First micro-ROS Application on Zephyr RTOS

This tutorial aims to create a new micro-ROS application on Olimex STM32-E407 evaluation board with Zephyr RTOS. It originally ran on the micro-ROS website. For more content like this, click here.

Required hardware

This tutorial uses the following hardware:

Olimex STM32-E407
USB-Serial Cable Female

What is micro-ROS?

micro-ROS is an open source robotic operating system which bridges extremely resource constrained platforms to more complex robotic architectures of ROS 2, the de facto standard robotic framework. It runs on Real-Time Operating Systems (RTOS), and uses the DDS middleware Micro XRCE-DDS, that is, DDS for eXtremely Resource-Constrained Environments.

Adding a new micro-ROS app

First of all, make sure that you have a ROS 2 installation.

TIP: if you are familiar with Docker containers, this image may be useful: ros:dashing

On the ROS 2 installation open a command line and follow these steps:

# Source the ROS 2 installation
source /opt/ros/$ROS_DISTRO/setup.bash

# Create a workspace and download the micro-ROS tools
mkdir microros_ws 
cd microros_ws
git clone -b $ROS_DISTRO src/micro-ros-build

# Update dependencies using rosdep
sudo apt update && rosdep update
rosdep install --from-path src --ignore-src -y

# Build micro-ROS tools and source them
colcon build
source install/local_setup.bash

Now, let’s create a firmware workspace that targets all the required code and tools for Olimex development board and Zephyr:

# Create step
ros2 run micro_ros_setup zephyr olimex-stm32-e407

Now you have all the required tools to crosscompile micro-ROS and Zephyr for Olimex STM32-E407 development board. At this point, you must know that the micro-ROS build system is a four-step workflow:

  1. Create: retrieves all the required packages for a specific RTOS and hardware platform.
  2. Configure: configures the downloaded packages with options such as the micro-ROS application, the selected transport layer or the micro-ROS agent IP address (in network transports).
  3. Build: generates a binary file ready for being loaded in the hardware.
  4. Flash: load the micro-ROS software in the hardware.

micro-ROS apps for Olimex + Zephyr are located at firmware/zephyr_apps/apps. In order to create a new application, create a new folder containing two files: the app code (inside a src folder) and the RMW configuration.

# Creating a new app
pushd firmware/zephyr_apps/apps
mkdir my_brand_new_app
cd my_brand_new_app
mkdir src
touch src/app.c app-colcon.meta

You will also need some other Zephyr related files: a CMakeLists.txt in order to define the building process and a prj.conf where Zephyr is configured. You have these two files here, for now it is ok to copy them.

For this example we are going to create a ping pong app where a node sends a ping package with a unique identifier using a publisher and the same package is received by a pong subscriber. The node will also answer to pings received from other nodes with a pong message:


To start creating this app, let’s configure the RMW with the required static memory. You can read more about RMW and Micro XRCE-DDS Configuration here. The app-colcon.meta should look like:

    "names": {
        "rmw_microxrcedds": {
            "cmake-args": [

Meanwhile src/app.c should look like the following code:

#include <rcl/rcl.h>
#include <rcl_action/rcl_action.h>
#include <rcl/error_handling.h>
#include "rosidl_generator_c/string_functions.h"
#include <std_msgs/msg/header.h>

#include <rmw_uros/options.h>

#include <stdio.h>
#include <unistd.h>

#include <zephyr.h>


// App main function
void main(void)
  //Init RCL options
  rcl_init_options_t options = rcl_get_zero_initialized_init_options();
  rcl_init_options_init(&options, rcl_get_default_allocator());
  // Init RCL context
  rcl_context_t context = rcl_get_zero_initialized_context();
  rcl_init(0, NULL, &options, &context);

  // Create a node
  rcl_node_options_t node_ops = rcl_node_get_default_options();
  rcl_node_t node = rcl_get_zero_initialized_node();
  rcl_node_init(&node, "pingpong_node", "", &context, &node_ops);

  // Create a reliable ping publisher
  rcl_publisher_options_t ping_publisher_ops = rcl_publisher_get_default_options();
  rcl_publisher_t ping_publisher = rcl_get_zero_initialized_publisher();
  rcl_publisher_init(&ping_publisher, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/ping", &ping_publisher_ops);

  // Create a best effort pong publisher
  rcl_publisher_options_t pong_publisher_ops = rcl_publisher_get_default_options();
  pong_publisher_ops.qos.reliability = RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;
  rcl_publisher_t pong_publisher = rcl_get_zero_initialized_publisher();
  rcl_publisher_init(&pong_publisher, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/pong", &pong_publisher_ops);

  // Create a best effort pong subscriber
  rcl_subscription_options_t pong_subscription_ops = rcl_subscription_get_default_options();
  pong_subscription_ops.qos.reliability = RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;
  rcl_subscription_t pong_subscription = rcl_get_zero_initialized_subscription();
  rcl_subscription_init(&pong_subscription, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/pong", &pong_subscription_ops);

  // Create a best effort ping subscriber
  rcl_subscription_options_t ping_subscription_ops = rcl_subscription_get_default_options();
  ping_subscription_ops.qos.reliability = RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;
  rcl_subscription_t ping_subscription = rcl_get_zero_initialized_subscription();
  rcl_subscription_init(&ping_subscription, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/ping", &ping_subscription_ops);

  // Create a wait set
  rcl_wait_set_t wait_set = rcl_get_zero_initialized_wait_set();
  rcl_wait_set_init(&wait_set, 2, 0, 0, 0, 0, 0, &context, rcl_get_default_allocator());

