MATLAB Answers

MQTT subscribe gets no data back

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Mar Ron
Mar Ron on 2 May 2020
I have a channel that has several feeds with live data. I am trying to use a particle Argon and thingspeak MQTT broker to subscribe to this data. I used the example provided and got nothing back from the subscription. I am pasting my code below. If anyone has a working example, that would be great!
#include "MQTT.h"
const long channelID = 10X4XXX;
String ReadAPIKey = "XXXXXXXXX";
String subscribeTopic = "channels/" + String( channelID ) + "/subscribe/fields/field1/"+String(ReadAPIKey);
void callback( char* topic, byte* payload, unsigned int length );
MQTT client( "" , 1883 , callback );
// This function processes the messages relayed by the MQTT broker.
void callback( char* topic, byte* payload, unsigned int length ) {
Serial.println("in callback");
char p[ length + 1 ]; // Leave an extra space to null terminate the string.
memcpy( p, payload, length );
p[ length ] = NULL; // Terminate the string.
void setup() {
Serial.println("in void setup");
// Connect to the server.
void loop() {
Serial.println("in void loop");
int timeHour = Time.hour();
if (client.isConnected()){
if ( ( timeHour > 23 ) or ( timeHour < 4 ) ){
Particle.publish( "Sleep" );
System.sleep( SLEEP_MODE_DEEP , 7200 );
void subscribeMQTT(){
Serial.println("in subscribe MQTT");
if (!client.isConnected()) {
client.connect( " MGSubscribeArgon" , "MGUsername" , MQTTAPIKey , NULL , MQTT::QOS0 , 0 , NULL , true );
Particle.publish( " Connect " );
Serial.println("Connected to broker");
delay( 1000 );
if ( client.isConnected()) {
Serial.println("MQTT connected");
client.subscribe( subscribeTopic );
Particle.publish( "subs" );

Answers (2)

Vinod on 4 May 2020
Have you taken a look at this example? It should be possible to modify it for a Particle Argon quite easily.
  1 Comment
Mar Ron
Mar Ron on 5 May 2020
That IS the example I am using. In the code I posted above, the only part that I deleted was the "strncmp" portion. I deleted it in the above example by mistake, but I eventually just use serial.println() in the callback function to see what is being received. That callback function that is supposed to be called by the receipt of a packet on MQTT subscribe, is never triggered.

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Christopher Stapels
Christopher Stapels on 6 May 2020
Edited: Christopher Stapels on 6 May 2020
Can you try connecting with another desktop based broker (MATLAB has one) or MQTTFx to make sure you have all the api keys correct? If you verify that, I have an Argon here at home that I can re verify the code with. Its previously been verified with an photon.
Christopher Stapels
Christopher Stapels on 8 May 2020
Publish and subscribe have slightly different requirements, I have found using a different client can be a really good check to make sure everything is correct. I've failed to match the appropriate channel ID for example and the subscription broker just disconnects me without a messge. Unofrtunately its the nature of the protocol.
Ive done pub and sub MQTT on a photon previously so im pretty confident it will work. Have a look a the serial ourput, especially during the subscription method. Do you see all the test messages?
Ill see If I can get a test going on my argon soon, but my first step will be to check the connection with MQTTfx.

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