🛠 Publier et s'abonner en MQTT

Objectives

Step up to IoT messaging: connect the GIGA R1 WiFi to WiFi, then use the MQTT protocol with the PubSubClient library to publish a value every 5 seconds and receive messages from another client. MQTT is the native protocol of Arduino IoT Cloud, which the board officially supports — this tutorial gives you the basics to understand what happens "under" the cloud.

Hardware

• Arduino GIGA R1 WiFi board + u.FL antenna
• WiFi 2.4 GHz WPA2
• PubSubClient library (Michael Schwartzkopff / Nick O'Leary) installed via the IDE Library Manager
• An accessible MQTT broker: the public test broker test.mosquitto.org on port 1883, or your own local Mosquitto

Complete Code

// GIGA R1 WiFi — MQTT publish/subscribe (MQTT brut port 1883, sans TLS)
#include <WiFi.h>
#include <PubSubClient.h>

const char SSID[] = "your-ssid";
const char PASS[] = "your-pass";
const char BROKER[] = "test.mosquitto.org";
const int  PORT_BROKER = 1883;
const char SUJET_PUB[]  = "giga/maison/salle/capteur";
const char SUJET_SUB[]  = "giga/maison/salle/consigne";

WiFiClient reseau;
PubSubClient mqtt(reseau);
unsigned long dernierEnvoi = 0;

void callback(char* sujet, byte* payload, unsigned int longueur) {
  Serial.print("Recu sur "); Serial.print(sujet); Serial.print(" : ");
  for (unsigned int i = 0; i < longueur; i++) Serial.write(payload[i]);
  Serial.println;
  digitalWrite(LED_BUILTIN, mqtt.payload[0] == '1' ? HIGH : LOW);
}

void setup {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
  WiFi.begin(SSID, PASS);
  while (WiFi.status != WL_CONNECTED) { Serial.print("."); delay(200); }
  Serial.print("IP "); Serial.println(WiFi.localIP);

  mqtt.setServer(BROKER, PORT_BROKER);
  mqtt.setCallback(callback);
  while (!mqtt.connected) {
    Serial.print("Connexion MQTT...");
    if (mqtt.connect("giga-r1")) {                 // client id "giga-r1"
      Serial.println("connecte");
      mqtt.subscribe(SUJET_SUB);
      Serial.print("abonne a "); Serial.println(SUJET_SUB);
    } else {
      Serial.print("echec rc="); Serial.println(mqtt.state);
      delay(2000);
    }
  }
}

void loop {
  mqtt.loop;                                     // indispensable : keepalive + reception
  if (WiFi.status != WL_CONNECTED) { mqtt.disconnect; WiFi.reconnect; return; }
  if (!mqtt.connected) { mqtt.connect("giga-r1"); mqtt.subscribe(SUJET_SUB); return; }
  if (millis - dernierEnvoi > 5000) {
    dernierEnvoi = millis;
    String v = String(analogRead(A0));             // 12 entrees analogiques 3.3 V
    mqtt.publish(SUJET_PUB, v.c_str);
    Serial.print("publie "); Serial.println(v);
  }
}

Expected Verification

The monitor displays the WiFi connection followed by "connected / subscribed to ...", then "published <value>" every 5 seconds. By publishing from another tool (mosquitto_pub -t giga/maison/salle/consigne -m 1), the board's LED turns on and the message appears in the console. On test.mosquitto.org, you can also subscribe to the published topic from any client.

Common Errors

• mqtt.loop forgotten in loop: without regular calls, the keepalive expires and the broker disconnects the board every 60 s; received messages are never distributed.
• Port 8883 used without TLS: mosquitto's port 8883 uses TLS; with a simple WiFiClient, use port 1883 (see the TLS tutorial for port 8883).
• Overloaded public broker: test.mosquitto.org is shared and sometimes slow; for a serious project, set up Mosquitto locally.
• Non-ASCII or too long topics: keep identifiers simple (letters, numbers, /); accents in topics break some tools.
• Poorly handled reconnection: after a WiFi drop, you must reconnect MQTT AND resubscribe — the subscription does not survive reconnection.