Networking and Communications

Important — WiFi Antenna: Both XIAO boards have a connector for an external WiFi antenna (IPEX/U.FL connector). The antenna must be physically plugged in before trying to connect to a WiFi network. Without it, the signal is too weak to reach the router reliably, and the board will fail to connect or drop the connection repeatedly. This was one of the problems I encountered during this week (see the Problems section). Connecting the antenna solved the issue immediately.

Key Concepts Summary:

The only lines you need to change in both sketches are the WiFi credentials. Everything else (broker, port, topic, logic) works as-is.

const char* WIFI_SSID     = "TU_RED_WIFI";      // ← your 2.4 GHz network name
const char* WIFI_PASSWORD = "TU_PASSWORD_WIFI"; // ← your WiFi password

Remember: both boards must be on the same 2.4 GHz network, and the antenna must be physically connected to the IPEX connector on each board.

#include <WiFi.h>              // ESP32 built-in WiFi library
#include <PubSubClient.h>      // MQTT client library — install via Library Manager

// ── WiFi credentials ─────────────────────────────────────────────────────────
const char* WIFI_SSID     = "TU_RED_WIFI";      // CHANGE: your 2.4 GHz network name
const char* WIFI_PASSWORD = "TU_PASSWORD_WIFI"; // CHANGE: your WiFi password

// ── MQTT broker settings ──────────────────────────────────────────────────────
const char* MQTT_BROKER = "broker.hivemq.com";  // Free public HiveMQ broker
const int   MQTT_PORT   = 1883;                 // Standard TCP port for MQTT (not WebSocket)

// ── Topic ─────────────────────────────────────────────────────────────────────
const char* TOPIC_LED = "ibero/led/control";    // Channel used to send commands to the LED board

// ── MQTT client objects ───────────────────────────────────────────────────────
WiFiClient   wifiClient;                        // Underlying TCP connection object
PubSubClient mqttClient(wifiClient);            // MQTT client built on top of the WiFi connection

// ── WiFi connection function ──────────────────────────────────────────────────
void conectarWiFi() {
  Serial.print("Connecting to WiFi...");
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);          // Start the WiFi handshake
  while (WiFi.status() != WL_CONNECTED) {        // Block here until connected
    delay(500);
    Serial.print(".");                           // Print dots as a progress indicator
  }
  Serial.println(" Connected!");                 // Success message
}

// ── MQTT reconnection function ────────────────────────────────────────────────
void reconectarMQTT() {
  while (!mqttClient.connected()) {              // Keep trying until the connection succeeds
    Serial.print("Connecting to MQTT...");
    // Generate a random client ID to avoid conflicts with other users of the same broker
    String clientId = "placa_ayuda_" + String(random(0xffff), HEX);
    if (mqttClient.connect(clientId.c_str())) {  // Attempt connection with the random ID
      Serial.println(" Connected!");             // Broker accepted the connection
    } else {
      Serial.print(" Failed (rc=");
      Serial.print(mqttClient.state());          // Print the error code for debugging
      Serial.println("). Retrying in 3s...");
      delay(3000);                               // Wait 3 seconds before the next attempt
    }
  }
}

// ── setup() — runs once on power-on ──────────────────────────────────────────
void setup() {
  Serial.begin(115200);                          // Open Serial Monitor at 115200 baud
  conectarWiFi();                                // Connect to WiFi first
  mqttClient.setServer(MQTT_BROKER, MQTT_PORT);  // Point the MQTT client at the broker
  Serial.println("Type 1 to turn the LED ON, 2 to turn it OFF:");
}

// ── loop() — runs continuously ────────────────────────────────────────────────
void loop() {
  if (!mqttClient.connected()) reconectarMQTT(); // Reconnect if the broker dropped us
  mqttClient.loop();                             // Process incoming MQTT events (mandatory)

  if (Serial.available()) {                      // Check if the user typed something
    char input = Serial.read();                  // Read exactly one character

    if (input == '1') {                          // User typed "1"
      mqttClient.publish(TOPIC_LED, "ENCENDER"); // Publish the ON command to the topic
      Serial.println("Message sent: Turn LED on");
    }
    else if (input == '2') {                     // User typed "2"
      mqttClient.publish(TOPIC_LED, "APAGAR");   // Publish the OFF command to the topic
      Serial.println("LED off");
    }
  }
}

#include <WiFi.h>              // ESP32 built-in WiFi library
#include <PubSubClient.h>      // MQTT client library — install via Library Manager

