Some time ago I got inspired by @ricard's post Running Axoloti on an Audiothingies P6 and "ported" Axoloti firmware onto a PreenFM2 which has the same STM32F405 MCU as the P6.
I did get Axoloti running on the stock hardware but as the PreenFM2 uses two MCP4922 as DACs for its audio output (that is the R5 PCB; the newer R6 PCB actually has SMD parts and a proper stereo audio DAC), the Axoloti object I wrote to support audio output using these via SPI was just too clumsy and ate too many cycles and I couldn't be bothered to optimize it using threads and thread syncing and whatnot (read as: I don't know how to write these properly).
So I got carried away and added a CS4354 DAC (same as P6) and an SD card connector (PreenFM2 uses a USB drive for patch storage), and while I was in there I experimented with adding the simple CV in circuit used in the PreenFM2 Eurorack conversion kit (the plastic taped bit sticking out of the case). The mods involved soldering wires to the PreenF405 module and even lifting a pin off the STM32F405 and soldering a connector to it. Like I said, I got carried away...
So basically this is not even a PreenFM2 anymore. I am not proud of the hot glued mess so if you're sensitive, have some sort of OCD or are simply the best engineer in the world who can't refrain from sharing his/her knowledge and telling people how to engineer right, best STOP SCROLLING AND CLOSE THIS TAB NOW.
Don't say I didn't warn you.
Ricard's objects to access the encoders, buttons and the LCD worked for the PreenFM2 with almost no modifications. Amazing work!
I do have the fork on my github but since this was some time ago I can't remember if it's up to date, and since there are these breakneck hardware hacks involved in getting it to run I wouldn't recommend anyone doing this to their Preen. In fact building a veroboard Axoloti using a STM32F4 Discovery board is more reasonable than this mess.