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4.3.2 Firmware for Original Prusa MINI/MINI+  

Same Old Shane
4.3.2 Firmware for Original Prusa MINI/MINI+

We are pleased to announce the final release of the new 4.3.2 firmware version for the Prusa MINI / MINI+. Kudos to the 32 bit development team on a lot of work on this upgrade!


  • Upgraded USB drivers
  • Upgraded Newlib library
  • FatFs library upgrade
  • Saving of self-test results
  • Fixed bugs

This is the final release of firmware 4.3.2. This changelog is a summary of the previous releases (beta1, RC1 and RC2). The main focus is on bug fixes related to the USB Flash drivers and performance-related optimizations.

Upgraded USB drivers

We’re constantly updating and optimizing the MINI+ firmware to further improve its performance and reliability. In this release, the developers focused on the USB driver responsible for handling all the data transfers from the connected USB flash drive (e.g., mounting drives, reading G-codes, ...). We received several reports that certain USB drives weren't working properly.

Together with the chipset manufacturer (STM company), developers have tested and deployed a new version of the USB driver. As a result, the Buddy board should now be able to communicate even with the problematic USB drives. Previously, when the drive wasn't properly recognized by the printer, the only option was to reinsert it. This is not necessary anymore, the system will handle it on its own.

Note that these fixes do not address a scenario when certain USB drives are not correctly detected by the bootloader. For example, while attempting to flash a new firmware.

Upgraded Newlib library

This firmware release brings an important upgrade to the Newlib library. The standard library is replaced with a more optimized Newlib-nano version. Also, the standard “printf” family functions are replaced with a different implementation.

This change results in several improvements and memory savings. Compared to the previous firmware 4.3.1:

98 kB of FLASH memory is saved 4 kB of RAM is saved (3 kB on heap/static allocations, 1 kB on stack) makes the printf() family functions fully reentrant and thread-safe (the previous implementation did not allow formatting of floats in ISRs)

Multiple tests revealed no negative effects related to these changes in the MINI+ firmware project. The measurements of RAM usage were done with release builds with metrics enabled while printing.

FatFs library upgrade

This build brings an upgrade of the FatFs library, which is supposed to fix synchronization issues of accessing the file system.

FatFs library has been upgraded from R0.12c to the latest version of R0.14b. The most important change is the fix of synchronization issues related to accessing the FAT file system. Please see the release notes of the FatFs library for details, where the synchronization fix is also discussed.

This also fixed several reported situations of BSOD after the removal of the USB drive from the MINI and MINI+.

Moreover, to improve the quality of testing of the USB drivers and file system fixes, we designed an automated device that simulates various edge cases while inserting and/or removing a USB drive. The testing device helped us to verify that the fix works as expected.

Saving of self-test results

The upgrade of the FatFs library also brings back the possibility of saving results of self-tests onto the USB drive (if present), which has been removed in previous versions due to random file system-related problems.

If you intend to use this feature, make sure there is a USB flash drive (FAT32) connected to the printer before the self-test is performed. Once the test is finished, a log file is saved onto the flash drive. Examples of use are provided in the release notes for firmware 4.3.0-RC1

Fixed bugs

Filament runout during serial print

When the print has been previously initiated from the serial line (e.g., OctoPrint) and a filament runout occurred, the printer did not react accordingly and the Change filament (M600) sequence was not initiated. Now, this behavior is fixed and even when the print is initiated by the serial interface, the Filament runout initiates the local Change filament sequence on the printer.

Extra head movements after the print ends

In rare cases, when the printer finishes the print, the print head performs additional random movements (repeated mesh bed leveling, additional purge line, etc.). From the reports, it seemed that this bug first occurred in firmware 4.1.0.

The developers have identified the issue caused by a long comment line in the G-code file (generated by PrusaSlicer merging all custom codes into one comment line), which is longer than the expected 95 characters. The characters exceeding the number 95 in this line were incorrectly parsed - text starting with the 96th character has been analyzed as a part of actual G-Code. In some cases, it may have contained G-Code commands, which have been interpreted and executed. The analysis was complicated due to users adding different Custom codes.

All concerned cases issued in Github are fixed now:



Supported printer:

Original Prusa MINI

Original Prusa MINI+

Download Link:  

How to flash the firmware on MINI:

  • Copy the firmware file (.bbf) onto the USB drive with FAT32 formatting
  • Insert the flash drive in the MINI's USB port
  • Turn the printer ON (or reboot it)
  • A new screen will appear
  • Confirm you want to flash the firmware

Please report any bug here:

As always, we wish you happy printing and look forward to your feedback!

Shane (AKA FromPrusa)...
Posted : 11/08/2021 7:30 pm
New Member
Alternative method to flashing the latest 4.3.2 firmware for the Mini?

Hey guys, 

I have a Prusa Mini and the USB-A port has died and NO USB thumb drives are recognized when plugged into this port on the Mini... I would like to note that ALL the USB thumb drives I was using on the Mini work just fine in my desktop PC and I have reformatted several of them with FAT 32 as specified (I've covered ALL the obvious things that could go wrong).

I am able to get a Raspberry Pi4b to operate fine over the micro-USB port and was wanting to flash this firmware through an Octoprint plugin called "Firmware Updater" that has worked on others Prusa MK3's... has anyone been able to get this to work for the Mini? If so, what are the settings supposed to look like inside the firmware updater plugin.

I'm hoping this firmware update will resurrect my USB-A port as it was working just fine. 

Thanks for your assistance. 

Posted : 31/08/2021 7:50 am

Can you please provide any available detail, how it died? I mean when and how did you discover it is not working - during print, during flashing, after octoprint actions etc.... ?

even an old man can learn new things 🙂
Standard I3 mk3s, FW 3.10.1 RC1, MMU2S, no closed box, Fusion 360, PrusaSlicer, Windows 10
PRUSA MINI FW 4.3.4, Prusa Connect Beta...
Posted : 31/08/2021 10:45 am
New Member
Mini USB port dead?

It seems to have stopped working as I was getting ready to insert one of my USB thumb drives to start another print job, same USB drive that I had been using for several months with no issues at all. ALL usb drives I have been using are recognized perfectly in my Windows 10 desktop a few feet away. There is a possibility that it was static electricity but very doubtful... I do have a USB extension cable to relocate the USB port just behind the LCD display. I have tried to use it and\or the main USB-A port on the Prusa Mini to NO avail. When I perform the USB self test on power up, after the bootloader version appears, I get an "Error #2 - unable to mount USB device" with a RED screen. I chatted with Prusa support the other day and they suggested I flash the firmware to the latest version. Strange in that the reason for contacting was because the USB is inoperable. I'd really like to know of how to flash the latest firmware and possibly restore USB functionality to the Mini. Anyone?

Posted : 01/09/2021 1:51 am
RE: re

How long do you have your MINI?

Seems you have an unique error. 

Can you let me know what is your current FW version?

Is your printer still under warranty?


even an old man can learn new things 🙂
Standard I3 mk3s, FW 3.10.1 RC1, MMU2S, no closed box, Fusion 360, PrusaSlicer, Windows 10
PRUSA MINI FW 4.3.4, Prusa Connect Beta...
Posted : 01/09/2021 7:12 am