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Seeing as I am going to be working from home the next couple of weeks (or possibly longer...) I purchased some USB-C to HDMI cables to plug my work Mac into my desk monitors.

They display correctly, but every so often (maybe every 5 minutes) one of them will turn off for a split second which will make the laptop behave like a monitor is being unplugged and plugged back in. I tried only having one monitor plugged in and the issue still persisted.

When running the Intel Power Gadget I notice a spike in the "PKG" and "Core" power every time one of these monitor incidents occur. I have included an example picture, when they hit 50 the monitor went off and on.

Does anyone have any recommendations of how to stop this? I know the monitors are fine as I have them plugged in my PC without issue.

This is a 2018 Mac, 2.2GHz i7 with Radeon Pro 555X

Running system_profiler SPDisplaysDataType yields

Graphics/Displays:

    Intel UHD Graphics 630:

      Chipset Model: Intel UHD Graphics 630
      Type: GPU
      Bus: Built-In
      VRAM (Dynamic, Max): 1536 MB
      Vendor: Intel
      Device ID: 0x3e9b
      Revision ID: 0x0000
      Automatic Graphics Switching: Supported
      gMux Version: 5.0.0
      Metal: Supported, feature set macOS GPUFamily2 v1

    Radeon Pro 555X:

      Chipset Model: Radeon Pro 555X
      Type: GPU
      Bus: PCIe
      PCIe Lane Width: x8
      VRAM (Total): 4 GB
      Vendor: AMD (0x1002)
      Device ID: 0x67ef
      Revision ID: 0x00e3
      ROM Revision: 113-C980AL-075
      VBIOS Version: 113-C97501P-005
      EFI Driver Version: 01.A1.075
      Automatic Graphics Switching: Supported
      gMux Version: 5.0.0
      Metal: Supported, feature set macOS GPUFamily2 v1
      Displays:
        VG270U:
          Resolution: 2560 x 1440 (QHD/WQHD - Wide Quad High Definition)
          UI Looks like: 2560 x 1440 @ 60 Hz
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Display Serial Number: TEHEE004852A
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Rotation: Supported
          Automatically Adjust Brightness: No
          Connection Type: DVI or HDMI
          Adapter Firmware Version: 0.00
        VG270U:
          Resolution: 2560 x 1440 (QHD/WQHD - Wide Quad High Definition)
          UI Looks like: 2560 x 1440 @ 60 Hz
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Display Serial Number: TEHEE004852A
          Mirror: Off
          Online: Yes
          Rotation: Supported
          Automatically Adjust Brightness: No
          Connection Type: DVI or HDMI
          Adapter Firmware Version: 0.00


  [1]: https://i.stack.imgur.com/0owqT.png
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More than likely, you need an active USB-C to HDMI adapter

The 2018 MacBook Pro has four (4) Thunderbolt 3 ports that utilize a USB Type C connector (this is where the confusion comes in). Since it's Thunderbolt 3, it's actually several signals in one port:

  • PCIe
  • DisplayPort
  • USB 3.1 Gen 2
  • Power Delivery

So, when you get one of those "USB-C to HDMI cables" you're actually getting a passive DisplayPort adapter and not an active adapter. Basically, it comes down to the fact that DisplayPort and HDMI are very different signals - most notably, the clock sync signal isn't present. So, to get a good reliable signal, you can do the following:

  • USB-C (DisplayPort) to DisplayPort - This is the best option because there's no conversion
  • USB-C (DisplayPort) Active HDMI Adapter. If you must convert the signal, use an active adapter that actually re-creates the HDMI signaling.
| improve this answer | |
  • Thanks for that very detailed answer. I have purchased some USB-C to DisplayPort cables and now the problem has disappeared – user3213262 Mar 20 at 18:59
  • That’s my preferred method; there’s no conversion of signals. Glad you got it all sorted out. – Allan Mar 20 at 19:05

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