To enter into bootloader mode you must press a key while powering on the PreenFM2.

The bootloader allows you to upgrade your PreenFM2 firmware and to access your USB drive from your computer.
PreenFM2 firmwares are available it the github release folder.

The fastest way to upgrade your firmware is to use (1) to upload your firmware on the USB drive then (2) to flash it.

Once you are in the bootloader, the display offers you several option.

(1) : Access Usb Stick.

Pressing this button will allow you to access your Usb Stick from your computer. I should be accessible as a regular mass storage device (read regular usb drive).
Each I/O access goes to the ARM MCU that responses as a mass storage device and redirect to the actual USB stick all low level I/Os.
The performance is really slow compared to if you were pluging directly your usb stick in your computer. But it can be usefull to copy last firmware or banks on the drive.

(2) : Upgrade from the firmware in the USB Key.

Once the firmware is copied in your  »/pfm2/ » directory, you’ll want to flash it.
This is the option to use.
Press (1),(2),(3),(4) or (5) to select next firmware.
Pree (6) or (7) to flash.

(3 ): DFU (device firmware upgrade)

The code used to upgrade through DFU is contained inside the STM32F4 microcontroller and has been written by STMicroelectronics. So you cannot brick your PreenFM.

I you press the button 3 while powering on your PreenFM it will display a message on the screen than jump to integrated DFU mode.
The process is then the the same as when jmp3 is shortcut so  see burn firmware for more information.