  // Create and allocate the pingpong publication message
  std_msgs__msg__Header msg;
  char msg_buffer[STRING_BUFFER_LEN]; = msg_buffer;
  msg.frame_id.capacity = STRING_BUFFER_LEN;

  // Create and allocate the pingpong subscription message
  std_msgs__msg__Header rcv_msg;
  char rcv_buffer[STRING_BUFFER_LEN]; = rcv_buffer;
  rcv_msg.frame_id.capacity = STRING_BUFFER_LEN;

  // Set device id and sequence number;
  int device_id = rand();
  int seq_no;
  int pong_count = 0;
  struct timespec ts;
  rcl_ret_t rc;

  uint32_t iterations = 0;

  do {
    // Clear and set the waitset
    size_t index_pong_subscription;
    rcl_wait_set_add_subscription(&wait_set, &pong_subscription, &index_pong_subscription);

    size_t index_ping_subscription;
    rcl_wait_set_add_subscription(&wait_set, &ping_subscription, &index_ping_subscription);
    // Run session for 100 ms
    rcl_wait(&wait_set, RCL_MS_TO_NS(100));

    // Check if it is time to send a ping
    if (iterations++ % 50 == 0) {
      // Generate a new random sequence number
      seq_no = rand();
      sprintf(, "%d_%d", seq_no, device_id);
      msg.frame_id.size = strlen(;
      // Fill the message timestamp
      clock_gettime(CLOCK_REALTIME, &ts);
      msg.stamp.sec = ts.tv_sec;
      msg.stamp.nanosec = ts.tv_nsec;

      // Reset the pong count and publish the ping message
      pong_count = 0;
      rcl_publish(&ping_publisher, (const void*)&msg, NULL);
      printf("Ping send seq %s\n",;  
    // Check if some pong message is received
    if (wait_set.subscriptions[index_pong_subscription]) {
      rc = rcl_take(wait_set.subscriptions[index_pong_subscription], &rcv_msg, NULL, NULL);

      if(rc == RCL_RET_OK && strcmp(, == 0) {
          printf("Pong for seq %s (%d)\n",, pong_count);

    // Check if some ping message is received and pong it
    if (wait_set.subscriptions[index_ping_subscription]) {
      rc = rcl_take(wait_set.subscriptions[index_ping_subscription], &rcv_msg, NULL, NULL);

      // Dont pong my own pings
      if(rc == RCL_RET_OK && strcmp(, != 0){
        printf("Ping received with seq %s. Answering.\n",;
        rcl_publish(&pong_publisher, (const void*)&rcv_msg, NULL);
  } while (true);

Once the new folder is created, let’s configure our new app with a serial transport on the USB:

# Configure step
ros2 run micro_ros_setup my_brand_new_app --transport serial-usb

When the configuring step ends, just build the firmware:

# Build step
ros2 run micro_ros_setup

Once the build has successfully ended, let’s power and connect the board. First, connect Olimex ARM-USB-TINY-H JTAG programmer to the board’s JTAG port:

Make sure that the board power supply jumper (PWR_SEL) is in the 3-4 position in order to power the board from the JTAG connector:

You should see the red LED lighting. It is time to flash the board:

# Flash step
ros2 run micro_ros_setup

Running the micro-ROS app

The micro-ROS app is ready to connect to a micro-ROS-Agent and start talking with the rest of the ROS 2 world.

First of all, create and build a micro-ROS agent:

# Download micro-ROS-Agent packages
ros2 run micro_ros_setup

# Build micro-ROS-Agent packages, this may take a while.
colcon build
source install/local_setup.bash

Then connect the Olimex development board to the computer using the USB OTG 2 connector (the miniUSB connector that is furthest from the Ethernet port).

TIP: Color codes are applicable to this cable. Make sure to match Olimex Rx with Cable Tx and vice-versa. Remember GND!

Then run the agent:

# Run a micro-ROS agent
ros2 run micro_ros_agent micro_ros_agent serial --dev [device]

TIP: you can use this command to find your serial device name: ls /dev/serial/by-id/*. Probably it will be something like /dev/serial/by-id/usb-ZEPHYR_Zephyr_microROS_3536510100290035-if00

And finally, let’s check that everything is working in another command line. We are going to listen to ping topic to check whether the Ping Pong node is publishing its own pings

source /opt/ros/$ROS_DISTRO/setup.bash

# Subscribe to micro-ROS ping topic
ros2 topic echo /microROS/ping

You should see the topic messages published by the Ping Pong node every 5 seconds:

user@user:~$ ros2 topic echo /microROS/ping
  sec: 20
  nanosec: 867000000
frame_id: '1344887256_1085377743'
  sec: 25
  nanosec: 942000000
frame_id: '730417256_1085377743'

On another command line, let’s subscribe to the pong topic

source /opt/ros/$ROS_DISTRO/setup.bash

# Subscribe to micro-ROS pong topic
ros2 topic echo /microROS/pong

At this point, we know that our app is publishing pings. Let’s check if it also answers to someone else pings in a new command line:

source /opt/ros/$ROS_DISTRO/setup.bash

# Send a fake ping
ros2 topic pub --once /microROS/ping std_msgs/msg/Header '{frame_id: "fake_ping"}'

Now, we should see on the ping subscriber our fake ping along with the board pings:

user@user:~$ ros2 topic echo /microROS/ping
  sec: 0
  nanosec: 0
frame_id: fake_ping
  sec: 305
  nanosec: 973000000
frame_id: '451230256_1085377743'
  sec: 310
  nanosec: 957000000
frame_id: '2084670932_1085377743'

And in the pong subscriber, we should see the board’s answer to our fake ping:

user@user:~$ ros2 topic echo /microROS/pong
  sec: 0
  nanosec: 0
frame_id: fake_ping

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