For a project, I like to use 3.5mm type USB connector. This is for a closed system therefore I am not worried about USB approval but I am worried for signal integrity and insertion/removal problems.
Are there any common 3.5mm 4 contact point USB connectors that I can safely use? What is the usual practice for these connectors?
Some consumer electronic devices have occasionally used 3.5mm TRRS to USB connectors. For instance, the LG A155 mobile phone and several older MP3 players do this.
The most commonly used pin-out (iPod Shuffle 2G, Dolphin Swimmer MP3 player, Samsung YP-F1) is (source):
This is, however, not defined in any official standards document to my knowledge. Thus, calling it "usual practice" might be stretching things a bit.
From a short-circuit prevention perspective, moving away from the practice listed above has advantages:
Having VUSB and GND next to each other may cause an accidental short from power to ground during insertion or removal of the TRRS plug.
However, since D+ and D- ought to be in a single twisted pair in the USB cable to minimize common-mode noise pick-up, this segregation must be done at the TRRS connector, not at the other (USB connector) end of the cable.
Note that using a TRRS connector will only work for 4-contact USB, and will not propagate the USB ID line.
Before going off to devise a special connectoring system upon which to deploy USB you should think long and hard why that seems like such a good idea to you. I suspect that you will find feedback from most others that this is not a good idea and that you should stick with standardized connectors that have been already proven for reliable USB operation.
I can see that you are already wondering about whether this 3.5mm jack idea is going to be a problem or not. I suggest that it will be a problem because you not get the normal GND/VCC make/break behavior that you get for standard USB connectors. Additionally the idea of putting a USB power lead on a 3.5mm jack just sounds like a big short waiting to happen.
Sometimes there are good reasons to use a different connector for USB. One such example is Apple's new Lightning connector used on the iPad Mini and iPad 4. That solution is well thought out and well engineered. And it was done with some very good ideas in mind. One such being that the cable ends of the interface can be inserted either direction making usage much easier for the customers.
