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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.

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    \$\begingroup\$ USB is not true differential. There are states that require both D+ and D- to be low. \$\endgroup\$ – Tom Carpenter Jul 30 '18 at 21:00
  • \$\begingroup\$ What are you going to use the extra pin for? If it is a microcontroller with a built-in USB peripheral and a diff pair of signals, you won't be able to use it for anything else. \$\endgroup\$ – Eugene Sh. Jul 30 '18 at 21:00
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    \$\begingroup\$ @EugeneSh. V-USB is a "virtual USB" implementation - allows AVRs with no USB peripheral to emulate USB. \$\endgroup\$ – Tom Carpenter Jul 30 '18 at 21:02
  • \$\begingroup\$ @EugeneSh. as hinted by my mention of V-USB and "cheap", it's not a USB peripheral (because those usually cost more), but essentially low-speed USB1.1 bit banging. the Digisparks for example run on an ATtiny85 \$\endgroup\$ – nonchip Jul 30 '18 at 21:02
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    \$\begingroup\$ You could look at the ATtiny861 which has more pins and is still inexpensive. Unclear if it's an official V-USB target but there are indications people have gotten it to work. But realistically, consider something cheap with a true USB device engine. \$\endgroup\$ – Chris Stratton Jul 30 '18 at 21:10
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USB is not always differential. There are signalling states that require both D+ and D- to be low.

Additionally D+/D- are used for other purposes, such as performing reset and speed detection which require sense resistor configurations to be changed.

The pin isn't "wasted", it's "required". I suppose you should think of it less like a differential pair, and more like a clever two-wire communication protocol that is sometimes used differential to improve speed and immunity, and sometimes single ended to do control stuff.

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    \$\begingroup\$ i wasn't aware of that before (D+/- are usually just described as a differential pair, and the names also really hint towards that), but yeah ok, obviously i can't do it then (except by adding a dedicated "usb to 1 wire" ic or something like that, which... no. :P) \$\endgroup\$ – nonchip Jul 30 '18 at 21:05

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