> Is it all implemented in a single chip, or is there
> host chip and some serial flash roms ?

They are typically 2-chip solutions. The USB peripheral controller and the flash memory. Many of them seem to have the flash memory on a daughterboard that can be replaced with one of different memory density.

One solution, if you can modify the hardware, would be to write to the flash daughterboard directly with the 8051, then reattach it to the USB controller before hooking up to the PC. Obviously not friendly for consumers (depending on packaging), but might be OK for special purpose scenario.

GB