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BootloaderThe Bootloader is a part of the devices firmware to have the ability to program a new firmware into devices flash memory with just a USB connection and a special application on the computer side.
So how does it work?
Well, if you plug in your device into USB, the Bootloader gets control of the processor.
It checks to see if a certain Jumper is not set for, that’s mean update mode. ( Details for that you will get later)
If so, it runs its own USB code that enumerates the device as a "PIC18F4550 Family Device".
You then run the bootloader application from the computer, and you can reprogram the firmware, but only the part that doesn't contain the bootloader code.
So you can quickly and easily put new firmware on your device without having any extra hardware.