Most of the newer SoC chips like Freescale iMX and TI OMAP have the USB core hardware built into the chip, so it's easy to have the chip provide it's own USB bootloader that doesn't require any code out of flash or RAM to get started. The only trick is to make sure the chip can start in this mode when necessary, usually a little helper PIC or some custom gates takes care of that.
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Most of the newer SoC chips like Freescale iMX and TI OMAP have the USB core hardware built into the chip, so it's easy to have the chip provide it's own USB bootloader that doesn't require any code out of flash or RAM to get started. The only trick is to make sure the chip can start in this mode when necessary, usually a little helper PIC or some custom gates takes care of that.