OK, you want to see your STM32 board behaving like a USB memory device?
First of all: do you have memory (RAM) on STM32 board which you want to see on PC host?
It works this way:
You configure and build you STM32 board FW so that it will setup an USB device USB memory (USB class MSC).
You have to add this USB stack as MSC device to your STM32 FW project.
This USB MSC works like a SCSI device: it reads the memory via low level primitives like reading and writing sectors on a memory device. The filesystem "interpretation" is done by the host (PC).
You have to implement the read and write functions for sectors on STM32 board, e.g. using internal SRAM as the storage device.
When the USB stack as MSC device on STM32 is running - the PC will see it as an external USB memory device.
It can format, write and read your STM32 like a USB stick.
If you use on STM32 board an external flash (e.g. eMMC), you have to provide the read, write sector functions which goes to you eMMC memory. Bear in mind: if it is a flash (e.g. eMMC), for a write, you have to erase a sector first, whereby the write data is 'cached' (stored somewhere), after erasing this sector you can write the sector.
So, it means:
- besides adding USB FW stack as MSC to your STM322 project
- you have to provide also the correct Read, Write functions for handling sectors
- on flash devices - it is a bit more 'complicated' because you have to erase sectors first before you can write a new sector
- better, to consider to use a STM32 board which supports also your FW as Azure OS: it comes also with support for flash devices, filesystems on flash memory
BTW: just to connect STM32 board via USB and the 'wish' to see it as memory device does NOT work without an appropriate FW for the board built and running (USB MSC device, mapping the SCSI commands to a physical memory).
It is possible (I did it several times on different STM32 boards). It just needs to get familiar how to add USB MSC Device to your FW project.
