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?
Answer
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):
- Tip --> D-
- Ring1 --> D+
- Ring2 --> GND
- Shield --> VUSB
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.
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