MQTT Subscribe

R2026b

Receive messages from the MQTT broker for specified topic

Since R2024a

expand all in page
  • MQTT subscribe block

Libraries:
STM32 Microcontroller Blockset / STM32F4xx Based Boards
STM32 Microcontroller Blockset / STM32F7xx Based Boards
STM32 Microcontroller Blockset / STM32H7xx Based Boards

Description

The MQTT Subscribe block receives messages from the MQ Telemetry Transport (MQTT) broker for the specified topic.

At each time step, the block checks if a new message is available on the specified topic. If a new message is available, the block receives the message. The Message port outputs the new message. If a new message is not available, Message outputs the last received message. The Topic port outputs the topic of the received message. The string topic/level/# on the block icon is specified by the Topic parameter.

For more information on MQTT, see Introduction to MQTT.

Note

The MQTT Subscribe block supports MQTT only over TCP/IP sockets.

Examples

New
Get Started with MQTT on STM32 Boards

Get Started with MQTT on STM32 Boards

Use the MQTT Publish and MQTT Subscribe blocks with two MQTT brokers: ThingSpeak™ and HiveMQ®. The example also includes the required STM32CubeMX IOC configuration and is implemented for the STM32H743ZI2 board.

Process ECG Signals Using MQTT on STM32 Processor Boards

Process ECG Signals Using MQTT on STM32 Processor Boards

Use the STM32 Nucleo F767ZI board using STM32™ Microcontroller Blockset to process an ECG signal input from an ECG sensor, extract the heart rate in beats per minutes (BPM), and send the ECG signal and heart rate to the ThingSpeak™ internet of things (IoT) analytics platform service.

Limitations

Only a maximum of 10 MQTT Subscribe blocks are allowed in a Simulink® model.

Ports

Output

expand all

If the client receives a new message since the last time step, the IsNew port outputs 1. Otherwise, the port outputs 0.

Data Types: Boolean

The message received from the MQTT broker, specified as an N-by-1 vector. N is the length of the message specified by the Message length (N) parameter.

Data Types: double

ASCII values that correspond to the topic name of the received message. For example, if the topic of the received message is model/data, the port outputs the topic as 109 111 100 101 108 47 100 97 116 97.

To convert the ASCII values into the alphanumeric equivalent, attach an ASCII to String block to the output of the Topic port. For information on topics and their structure, see Topics in MQTT.

Note

A topic can have a maximum of 128 characters.

Dependencies

The Topic port is available only if the Topic parameter value specified in the Block Parameters dialog box includes a wildcard entry. For information on wildcards, see Wildcards in MQTT Topics.

Data Types: uint8

Parameters

expand all

Specify a topic to which the client subscribes. For information on topics and its structure, see Topics in MQTT.

Programmatic Use

Block Parameter: Topic
Type: character vector
Values: 'topic/level'|valid topic name
Default: 'topic/level'

Quality of Service (QoS) defines the reliability of the message delivery process in MQTT. MQTT provides three QoS levels for message delivery: QoS 0, QoS 1, and QoS 2. The MQTT broker that you are using might not support three levels of QoS. For example, ThingSpeak™ MQTT supports only QoS 0.

Select a QoS level from the list:

  • 0 (At most once): The MQTT broker sends the message to the client. The client does not send an acknowledgment of the receipt.

  • 1 (At least once): The MQTT broker sends the message to the client at least once. The broker resends the message if it does not receive an acknowledgment from the client.

  • 2 (Exactly once): The MQTT broker sends the message to the client exactly once. The broker and client exchange acknowledgments so that the message is received only once.

For more information on QoS levels, see Levels of QoS in MQTT.

Programmatic Use

Block Parameter: QoS
Type: character vector
Values: '0'|'1'|'2'
Default: '0'

Specify the length of the message to be received from the MQTT broker. If the length of the received message is greater than the length specified in Message length (N), the block outputs only the first N characters of the received message. If the length of the received message is less than the length specified in Message length (N), the block appends zeros to the received message.

Programmatic Use

Block Parameter: MessageLength
Type: character vector
Values: '1'|integer between 1 and 64 inclusive
Default: '1'

Specify how often the subscriber checks for a new message from the broker. When you specify Sample time as -1, Simulink determines the best sample time for the block based on the block context within the model.

Programmatic Use

Block Parameter: sampletime
Type: character vector
Values: scalar | vector
Default: '0.1'

Programmatic Use: Add Block

To add the block programmatically, use the add_block function with one of these library block paths as the source.

"stm32f4xxblockslib/MQTT Subscribe"
"stm32f7xxblockslib/MQTT Subscribe"
"stm32h7xxblockslib/MQTT Subscribe"

Example: add_block("stm32f4xxblockslib/MQTT Subscribe","myModel/myBlock",MakeNameUnique="on")

Version History

Introduced in R2024a

See Also

Blocks

Topics