtt_password = "public";
const int mqtt_port = 1883;

Open a serial connection to output the results of the program and connect to the Wi-Fi network.

void setup() {
    Serial.begin(115200);
    connectToWiFi();
    mqtt_client.setServer(mqtt_broker, mqtt_port);
    mqtt_client.setKeepAlive(60);
    mqtt_client.setCallback(mqttCallback); // Corrected callback function name
    connectToMQTT();
}

void connectToWiFi() {
    WiFi.begin(ssid, password);
    Serial.print("Connecting to WiFi");
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("\nConnected to WiFi");
}

Use PubSubClient to connect to the public MQTT broker.

void connectToMQTT() {
    while (!mqtt_client.connected()) {
        String client_id = "esp32-client-" + String(WiFi.macAddress());
        Serial.printf("Connecting to MQTT Broker as %s.....\n", client_id.c_str());
        if (mqtt_client.connect(client_id.c_str(), mqtt_username, mqtt_password)) {
            Serial.println("Connected to MQTT broker");
            mqtt_client.subscribe(mqtt_topic);
            mqtt_client.publish(mqtt_topic, "Hi EMQX I'm ESP32 ^^"); // Publish message upon successful connection
        } else {
            Serial.print("Failed, rc=");
            Serial.print(mqtt_client.state());
            Serial.println(" try again in 5 seconds");
            delay(5000);
        }
    }
}

After the MQTT server is successfully connected, ESP32 will publish messages on topic esp32/test to the MQTT server and subscribe to messages on topic esp32/test.

// publish and subscribe
mqtt_client.subscribe(mqtt_topic);
mqtt_client.publish(mqtt_topic, "Hi EMQX I'm ESP32 ^^"); // Publish message upon successful connection

Set the callback function to print the topic name to the serial port and print the message received from the esp32/test topic.

void mqttCallback(char *mqtt_topic, byte *payload, unsigned int length) {
    Serial.print("Message received on mqtt_topic: ");
    Serial.println(mqtt_topic);
    Serial.print("Message: ");
    for (unsigned int i = 0; i < length; i++) {
        Serial.print((char) payload[i]);
    }
    Serial.println("\n-----------------------");
}

The complete code is displayed as follows:

#include <WiFi.h>
#include <PubSubClient.h>

// WiFi Credentials
const char *ssid = "WIFI_SSID";            // Replace with your WiFi name
const char *password = "WIFI_PASSWORD";  // Replace with your WiFi password

// MQTT Broker Settings
const char *mqtt_broker = "broker.emqx.io";
const char *mqtt_topic = "emqx/esp32";
const char *mqtt_username = "emqx";
const char *mqtt_password = "public";
const int mqtt_port = 1883;

WiFiClient espClient;
PubSubClient mqtt_client(espClient);

// Function Declarations
void connectToWiFi();

void connectToMQTT();

void mqttCallback(char *mqtt_topic, byte *payload, unsigned int length);

void setup() {
    Serial.begin(115200);
    connectToWiFi();
    mqtt_client.setServer(mqtt_broker, mqtt_port);
    mqtt_client.setKeepAlive(60);
    mqtt_client.setCallback(mqttCallback); // Corrected callback function name
    connectToMQTT();
}

void connectToWiFi() {
    WiFi.begin(ssid, password);
    Serial.print("Connecting to WiFi");
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("\nConnected to WiFi");
}

void connectToMQTT() {
    while (!mqtt_client.connected()) {
        String client_id = "esp32-client-" + String(WiFi.macAddress());
        Serial.printf("Connecting to MQTT Broker as %s.....\n", client_id.c_str());
        if (mqtt_client.connect(client_id.c_str(), mqtt_username, mqtt_password)) {
            Serial.println("Connected to MQTT broker");
            mqtt_client.subscribe(mqtt_topic);
            mqtt_client.publish(mqtt_topic, "Hi EMQX I'm ESP32 ^^"); // Publish message upon successful connection
        } else {
            Serial.print("Failed, rc=");
            Serial.print(mqtt_client.state());
            Serial.println(" try again in 5 seconds");
            delay(5000);
        }
    }
}

void mqttCallback(char *mqtt_topic, byte *payload, unsigned int length) {
    Serial.print("Message received on mqtt_topic: ");
    Serial.println(mqtt_topic);
    Serial.print("Message: ");
    for (unsigned int i = 0; i < length; i++) {
        Serial.print((char) payload[i]);
    }
    Serial.println("\n-----------------------");
}


void loop() {
    if (!mqtt_client.connected()) {
        connectToMQTT();
    }
    mqtt_client.loop();
}

Connect over TLS/SSL Port

This section describes how to connect an ESP32 client to MQTT broker over TLS/SSL port in Arduino IDE. Settings for connections over TLS/SSL ports are different from those for connections over TCP port, but the code is the same in the publishing and subscribing functions.

Import libraries ESP32WiFi and PubSubClient.

#include <WiFi.h>
#include <PubSubClient.h>
#include <WiFiClientSecure.h>

Set Wi-Fi name and password, and connection address and port.

