One of the selling points of the USB-C plug is that, supposedly, there is no way to insert it incorrectly. How [Pim de Groot] shows with a ‘Cursed USB-C 2.0 device‘, the reality is a little more complicated when it comes to USB 2.0 compatibility on USB-C. He made a printed circuit board that elegantly demonstrates the simplicity of the problem, with two LEDs. Only one orientation of the USB-C plug will cause one of the LEDs to light green, with the other orientation leaving both LEDs flashing red.
The reason for this behavior is simple: how [Pim] explains, although the USB-C plug has only a single pair of data lines (D +/-) for USB 2.0 connectivity, the receiver duplicates them on both sides of its pins, taking out two pairs of D + / lines -. Normally, you would connect the corresponding lines in these pairs to ensure consistent behavior, regardless of the orientation of the plug, but it is not necessary.
When driving each USB 2.0 data pair to its own SAMD11C MCU, only one of the MCUs would be connected to the USB, resulting in the connected MCU flashing the LEDs. With a little more circuits it is possible to detect in which direction the plug is inserted and to use this information in a single MCU system, changing its behavior. Although at first glance this looks like little more than a fun trick for a party, but it also offers insight into a possible failure mode of USB-C 2.0 devices where only one plug orientation works, due to broken strokes or pads .
(Header image: Cursed USB-C 2.0 device, from Pim de Groot)