On Twitter, [whitequark] has  found and highlighted an intriguing design – a breakout board for the VL670, accompanied by an extensive yet very easy to digest write-up about its usefulness and inner workings. The VL670 is a chip that addresses a surprising problem – converting USB 2.0 signals into USB 3.0.

If you have a USB 2.0 device and a host with only USB 3.0 signals available, this chip is for you. It might be puzzling – why is this even needed? It’s about the little-known dark secret of USB3, that anyone can deduce if they ever have to deal with a 9-pin USB 3.0 connector where one of the three differential pairs doesn’t quite make contact.

When you see a blue “3.0” port, it’s actually USB 2 and USB 3 — two separate interfaces joined into a single connector. USB 3 uses two single-directional differential pairs, akin to PCI-E, whereas USB 2 uses a single bidirectional one, and the two interfaces on a blue connector operate basically independently of each other. There’s many implications to this that are counterintuitive if you simply take “USB 3.0” for “faster backwards-compatible USB”, and they have painful consequences.

For instance, USB 3 hub ICs have two separate hub entities inside – one for USB 3 and one for USB 2. Even if you have a USB 3 hub plugged into a USB 3 port, multiple USB 2 devices plugged into it still cannot break through the USB 2 uplink limit of 480 MBps. If you ever thought that a faster hub with a faster uplink would fix your USB 2 device speed problems – USB-IF engineers, apparently, thought differently; and you might have to find a workaround for your “many cheap SDRs and Pi 4 in a box” setup. Continue reading “A Chip To Bridge The USB 2 – USB 3 Divide” →