I have already mine PCB with my microcontroller on (Attiny2313) and for now I would like to have a USB interface to talk to the sensors that I control with this microcontroller
I mean - if you have free 4 port pins on your attiny2313 , you can connect it directly to USB bus without FTDI chip and add code to existing program read sensor value . I can not estimate memory usage to hold USB connectivity and your specific program- but it is worth to try.