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Why do USB-C hubs still have USB 2.0 ports?

 3 years ago
source link: https://dancharblog.wordpress.com/2021/08/19/why-do-usb-c-hubs-still-have-usb-2-0-ports/
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USB 2.0 is over 20 years old, so why do modern USB-C and USB 3.x hubs still have 2.0 ports? Shouldn’t they all be USB 3.x by now?

Some folks would assume this is simply a cost saving measure given that it is cheaper to implement 2.0 vs 3.x. But it is a little more nuanced than that. The answer lies in the pinout of USB connectors. USB 3.x and USB-C have both super speed data lines (SSTX+/-, SSRX+/-, TX+/-, and RX+/-) and low speed data lines (D+/D-):

  • USB 3.0 pinout [courtesy Wikipedia.com]
  • USB-C pinout [courtesy Wikipedia.com]

Physical ports have all the pins available for backwards compatibility with USB 2.0 devices like mice/keyboards. But devices embedded inside a hub only connect to the pins they actually need. For example, SD card readers and Gigabit Ethernet chipsets typically only use the super speed SSTX/SSRX pins, not the D+/D- pins. So what can we do with unused D+/D- pins?

Enter port splitting or port bifurcation.

According to Microchip, port splitting “allows for the USB3.1 Gen1 and USB2.0 portions of downstream ports… to operate independently and enumerate two separate devices in parallel…”. Here is part of Microchip’s guidance on how to implement port splitting:

[Image courtesy Microchip.com]

Essentially its getting a USB 2.0 port for free. There are some caveats with this approach. The USB 3.x device must be internal/embedded and not something that can be unplugged. The USB 2.0 device can either be embedded like an audio chipset, or an external port. Because of the way USB hub controllers are setup, if there is a fault on either the USB 3.x or USB 2.0 device, both may go down. So in this aspect, port splitting can be less reliable with problematic or non-compliant devices compared to truly distinct ports.

Other USB hub chipsets from ViaTech, Genesas, etc. also support this feature but do not provide full public documentation. But you can always tell when port splitting is in use because the total number of embedded devices plus downstream ports will exceed the hub chipset downstream port count. For example, a 4-port hub chipset might have a combined total of 5 or 6 downstream ports/devices. And who wouldn’t want to have a 6-in-1 instead of a 4-in-1 hub for less than $1 USD of extra parts?

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