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 the green LED:

  • Very fast blinking: Searching for a network

  • Fast blinking: Connecting to broker

  • Slow, long blinking: Sending a publish message

Note that before any MQTT messages can be sent or received you’ll need to configure an IPv6 connection to the device and assign a routable IPv6 address.

For details how to do this please refer to sections ‘Establishing an IPv6 connection’ and ‘Distributing routable IPv6 prefix’ in platform/nrf52dk/

Broker setup

By default the example will attempt to publish readings to an MQTT broker running on the IPv6 address specified as MQTT_DEMO_BROKER_IP_ADDR in project-conf.h. This functionality was tested successfully with mosquitto.

On Ubuntu you can install and run mosquitto broker using the following commands:

apt-get install mosquitto mosquitto_clients
killall mosquitto
mosquitto -p 1883 -v


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:quickstart:cc2538:<device-id>, where <device-id> gets constructed from the device’s IEEE address.


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 ‘Org ID’ (DEFAULT_ORG_ID). 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 undefine MQTT_DEMO_BROKER_IP_ADDR.

If you want to use IBM’s cloud service with a registered device, change ‘Org ID’ (DEFAULT_ORG_ID) and provide the ‘Auth Token’ (DEFAULT_AUTH_TOKEN), which acts as a ‘password’, but bear in mind that it gets transported in clear text.