HOWTO: Use FFI to call user C modules on EV3

[jaguilar]

Hi there! In #1451, it was suggested that I make an example project to show how to use the micropython ffi module on ev3dev.

This repository contains the results of that effort. It shows how to build C libraries, copy them to the brick, load them with ffi, and call them.

I found that C is (big surprise) quite a bit faster than Python. According to very, very unscientific tests, it looks like it offers around a 32x speedup on this toy problem. If Viper is 4x as fast as plain micropython, that means C can give you an additional 8x speedup on top of Viper. So it might be something to reach for if you want to program CPU-intensive stuff on EV3 but not give up Python for the glue code. I reckon that similar results can be obtained using the technique in pybricks/pybricks-micropython#149 on more recent hubs.

One sad thing: it appears to no longer be possible to build the docker container used to cross-compile pybricks-micropython from ev3dev. It looks like the Debian stretch repositories have gone away, so the container can no longer be built. So let's not lose that docker image okay!?

[jaguilar]

I replaced the PID module in my gyroboy implementation with a C FFI version, and it made a material difference in the steadiness of the robot. I noticed the average loop time went down from 7-8ms to 5ms. Which probably means the variance went down a ton too since I have the loop set to run at a maximum frequency of 200hz. You can see the video of the after and before on the README file.

I found it to be a lot of fun to program in a mix of C and Python.

If you're doing anything CPU-intensive in Micropython, I really recommend this approach!