// ── WiFi credentials ─────────────────────────────────────────────────────────
const char* WIFI_SSID     = "TU_RED_WIFI";      // CHANGE: your 2.4 GHz network name
const char* WIFI_PASSWORD = "TU_PASSWORD_WIFI"; // CHANGE: your WiFi password

// ── MQTT broker settings ──────────────────────────────────────────────────────
const char* MQTT_BROKER = "broker.hivemq.com";  // Must match the publisher's broker exactly
const int   MQTT_PORT   = 1883;                 // Must match the publisher's port exactly

// ── Topic ─────────────────────────────────────────────────────────────────────
const char* TOPIC_LED = "ibero/led/control";    // Must match the publisher's topic exactly

// ── LED pin ───────────────────────────────────────────────────────────────────
#define LED_PIN 43                              // GPIO 43 = D6 on the XIAO ESP32-C6 pinout

// ── MQTT client objects ───────────────────────────────────────────────────────
WiFiClient   wifiClient;                        // Underlying TCP connection object
PubSubClient mqttClient(wifiClient);            // MQTT client built on top of the WiFi connection

// ── Message callback — runs every time a subscribed message arrives ───────────
void onMessage(char* topic, byte* payload, unsigned int length) {
  String mensaje = "";
  for (unsigned int i = 0; i < length; i++) {
    mensaje += (char)payload[i];               // Convert byte array to String character by character
  }

  if (mensaje == "ENCENDER") {                  // Command received: turn LED on
    digitalWrite(LED_PIN, HIGH);               // Set GPIO 43 HIGH (LED on)
    Serial.println("Led encendido");           // Confirm in Serial Monitor
  }
  else if (mensaje == "APAGAR") {               // Command received: turn LED off
    digitalWrite(LED_PIN, LOW);                // Set GPIO 43 LOW (LED off)
    Serial.println("Led apagado");             // Confirm in Serial Monitor
  }
}

// ── WiFi connection function ──────────────────────────────────────────────────
void conectarWiFi() {
  Serial.print("Connecting to WiFi...");
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);          // Start the WiFi handshake
  while (WiFi.status() != WL_CONNECTED) {        // Block here until connected
    delay(500);
    Serial.print(".");
  }
  Serial.println(" Connected!");
}

// ── MQTT reconnection function ────────────────────────────────────────────────
void reconectarMQTT() {
  while (!mqttClient.connected()) {
    Serial.print("Connecting to MQTT...");
    // Random client ID prevents conflicts on the shared public broker
    String clientId = "placa_led_" + String(random(0xffff), HEX);
    if (mqttClient.connect(clientId.c_str())) {  // Attempt connection
      Serial.println(" Connected!");
      mqttClient.subscribe(TOPIC_LED);           // Subscribe to the topic AFTER connecting
                                                 // (subscriptions must be re-done after every reconnect)
    } else {
      Serial.print(" Failed (rc=");
      Serial.print(mqttClient.state());          // Error code: -2 = broker unreachable, -4 = timeout
      Serial.println("). Retrying in 3s...");
      delay(3000);
    }
  }
}

// ── setup() — runs once on power-on ──────────────────────────────────────────
void setup() {
  Serial.begin(115200);                          // Open Serial Monitor
  pinMode(LED_PIN, OUTPUT);                      // Configure GPIO 43 as a digital output
  digitalWrite(LED_PIN, LOW);                    // Start with LED off
  conectarWiFi();                                // Connect to WiFi
  mqttClient.setServer(MQTT_BROKER, MQTT_PORT);  // Configure the broker
  mqttClient.setCallback(onMessage);             // Register the message handler function
}

// ── loop() — runs continuously ────────────────────────────────────────────────
void loop() {
  if (!mqttClient.connected()) reconectarMQTT(); // Maintain the MQTT connection
  mqttClient.loop();                             // Process incoming messages (mandatory — without
                                                 // this line, the callback never fires)
}

Connecting to WiFi......... Connected!
Type 1 to turn the LED ON, 2 to turn it OFF:

[After typing "1":]
Message sent: Turn LED on

[After typing "2":]
LED off

Connecting to WiFi......... Connected!
Connecting to MQTT... Connected!

[When ENCENDER command arrives:]
Led encendido

[When APAGAR command arrives:]
Led apagado

Important: The call to mqttClient.loop() inside loop() is absolutely mandatory on the subscriber. Without it, the board never processes incoming packets and the callback never fires — the LED would never respond, even if the message arrives at the broker correctly.