Advanced LCM Guide

Last Updated: Feb 28, 2024

The guide introduces you how to leverage LCM for your project.

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How to add new LCM type and channel

How to add new LCM type and channel

Assuming you have followed the MBot System Setup guide, you should already have cloned both mbot_lcm_base and mbot_firmware to your Mbot Jetson.

All the LCM message types are stored in the mbot_lcm_base/mbot_msgs/lcmtypes directory. To add a new LCM type, define your customized LCM message in that location.
Assign a channel name in mbot_lcm_base/mbot_lcm_serial/include/mbot_lcm_serial/lcm_config.h.
```
 #define MBOT_EXAMPLE_CHANNEL "MBOT_EXAMPLE"
```
- Here MBOT_EXAMPLE_CHANNEL is a constant that stores the string "MBOT_EXAMPLE". Use of these defined constants instead of the raw strings in the rest of the code base helps to avoid errors due to mistyping.
Add the channel to the enum in mbot_lcm_base/mbot_lcm_serial/include/mbot_lcm_serial/lcm_config.h.
```
 enum message_topics{
     ...
     MBOT_EXAMPLE = 200
 };
```
The channel number is limited to a maximum of 255 due to the 8-bit limit. The numbers 0 and 255 might reserved for special purposes. Therefore, 254 is your highest usable value for channel identifiers.

Add the new defined type to mbot_msgs/CMakeLists.txt:

set(LCM_FILES
  ...  
  lcmtypes/slam_status_t.lcm
  lcmtypes/exploration_status_t.lcm
  ...
  lcmtypes/mbot_example_t.lcm
)

Make and install mbot_lcm_base by running
```
 $ cd ~/mbot_ws/mbot_lcm_base
 $ ./scripts/install.sh
```
Now your lcm base is good to go. Next we want to update the firmware base.
Add the same channel and number from step 3 to the enum in mbot_firmware/comms/include/comms/mbot_channels.h
Register new LCM channel by adding it to register_topics() in mbot_firmware/src/mbot.c, remember to define the callback functions for subscriptions.

Make and upload the firmware by executing the following commands same as introduced in the system setup:

 # Compile the firmware
 $ cd ~/mbot_ws/mbot_firmware/build
 $ cmake ..
 $ make  
 # Upload to the control board
 $ cd ~/mbot_ws/mbot_firmware
 $ sudo ./upload.sh flash build/src/mbot.uf2

To publish your newly defined message from mbot_autonomy, you need to use an LCM instance to either publish to or subscribe from channels with the messages or callback functions.

Check the code in mbot_autonomy/src/planning/motion_planner_server.cpp as example
```
 //...
 lcm_.subscribe(PATH_REQUEST_CHANNEL, &MotionPlannerServer::handleRequest, this);
 //...
 lcm_.publish(CONTROLLER_PATH_CHANNEL, &path);
```
- lcm_.subscribe listens for messages on PATH_REQUEST_CHANNEL. When a message is received, it triggers the callback function handleRequest. The this pointer refers to the current instance of the class.
  - The MotionPlannerServer::handleRequest callback function is where we define the preferred actions to take when data arrives.
- lcm_.publish sends the path message on CONTROLLER_PATH_CHANNEL.

Advanced LCM Guide

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How to add new LCM type and channel