Several years ago in 2015 we posted about the "ARM Radio" by Alberto I2PHD which is a minimalist SDR implementation based on the ARM processor on the STM32F429 discovery board. It was implemented with nothing more than a basic low pass front end, a reconstruction filter for the audio output and some DSP code. With it's low cost ADC it's only able to tune from 8 kHz to 900 kHz, but this is enough to get broadcast AM signals and NDBs. While it may not have the best specs, it's an excellent learning project for SDR DSP and microcontroller programming, and the code is completely open source, although a non-free toolchain is required.
Recently Alberto Garlassi wrote in and wanted to share a re-implementation of the code on a free toolchain. He writes:
Unfortunately the author used the Keil MDK toolchain, this means that it is not possible to change the code without paying for a license. The free version is limited to 32K and this is not enough.
I ported it to the free (don't know how much, certainly GCC + Eclipse) System Workbench, now it is easy for everybody to start where I2PHD left.
I did this several years ago and in the meantime ST and ARM changed many things in their tools and libraries, but it still works ok, I checked.
The complete project is on Github, it should be a matter of downloading the IDE, the libraries and press the debug icon. I'm in touch with Alberto Di Bene I2PHD, he has no objections and told me he's happy about this.