The sample code will use a public MQTT server to connect, and the public MQTT server does not require a username and password. If you create a deployment, please refer to Default Authentication to set username and password.

// WiFi credentials
const char *ssid = "WIFI_SSID";             // Replace with your WiFi name
const char *password = "WIFI_PASSWORD";   // Replace with your WiFi password

// MQTT Broker settings
const char *mqtt_broker = "broker.emqx.io";
const char *mqtt_topic = "emqx/esp32";
const char *mqtt_username = "emqx";
const char *mqtt_password = "public";
const int mqtt_port = 8883;

Set server-side certificate.
If you are using a public MQTT broker, download CA certificate here for TLS/SSL port connection with Serverless deployment. If you are using Dedicated deployment, use your own server-side certificate.

// Root CA Certificate
// Load DigiCert Global Root G2, which is used by EMQX Public Broker: broker.emqx.io
const char *ca_cert = R"EOF(
-----BEGIN CERTIFICATE-----
MIIDjjCCAnagAwIBAgIQAzrx5qcRqaC7KGSxHQn65TANBgkqhkiG9w0BAQsFADBh
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3
d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBH
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pLiaWN0bfVKfjllDiIGknibVb63dDcY3fe0Dkhvld1927jyNxF1WW6LZZm6zNTfl
MrY=
-----END CERTIFICATE-----
)EOF";

// Load DigiCert Global Root CA ca_cert, which is used by EMQX Serverless Deployment
/*
const char* ca_cert = R"EOF(
-----BEGIN CERTIFICATE-----
MIIDrzCCApegAwIBAgIQCDvgVpBCRrGhdWrJWZHHSjANBgkqhkiG9w0BAQUFADBh
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3
d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBD
QTAeFw0wNjExMTAwMDAwMDBaFw0zMTExMTAwMDAwMDBaMGExCzAJBgNVBAYTAlVT
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CSDMAZOnTjC3U/dDxGkAV53ijSLdhwZAAIEJzs4bg7/fzTtxRuLWZscFs3YnFo97
nh6Vfe63SKMI2tavegw5BmV/Sl0fvBf4q77uKNd0f3p4mVmFaG5cIzJLv07A6Fpt
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YSEY1QSteDwsOoBrp+uvFRTp2InBuThs4pFsiv9kuXclVzDAGySj4dzp30d8tbQk
CAUw7C29C79Fv1C5qfPrmAESrciIxpg0X40KPMbp1ZWVbd4=
-----END CERTIFICATE-----
*/

Open a serial connection to output of the result of the program and connect to the Wi-Fi network.

void setup() {
    Serial.begin(115200);
    connectToWiFi();

    // Set Root CA certificate
    esp_client.setCACert(ca_cert);

    mqtt_client.setServer(mqtt_broker, mqtt_port);
    mqtt_client.setKeepAlive(60);
    mqtt_client.setCallback(mqttCallback);
    connectToMQTT();
}

void connectToWiFi() {
    WiFi.begin(ssid, password);
    Serial.print("Connecting to WiFi");
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("\nConnected to WiFi");
}

Set certificate, and MQTT broker, write callback function, and print connection information on the serial monitor at the same time.

void connectToMQTT() {
    while (!mqtt_client.connected()) {
        String client_id = "esp32-client-" + String(WiFi.macAddress());
        Serial.printf("Connecting to MQTT Broker as %s...\n", client_id.c_str());
        if (mqtt_client.connect(client_id.c_str(), mqtt_username, mqtt_password)) {
            Serial.println("Connected to MQTT broker");
            mqtt_client.subscribe(mqtt_topic);
            mqtt_client.publish(mqtt_topic, "Hi EMQX I'm ESP32 ^^");  // Publish message upon connection
        } else {
            Serial.print("Failed to connect to MQTT broker, rc=");
            Serial.print(mqtt_client.state());
            Serial.println(" Retrying in 5 seconds.");
            delay(5000);
        }
    }
}

After successfully connecting to the MQTT broker, ESP32 will publish messages and subscribe to the MQTT broker.

// publish and subscribe
mqtt_client.subscribe(mqtt_topic);
mqtt_client.publish(mqtt_topic, "Hi EMQX I'm ESP32 ^^");  // Publish message upon connection

Print the topic name to the serial port and then print every byte of received messages.

void callback(char *topic, byte *payload, unsigned int length) {
    Serial.print("Message arrived in topic: ");
    Serial.println(topic);
    Serial.print("Message:");
    for (int i = 0; i < length; i++) {
        Serial.print((char) payload[i]);
    }
    Serial.println();
    Serial.println("-----------------------");
}

Reconnect function

void mqttCallback(char *topic, byte *payload, unsigned int length) {
    Serial.print("Message received on topic: ");
    Serial.println(topic);
    Serial.print("Message: ");
    for (unsigned int i = 0; i < length; i++) {
        Serial.print((char) payload[i]);
    }
    Serial.println("\n-----------------------");
}

The complete code is displayed as follows:

