Sometimes we get the question of where to modify or add code to change some behavior of the Crazyflie. There is no quick answer to this question but we thought that we should write a post to clear up some question marks and give a better idea of how to approach the problem.
There are quite a few repositories on the Bitcraze github page but there are two that are the main focal point for almost any Crazyflie work, those are the crazyflie-firmware and the crazyflie-lib-python. The crazyflie-firmware contains the source code (written in C) for the firmware that runs in the Crazyflie, that is the code responsible for flying, blinking LEDs, communicating with the radio, scanning sensors and so on. The crazyflie-lib-python (often called the python lib) on the other hand is running on the PC side and is the API to use to communicate with the Crazyflie from a script. The crazyflie-lib-python is also used by the python client which means that anything you see in the client can be done by a script using the python lib.
Let’s assume we have a system of one Crazyflie connected to a computer using a Crazyradio. Now we want to control the Crazyflie and make it take off for instance, how should this be done?
The easiest way would be to use the python lib. The python lib is used to communicate with the Crazyflie and we can use it to send instructions to the Crazyflie, for instance to take off or fly a trajectory. It is also possible to use the parameter framework to change values in the Crazyflie. The main way of monitoring what is going on in the Crazyflie is to use the log framework to read variables from the Crazyflie. The python lib is perfect for controlling the Crazyflie or prototype ideas as it is very fast to make changes and try things out. The best to get started with the python lib is to start from an example that already uses functionalities you want to use.
Another option is to add code in the firmware. Originally this has been quite hard since the firmware has not been initially designed to accept user code. This means that unless you want to modify an already existing code, it is quite hard to find where to add your code so that it runs in the Crazyflie firmware and you would have to make a fork of the firmware which can he hard to maintain and keep up to date in the long run. This is one of the things the out-of-tree build and the app layer is solving, it is now quite easy to add and run your own C files to the firmware in your own project without having to fork the Crazyflie firmware. There is a bunch of examples in the firmware that shows how to implement autonomous behavior as an app. The easiest is to start with the hello world example. When it comes to modifying exiting functionality in the firmware code, most of the time forking and modifying the official firmware unfortunately is the only solution. We are however working our way to make more and more of the firmware modular so that it can be expanded out of tree. For example there has been work to make an out of tree estimator possible to implement.
A prototype written as a python script is often pretty easy to move to the Crazyflie firmware. This is a good pattern when writing an application, rapid prototyping in python and then finalizing in firmware if needed. The best example of that is the push demo. It is a demo where the Crazyflie can be pushed-around with the help of the flow deck for autonomous flight and the multiranger deck for detecting obstacle/hands. We have a python cflib push demo as well as a Crazyflie firmware push demo app.
There is some support in the python lib for interacting with multiple Crazyflies and it is probably a good start point for simple swarms. For more advanced swarm work Crazyswarm may be a better option.
If you would like to see some of the process in action, we have made a workshop during our BAM days about implementing functionality both using the python lib and in the firmware as an out of tree app: