[ajsnigrutin]

USB is hard... you need a chip to deal with it. "Back in my day", you could bitbang stuff with a parallel port, but those days are gone.

I would've picked a more lazy approach, just pull the atmega chip from an arduino (to not cause issues), and use the rx and tx pins on the (now empty) arduino board, since they're both connected to ftdi chip already, and no de/soldering is needed.

[magnat]

You can actually bit-bang low speed USB on Arduino [0]. There is even a bootloader [1] for ATtiny visible to host as HID USB device. It is obviously not compatible with Arduino, but those tiny boards look too cute to ignore [2]. Also, you can use USB port both for downloading firmware and your application.

[0] https://github.com/obdev/v-usb

[1] https://github.com/micronucleus/micronucleus

[2] https://cpldcpu.wordpress.com/2014/04/25/the-nanite-85/