I'd like to make a device similar to the "Digispark", but what really bugs me about those (and most, if not all, V-USB designs, at least all I could find online), is the "waste" of a GPIO pin which is just the negative of another pin for the differential pair. so I wondered if there's an easy (and cheap+small, so low chip count) way to do the differential pair to single ended full duplex conversion?
methods I know of:
- using an opamp to convert differential pair to single ended
- using a NOT gate to convert single ended to differential pair
the problem is I obviously couldn't just put those two in parallel, or I'd produce a feedback loop.
Is this even possible without having a dedicated IC that understands USB or another "wasted" pin to indicate the direction? the circuit would have to distinguish between a driven output and a high-impedance input pin in real time essentially, and I have no idea if or how that could be achieved.