USB is both the simplest and most complex interface to use. It is simple to plug in and let the computer handle. It is complex to create and run a microcontroller with it.
— Links —
My Website: https://sinelab.net
Schematic: https://sinelab.net/pdf/usb_test.pdf
Code: https://sinelab.net/code/usbify.zip
ASF-USB: https://asf.microchip.com/docs/latest/asf_usb.html
LUFA: http://www.fourwalledcubicle.com/LUFA.php
USB Datasheet: https://sinelab.net/pdf/usb-20-specification.pdf
ATmega16u2 Datasheet: https://sinelab.net/pdf/ATmega16u2.pdf
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