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can't we download hex file without the help of computer. we can use some othe device like pen drive or sd card to carry the hex file and download it into mcu.
"we can use some othe device like pen drive or sd card to carry the hex file..."
Yes, you could use these devices to carry the data (whether as an Intel Hex file or otherwise)
"...and download it into mcu."
No: they are just storage devices - they do not have any means to actually load the file into the chip, do they?
You would have to provide some sort of bootloader in the chip that could read the data from the storage device, write it into CODE memory, and then start to run it.
Of course, for a pen drive, you would also have to provide a USB Host interface...