The article describes a device that transmits data and processes commands over the MQTT protocol on the Internet. A small batch was made to assess the performance in real practice. The software for the microcontroller is written in C using a real-time operating system (RTOS), which allows you to streamline software development through pseudo-parallel code execution, task synchronization mechanisms (semaphores, mutexes), and a queue mechanism. The paper presents an example of an algorithm for the interaction of FreeRTOS tasks: the tasks of waiting for a command from the server, the tasks of measuring temperature, the main thread that sends a message to the server in response to the appearance of a line in the message queue. The core of the MQTT client being developed is the STM32 microcontroller and the GSM/GPRS SIM800 module. A large number of interfaces are displayed on the device, which allows you to connect a wide variety of peripherals (temperature, humidity, pressure sensors, servos, etc.). The device is able to independently restore the connection to the network in case of its interruption. The proprietary implementation of the MQTT protocol version 3.1 QoS 0 is used in the software of the control microcontroller. MQTT messages are transmitted as part of a TCP connection established by means of a GSM module. The control microcontroller communicates with the GSM module via AT commands. The possibility of two-way real-time messaging is implemented, which allows using the developed microprocessor system both in the mode of an autonomous control device and in the interactive mode of executing commands received wirelessly and sending the execution statuses of these commands. The main feature of the MQTT client is the ability to remotely update the microcontroller software (OTA), implemented using an auxiliary microcontroller and a flash memory chip. The developed MQTT client is intended to be used as the basis of microprocessor systems - clients of the Internet of Things.
