Beginner Tutorial: Your First MQTT Lua Program on the ESP32 Using Xedge32 (Part 3)

Combining the Lua scripting language with MQTT (Message Queuing Telemetry Transport) and the Xedge32 IoT platform offers an efficient and simplified approach to IoT programming. Lua’s lightweight syntax and flexibility make it an ideal choice for crafting MQTT client code, while Xedge32 enhances the capabilities of the ESP32 microcontroller, enabling seamless integration with sensors and other devices. This tutorial will guide you through setting up your first MQTT Lua program using Xedge32 on the ESP32.

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What is MQTT?

MQTT is a lightweight messaging protocol designed for minimal bandwidth consumption, making it ideal for IoT applications. It operates on a publish-subscribe model, allowing devices to publish messages on topics and subscribe to topics to receive messages. MQTT supports various Quality of Service (QoS) levels, ensuring reliable message delivery. It uses TCP/IP for transport and can employ TLS encryption for secure communication.

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The ESP32 microcontroller, with its dual-core CPU, integrated WiFi, and extensive GPIOs, stands out in the IoT realm. However, the true magic unfolds with the Xedge32 IoT platform, which allows programming the ESP32 using the Lua programming language. Xedge32 simplifies the development of robust, commercial IoT products, making advanced programming more approachable and affordable.

Xedge32 Installation

To install Xedge32 on your ESP32, follow these steps:

Connect Your ESP32:

• Connect the ESP32’s USB cable to your computer.
• Click the ‘Install Xedge32’ button on the Xedge32 website.

Select Your Device:

• A list of devices will appear. Select the device that includes ‘serial’ in its name.
• Follow the installation wizard to complete the installation.

Reboot Your ESP32:

• Once the installation is complete, unplug and replug the USB cable to reboot the ESP32.
• The ESP32 will now be in access point mode. Follow the ESP32 Access Point Mode documentation for configuration.

For a visual guide on setting up Xedge32, refer to this video tutorial.

With Xedge32 installed, let’s create an MQTT client in Lua.

-- Define the onstatus callback function
-- This function handles the status updates of the MQTT connection.
local function onstatus(type, code, status)
   print("Status Callback Triggered:", type, code)
   if type == "mqtt" and code == "connect" then
      -- Check if the connection was successful
      if status.reasoncode == 0 then
         print("Successful new connection")
         print("Server properties:", ba.json.encode(status.properties))
         return true -- Accept the connection
      else
         print("Connection failed with reason code:", status.reasoncode)
         return false -- Deny reconnect
      end
   elseif type == "mqtt" and code == "disconnect" then
      -- Handle disconnection
      print("Disconnected with reason code:", status and status.reasoncode or "unknown")
   elseif type == "sock" then
      -- Handle socket errors
      print("Socket error occurred")
   else
      -- Handle unknown errors
      print("Unknown error occurred")
   end
   return false -- Deny reconnect by default
end

-- Define the onpublish callback function
-- This function handles incoming messages from the subscribed topics.
local function onpublish(topic, payload, prop)
   print("Received:", topic, payload, ba.json.encode(prop))
end

-- Create the MQTT client instance with TLS and authentication
-- Replace the placeholder values with your actual credentials and broker information.
local mqtt = require("mqttc").create(
   "<>", -- Your broker URL
   onstatus, 
   onpublish, 
   {
      secure = true,           -- Enable TLS for a secure connection
      username = "<>", -- Your username
      password = "<>", -- Your password
      port = 8883,              -- Default port for secure MQTT connections
      keepalive = 60,           -- Keep-alive interval in seconds
      clientidentifier = "my_mqtt_client" -- Unique client ID, can be any string
   }
)

print("Subbing")

-- Subscribe to a topic
-- This subscribes to the topic "mytopic". Replace "mytopic" with your desired topic.
mqtt:subscribe("mytopic")

-- Publish a message to the topic
-- This publishes the message "testing" to the topic "mytopic". Replace with your desired topic and message.
mqtt:publish("mytopic", "testing")

-- Give it some time to process messages
-- The sleep time can be adjusted as necessary to allow the client to receive messages.
ba.sleep(5000)

-- Disconnect the MQTT client
-- This gracefully disconnects the client from the broker.
mqtt:disconnect(0)

Script Details

onstatus Callback Function:

• This function monitors the MQTT connection status.
• It handles different connection events such as connection success, failure, disconnection, and socket errors.
• It prints appropriate messages based on the connection status and decides whether to accept or deny reconnections.

onpublish Callback Function:

• This function handles incoming messages from subscribed topics.
• It prints the received topic and payload.

Creating the MQTT Client:

• The mqtt client is created with TLS and authentication for a secure connection.
• Replace the placeholder values (<>) with your actual broker URL, username, and password.

Subscribing to a Topic:

• The script subscribes to a topic named “mytopic”.
• Replace “mytopic” with your desired topic.

Publishing a Message:

• The script publishes a message “testing” to the topic “mytopic”.
• Replace “mytopic” and “testing” with your desired topic and message.

Processing Messages:

• The script pauses for 5 seconds (ba.sleep(5000)) to allow the client to process incoming messages.

Disconnecting the Client:

• The script gracefully disconnects the MQTT client from the broker using mqtt:disconnect(0).

Conclusion

With this tutorial, you have successfully created your first MQTT client program using Lua and Xedge32. You’ve learned how to set up Xedge32, connect to an MQTT broker, publish messages, subscribe to topics, and ensure secure communication using TLS. This foundational knowledge will enable you to build more complex and secure IoT applications in the future.

Xedge32’s integration with Lua and the ESP32 microcontroller simplifies IoT development, making it accessible to a broad range of users, from novices to seasoned developers. Dive into your next IoT project with confidence, leveraging the power of Xedge32 and MQTT.