You don't even need that. The attiny85 is sufficient. See https://github.com/Bluebie/micronucleus-t85 https://github.com/Bluebie/micronucleus-t85 It's a bootl

You don't even need that. The attiny85 is sufficient. See https://github.com/Bluebie/micronucleus-t85 https://github.com/Bluebie/micronucleus-t85 It's a bootloader that detects usb in the boot process. If usb is hooked up, you can upload programs, then it will run whatever code is loaded. This bootloader is the same one used on the digistump. You need 3 diodes and 2 resistors along with the attiny85 chip to make your own usb to serial converter.