Welcome to MicroROS V2 Robot Car repository

2. Subscribe To Topics

Subscribe To Topics

Subscribing to Topics

1. Experiment Objective

In this experiment, you will learn how to create a ROS2 topic subscriber on the MicroROS-V2 control board using the micro-ROS framework. Through this example, you will master the subscriber initialization method of micro-ROS, the writing of subscription callback functions, and the use of the executor. This example subscribes to Int32 messages on the topic /subscriber and prints the received data to the serial port.

2. Experiment Principle

1. Keyword Explanation

Keyword	Description
micro-ROS Agent	A proxy program running on the ROS2 host side, responsible for bridging communication between the ESP32 firmware and the ROS2 system, connecting with the microcontroller via the UDP protocol
subscriber	The subscriber, the message-receiving end in ROS2 communication, subscribes to messages on a specified topic and triggers the callback function to process data when a new message arrives
rclc	A ROS2 client library for microcontrollers, providing lightweight node, publisher, subscriber and other APIs, adapted to resource-constrained embedded devices
callback (ON_NEW_DATA)	The subscription callback trigger mode, which invokes the callback function only when a new message is received, avoiding wasted CPU resources from empty polling
QoS (Best Effort)	The "best effort" mode in the quality-of-service policy, which does not guarantee message delivery and ordering, suitable for scenarios with high real-time requirements but tolerant of a small amount of packet loss
executor	The executor, an event-driven scheduler in micro-ROS, responsible for polling subscriber handles and invoking the corresponding callback functions when messages are received

2. Technical Architecture

This experiment builds a one-way data channel from a ROS2 host to an ESP32 firmware subscriber:

Hardware layer: The MicroROS-V2 control board (ESP32) connects to the local network via Wi-Fi
Transport layer: Uses the UDP protocol, communicating with the ROS2 host through the micro-ROS Agent
Node configuration: Node name esp32_subscriber, subscribing to topic /subscriber
Message type: std_msgs/msg/Int32, carrying a 32-bit signed integer
QoS mode: Best Effort, adapting to the unreliable characteristics of the Wi-Fi UDP link
Data flow: ROS2 host publishes a message → Agent → UDP → micro-ROS stack → executor polling → subscription callback → serial port printing

3. Core Topic Quick Reference

Firmware Downlink (Control Commands)

Topic	Message Type	QoS	Field	Range	Direction
`/subscriber`	`std_msgs/msg/Int32`	Best Effort	`data` (int32)	Any integer value	Input, printed to serial port

Usage Example:


ros2 topic pub /subscriber std_msgs/msg/Int32 "data: 42"

4. Middleware Configuration

micro-ROS uses a static memory model (RCUTILS_AVOID_DYNAMIC_ALLOCATION=ON), all entity memory is pre-allocated at compile time, and runtime dynamic creation is not supported.

Configuration file: ~/esp/extra_components/micro_ros_espidf_component/colcon.meta

Key Constraints:

MAX_HISTORY=1 means there is no message buffer queue, so uplink topics (firmware → host) uniformly use best_effort QoS
MAX_SERVICES=0, MAX_CLIENTS=0 means only topic communication is supported, not Service/Client
After modifying colcon.meta, the micro-ROS library must be recompiled for the changes to take effect: run idf.py clean-microros in the project directory to clean and rebuild the micro-ROS library, then idf.py build to compile the firmware

3. Experiment Steps

1. Hardware Preparation

Hardware	Requirement
MicroROS-V2 control board	✅ Required
USB Type-C cable	✅ Required
Wi-Fi network (2.4 GHz)	✅ Required

2. Prerequisites

The ESP32-IDF development environment has been set up
The MicroROS-V2 control board is connected to the computer via USB

Configure the Wi-Fi SSID, password, Agent IP and port via idf.py menuconfig (the micro_ros menu item)


xxxxxxxxxx
cd ~/esp/microROS_samples/subscriber
source ~/esp/esp-idf/export.sh
idf.py menuconfig

Select example-app setting to set the domain ID to be consistent with the virtual machine

After configuration, press "s" to save and press "q" to exit the configuration interface.

Publish Topics

Multi Topic Subscription And Publishing