MQTT Client Example

The MQTT client can be used to:

  • Publish sensor readings to an MQTT broker.

  • Subscribe to a topic and receive commands from an MQTT broker

The demo will give some visual feedback with a LED (configurable):

  • Very fast blinking: Searching for a network

  • Fast blinking: Connecting to broker

  • Slow, long blinking: Sending a publish message

This example is known to work with all platforms that support the new button API.

This example can operate in two modes: A default mode to be used with the mosquitto MQTT broker and a second mode to be used with the IBM Watson IoT platform.

To enable Watson mode, define MQTT_CLIENT_CONF_WITH_IBM_WATSON as 1 in the example’s project-conf.h.


By default the example will attempt to publish readings to an MQTT broker running on the IPv6 address specified as MQTT_CLIENT_CONF_BROKER_IP_ADDR in project-conf.h. This functionality was tested successfully with mosquitto. This define will be ignored in IBM Watson mode.

The publish messages include sensor readings but also some other information, such as device uptime in seconds and a message sequence number. The demo will publish to topic iot-2/evt/status/fmt/json. The device will connect using client-id d:<org-id>:mqtt-client:<device-id>, where <device-id> gets constructed from the device’s IEEE address. <org-id> can be controlled through the MQTT_CLIENT_CONF_ORG_ID define.


You can also subscribe to topics and receive commands, but this will only work if you use “Org ID” != ‘quickstart’. To achieve this, you will need to change MQTT_CLIENT_CONF_ORG_ID in project-conf.h. In this scenario, the device will subscribe to:


You can then use this to toggle LEDs. To do this, you can for example use mosquitto client to publish to iot-2/cmd/leds/fmt/json. So, to change the state of an LED, you would do this:

mosquitto_pub -h <broker IP> -m "1" -t iot-2/cmd/leds/fmt/json

Where broker IP should be replaced with the IP address of your mosquitto broker (the one where you device has subscribed). Replace -m "1' with -m "0" to turn the LED back off.

Bear in mind that, even though the topic suggests that messages are of json format, they are in fact not. This was done in order to avoid linking a json parser into the firmware. This comment only applies to parsing incoming messages, outgoing publish messages use proper json payload.

IBM Quickstart Service

It is also possible to publish to IBM’s quickstart service. To do so, you need to enable this mode by setting MQTT_CLIENT_CONF_WITH_IBM_WATSON to 1 in project-conf.h.

The device will then try to connect to IBM’s quickstart over NAT64, so you will need a NAT64 gateway in your network to make this work. A guide on how to setup NAT64 is out of scope here, but you can find one in the documentation.

If you want to use IBM’s cloud service with a registered device, you will need to set MQTT_CLIENT_CONF_ORG_ID and then also to provide the ‘Auth Token’ (MQTT_CLIENT_CONF_AUTH_TOKEN), which acts as a ‘password’. You will also need to configure your Organisation / Registered device on Watson such that TLS is optional.

Note: The token will be transported in cleartext.