#include <WiFi.h>
#include <PubSubClient.h>
#include <WiFiClientSecure.h>

// WiFi credentials
const char *ssid = "WIFI_SSID";             // Replace with your WiFi name
const char *password = "WIFI_PASSWORD";   // Replace with your WiFi password

// MQTT Broker settings
const char *mqtt_broker = "broker.emqx.io";
const char *mqtt_topic = "emqx/esp32";
const char *mqtt_username = "emqx";
const char *mqtt_password = "public";
const int mqtt_port = 8883;

// WiFi and MQTT client initialization
WiFiClientSecure esp_client;
PubSubClient mqtt_client(esp_client);

// Root CA Certificate
// Load DigiCert Global Root G2, which is used by EMQX Public Broker: broker.emqx.io
const char *ca_cert = R"EOF(
-----BEGIN CERTIFICATE-----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5uNu5g/6+rkS7QYXjzkwDQYJKoZIhvcNAQELBQADggEBAGBnKJRvDkhj6zHd6mcY
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pLiaWN0bfVKfjllDiIGknibVb63dDcY3fe0Dkhvld1927jyNxF1WW6LZZm6zNTfl
MrY=
-----END CERTIFICATE-----
)EOF";

// Load DigiCert Global Root CA ca_cert, which is used by EMQX Serverless Deployment
/*
const char* ca_cert = R"EOF(
-----BEGIN CERTIFICATE-----
MIIDrzCCApegAwIBAgIQCDvgVpBCRrGhdWrJWZHHSjANBgkqhkiG9w0BAQUFADBh
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3
d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBD
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06O/nVsJ8dWO41P0jmP6P6fbtGbfYmbW0W5BjfIttep3Sp+dWOIrWcBAI+0tKIJF
PnlUkiaY4IBIqDfv8NZ5YBberOgOzW6sRBc4L0na4UU+Krk2U886UAb3LujEV0ls
YSEY1QSteDwsOoBrp+uvFRTp2InBuThs4pFsiv9kuXclVzDAGySj4dzp30d8tbQk
CAUw7C29C79Fv1C5qfPrmAESrciIxpg0X40KPMbp1ZWVbd4=
-----END CERTIFICATE-----
*/


// Function Declarations
void connectToWiFi();

void connectToMQTT();

void mqttCallback(char *topic, byte *payload, unsigned int length);


void setup() {
    Serial.begin(115200);
    connectToWiFi();

    // Set Root CA certificate
    esp_client.setCACert(ca_cert);

    mqtt_client.setServer(mqtt_broker, mqtt_port);
    mqtt_client.setKeepAlive(60);
    mqtt_client.setCallback(mqttCallback);
    connectToMQTT();
}

void connectToWiFi() {
    WiFi.begin(ssid, password);
    Serial.print("Connecting to WiFi");
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("\nConnected to WiFi");
}

void connectToMQTT() {
    while (!mqtt_client.connected()) {
        String client_id = "esp32-client-" + String(WiFi.macAddress());
        Serial.printf("Connecting to MQTT Broker as %s...\n", client_id.c_str());
        if (mqtt_client.connect(client_id.c_str(), mqtt_username, mqtt_password)) {
            Serial.println("Connected to MQTT broker");
            mqtt_client.subscribe(mqtt_topic);
            mqtt_client.publish(mqtt_topic, "Hi EMQX I'm ESP32 ^^");  // Publish message upon connection
        } else {
            Serial.print("Failed to connect to MQTT broker, rc=");
            Serial.print(mqtt_client.state());
            Serial.println(" Retrying in 5 seconds.");
            delay(5000);
        }
    }
}

void mqttCallback(char *topic, byte *payload, unsigned int length) {
    Serial.print("Message received on topic: ");
    Serial.println(topic);
    Serial.print("Message: ");
    for (unsigned int i = 0; i < length; i++) {
        Serial.print((char) payload[i]);
    }
    Serial.println("\n-----------------------");
}


void loop() {
    if (!mqtt_client.connected()) {
        connectToMQTT();
    }
    mqtt_client.loop();
}

Test Connection

After the ESP32 client has successfully connected to the MQTT broker, you can use the Arduino IDE and MQTTX to test the connection.

Open the serial monitor, select 115200 baud rate, and check the ESP32 connection status.
Establish the connection between MQTTX client and MQTT broker, and send messages to ESP32.

In summary, we have created an MQTT connection in an ESP32 project, and simulated the connecting, subscribing, sending and receiving messages between the client and MQTT broker. You can download the source code of the example here, and you can also find more demo examples in other languages on GitHub.

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Platform API

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Connect with ESP32

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Get MQTT Broker

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Serverless Deployment API

Connect with ESP32 ​

Prerequisites ​

Get MQTT Broker ​

Arduino IDE ​

Installation Dependencies ​

Connect over TCP Port ​

Connect over TLS/SSL Port ​

Test Connection ​

More ​

Connect with ESP32

Prerequisites

Get MQTT Broker

Arduino IDE

Installation Dependencies

Connect over TCP Port

Connect over TLS/SSL Port

Test Connection

More