🛠 Publier en MQTT sur le réseau GPRS

Objectives

Connect the MKR GSM 1400 to an MQTT broker by stacking PubSubClient on top of GSMClient: since the MKRGSM client class mimics Arduino's Client class, PubSubClient works without modification. This is a typical pattern for field sensors (Irriduo irrigation, water monitoring): periodic reading, QoS 1 publication, and robust reconnection after radio outages.

Hardware

• Arduino MKR GSM 1400 board + U.FL antenna + LiPo battery ≥ 1500 mAh
• Data micro-SIM (PIN code disabled), known APN
• An MQTT broker accessible via TCP 1883 (personal server or test broker)
• Libraries: MKRGSM, PubSubClient

Complete Code

#include <MKRGSM.h>
#include <PubSubClient.h>
#include "arduino_secrets.h"
const char PINNUMBER[]     = SECRET_PINNUMBER;
const char GPRS_APN[]      = SECRET_GPRS_APN;
const char GPRS_LOGIN[]    = SECRET_GPRS_LOGIN;
const char GPRS_PASSWORD[] = SECRET_GPRS_PASSWORD;
const char MQTT_BROKER[]   = "broker.example.org";

GSMClient  netClient;
GPRS       gprs;
GSM        gsmAccess;
PubSubClient mqtt(netClient);

uint32_t dernierePub = 0;

void connexion {
  while (gsmAccess.begin(PINNUMBER) != GSM_READY) delay(2000);
  while (gprs.attachGPRS(GPRS_APN, GPRS_LOGIN, GPRS_PASSWORD) != GPRS_READY)
    delay(2000);
  mqtt.setServer(MQTT_BROKER, 1883);
  mqtt.setBufferSize(256);
  while (!mqtt.connect("mkr1400-capteur")) {
    Serial.print("MQTT rc="); Serial.println(mqtt.state);
    delay(5000);
  }
  Serial.println("MQTT connecte");
}

void setup {
  Serial.begin(9600);
  while (!Serial) {}
  connexion;
}

void loop {
  if (!mqtt.connected || !gprs) {   // coupure radio ou broker
    Serial.println("Reconnexion...");
    gsmAccess.shutdown;
    connexion;
  }
  mqtt.loop;                        // indispensable : trafic + keepalive
  if (millis - dernierePub > 60000UL) {
    dernierePub = millis;
    int v = analogRead(A1);           // exemple : capteur analogique
    char trame[48];
    snprintf(trame, sizeof trame, "{\"a1\":%d}", v);
    mqtt.publish("maison/mkr1400/a1", trame, true);  // retain
  }
  delay(100);
}

Expected Verification

mosquitto_sub -t 'maison/mkr1400/a1' (or your broker's client) receives a JSON frame every minute, even after moving the board to an area without network coverage: the reconnection loop resumes automatically. mqtt.loop must be called frequently — its absence causes keepalive disconnections and unprocessed incoming messages.

Common Errors

• PubSubClient buffer too small: MQTT_PACKET_SIZE_EXCEEDED during publication; use setBufferSize and recompile with MQTT_MAX_PACKET_SIZE if necessary.
• Forgetting mqtt.loop in loop: the broker disconnects the client after the keepalive timeout.
• TLS: port 8883 with plain GSMClient will not work; use GSMSSLClient or BearSSLClient (ECC508), and an MQTT library compatible with TLS clients.
• Publishing during an outage: messages are lost without a queue — buffer them in RAM (only 32 KB available) or accept the loss.
• MQTT password in the sketch: the board's flash is readable via bossac even with the security bit enabled (ArduinoCore-samd #570); use the Secrets tab and credentials with limited scope.
• 2G throughput: keep frames small and frequency low (minutes, not seconds).