Usb Mass Storage + eeprom (or flash pic, or eeprom pic)

fkl · Joined: 20 Nov 2010 Posts: 44

Please help me how to make a device that is in the OS defined as a regular USB flash drive, and then works with the external/internal eeprom memory or free space of the flash microcontroller.

Ttelmah · Joined: 11 Mar 2010 Posts: 19470

Don't do either....

On the USB side, you need the bulk device (as in the scope example), with the descriptor type set to 8 (mass storage).
However you need to also be aware of the difficulties this imposes. Remember if Windows is talking to a mass storage device, it will generate a FAT file-system. Now this immediately means a directory, and file allocation table, so several KB used. Windows in recent releases, gives problems handling volumes smaller than about 160KB with any FAT system (FAT12). This was the size of the smallest floppy the system was built to handle. So this is the smallest size the 'drive' can be. You are not going to fit that into a small EEPROM, or the memory of the PIC.
The smallest it can handle with FAT16, is 4.1MB.

You need a lot more memory.

Best Wishes

fkl · Joined: 20 Nov 2010 Posts: 44

Ttelmah · Joined: 11 Mar 2010 Posts: 19470

Great, but in which case it can't be handled as a 'mass storage'' device....

The point is that the Windows drivers make 'assumptions' about what is attached. One of the assumptions, is that a 'mass storage' device _can_ store a 'mass' of data.

You can handle it yourself as a bulk transfer device if you want (this is what the scope example does), or just talk to it as CDC.

Thinking more about it, I'd suggest using the Chip Card Interface Device Class. This is what cards for things like dongles, with a small amount of ROM/RAM use.

Best Wishes