ATtiny85. How do I burn a bootloader? What is the process?

  • Yes, there are USB interfaces for ATtiny85. Just look at one of the open hardware designs out there, e.g. Digispark.
  • An USB interface is going to require two pins, so using one to save one pin doesn’t strike me as cost effective.
  • I agree with @FAD, that you don’t really need an USB stack, and simply could do a time-delay-at-boot thing with the UART pins.
  • HVSP (the high voltage programming) is not hard to do. If you’re already comfortable with ArduinoISP, you can try ScratchMonkey, which uses the same approach (with a 12V power supply and one transistor) to do HVSP programming.