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Based on what I'm reading in the USB4 V2 spec (section 2.1.1.4), in addition to less technical references I've been able to find online, all USB4 hosts, hubs, and devices are required to support 20G (i.e., 20 Gbit/s) operation. This effectively means that any and all USB4-compliant hardware can operate at 20 Gbit/s, in addition to other operation modes they may support, including other modes they are required to support (such as 12 Mbit/s Full Speed mode, which the spec also requires for all hosts, hubs, and devices).

This seems to imply that a device with very minimal transfer speed needs (a mouse, for example), in order to be USB4 compliant, would be required to support operation at 20 Gbit/s.

What's not clear to me from the spec is whether a device like a mouse could be considered to be USB4 standards compliant even if it never actually operates at 20 Gbit/s.

There is, quite obviously, no reason for a standard computer mouse to operate at 20 Gbit/s, and some other operation mode, like 1.5 Mbit/s, would be more appropriate. But if someone did want to make a USB4-compliant mouse, I'm not understanding how the specification would accommodate that.

It seems like there are two possibilities:

  1. The mouse would have to include 20G hardware and actually operate over a 20G connection, even though only a tiny fraction of the available bandwidth would ever be used; or

  2. The mouse would have to include 20G hardware, but choose to operate at some much slower, more appropriate mode (like 1.5 Mbit/s).

With the first, the device would clearly be USB4 compliant. In the case of the second possibility, it's unclear what it would mean for a device to include 20G hardware but literally never operate at 20 Gbit/s. To me, that seems the same as not supporting 20G operation, which would mean the device is not USB4 compliant.

I understand that USB4 compliant devices all do support USB 1.1 and USB 2.0 signaling rates and, ignoring specified requirements, have the technical capacity to operate at speeds significantly less than 20 Gbit/s, even in a connection with USB4 hardware on both ends. My confusion is specifically related to a device's ability to choose to operate at those slower speeds, and whether that's allowed according to the spec.

Is there another interpretation of the USB4 spec that clarifies how this sort of situation should be handled? Or, perhaps, is it the case that one of the purposes of the USB4 spec is to completely differentiate low speed and high speed devices, with the intention being that if something is USB4 compliant, and labeled as such, it is a device the requires or benefits from high speeds (with lower speed support existing only for backwards compatibility, as opposed to being a single spec that is appropriate for both high speed and low speed devices).

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  • \$\begingroup\$ Is there anything that says a USB4 host can't communicate with a USB 1.1 device? I'm pretty sure nothing in the standard says that devices have to be USB4. \$\endgroup\$
    – Hearth
    Commented Sep 19, 2023 at 3:04
  • \$\begingroup\$ If you are making e.g. a mouse that will refuse to communicate at USB4 protocol even if it is capable of USB4 protocol, then it's not an USB4 device as it does not communicate at USB4 protocol and can't be tested and certified according to USB4 specs. For what purpose you are trying to find different interpretations to the standard? \$\endgroup\$
    – Justme
    Commented Sep 19, 2023 at 8:15
  • \$\begingroup\$ Is this question unique to USB4, or could you ask the same thing about USB3 too? If your USB mouse does not use the SuperSpeed pair, then it's not USB3 compliant either. You can slap a Type-C connector onto any USB device without making any attempt at labeling it as USB3. \$\endgroup\$
    – brhans
    Commented Sep 19, 2023 at 11:31
  • \$\begingroup\$ @brhans I believe this is unique to USB4. Based on my understanding of USB 2.0 and USB 3.2, USB 2.0 does not require a complaint device to implement the 480 Mbit/s signaling rate, and USB 3.2 does not require a compliant device to implement any of the SuperSpeed or SuperSpeedPlus operation modes. \$\endgroup\$
    – Farski
    Commented Sep 19, 2023 at 11:47
  • \$\begingroup\$ @Hearth The spec does require that a USB4 host be able to communicate with a USB 1.1 device. And you're correct in that there is no requirement that all new USB devices be USB4; a mouse designed an manufactured in 2025 could certainly implement only USB 2.0, and use a USB-C connector, and work perfectly well in a USB4 world. My question is mainly if that's the way the USB4 spec is intended to work (meaning, USB4, by design, should not be used for a low speed device like a mouse). \$\endgroup\$
    – Farski
    Commented Sep 19, 2023 at 11:51

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There is no such thing as "USB-4 only" setup. USB4 is a USB4 capable link that can reuse Tx-Rx lanes at USB3 protocol speed/format if USB4 fails to train at USB4 level or a cable does not support USB4. And USB4 standard mandates the use of a SEPARATE USB2 differential pair that works independently of super-speed wires and provides capability to serve legacy USB2 HS, FS, and LS devices. This is the same as for USB3 framework, LS/FS/HS devices are supported over the legacy two-wire interface, in parallel to super-speed wires. It is not a "practical solution", it is USB standard.

So, there is no such thing as a "low speed USB4" device. Just like there were no "low speed USB3" devices. USB2 devices are a special case however. Legacy USB2 devices do not need to be "USB4-compliant", it is a host that wants to be USB4-compliant must support USB2-compliant devices.

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