Dell U3818DW monitor USB-C hub speeds stuck at USB 2.0

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Dell U3818DW monitor USB-C hub speeds stuck at USB 2.0

Hi all,

I recently upgraded to an M1-based MacBook Air and purchased a Dell U3818DW ultra wide monitor, which I connect to the MacBook Air via a single USB-C cable (which was supplied with the monitor).

This single USB-C connection allows for the display data as well as power delivery and connection of the inbuilt USB 3.0 hub on the monitor, and is one of the primary reasons for having chosen this monitor.

I run the monitor at its native 3840x1600 resolution at 60hz.

I've noticed the following issue with this setup; I have a few peripherals connected to the monitor's USB ports, which should operate at USB 3.0 speeds (up to 5Gbps) but in actual fact I've observed only USB 2.0 speeds (up to 480Mbps) - most notably with a USB NIC which I have connected via the monitor.

Upon investigation, I noted that the MacBook's System Report shows that the Hub connected via the USB 3.1 Bus in question is indeed only recognised as operating up to 480 Mb/s (it is recognised as Product ID: 0x8442, Vendor ID: 0x0451 - Texas Instruments, and is utilising host controller driver AppleT8103USBXHCI.

The monitor is at its latest available firmware (M3B106) and having read through various Dell community threads it appears that others with the same issue have been pointed here. I'm aware that it may well be a firmware bug with the monitor but I thought I might as well also ask here (although I have also posted on the Dell community).

Are others able to reproduce this issue? Is there a solution?

Any help would be very much appreciated.

Thanks

Posted on Dec 9, 2020 6:44 AM

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Answer 1

be77y Author

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Dec 9, 2020 7:42 AM in response to be77y

I've had a reply on my post from the Dell community forum, which I'll add here rather than deleting the thread, in case anyone in the future happens upon this with the same issue:

The manual also states on Page 13 that the USB-C port only supports USB 2.0 data. If you want USB 3.0, you need to connect the display and the system via the upstream USB 3.0 port. The reason is that the display only supports DisplayPort 1.2. Due to the way USB-C works and the bandwidth available on DisplayPort 1.2, the only way to run that display's native resolution and refresh rate over USB-C is to set up the USB-C link so that all four high speed lanes are allocated to video. That means you can only run USB 2.0 data. Running USB 3.x over USB-C requires 2 high speed lanes, but that then cuts available video bandwidth in half, and half of a DisplayPort 1.2 link isn't enough bandwidth for that display's native resolution and refresh rate. So it seems Dell designed the display on the (probably reasonable) assumption that more people would want to run their display at its native resolution and refresh rate than get USB 3.x speeds and have to reduce resolution and/or refresh rate. Newer displays that support DisplayPort 1.4 over USB-C can run higher resolutions while still running USB 3.x when connected to a system that also supports it, but not the U3818DW.

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Dell U3818DW monitor USB-C hub speeds stuck at USB 2.0

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