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we are facing USB connection issues with our CM4, which opened a new question concerning the usage of USB with it.

To function properly, USB apparently needs Pullup/Pulldown resistors (see here and here). The USB hub used on the CM4 IO Board states in its Datasheet (on page 1, listed at "Features") that it implements all the necessary "USB termination and pull-up/pulldown resistors".

However, we are using the USB port of the CM4 directly to connect with a High-Speed device without a USB hub. Initially we were connecting to our downstream device without any resistors, just as the USB hub is connected on it CM4 IO board (with the exeption of the mux, which should not affect the signal). After debugging for quite some time, we found out that our problems went away after patching 15k Pulldown resistors on the data lines to GND (as seen on other applaction notes to other hubs or on the link before) externally to the CM4 on our PCB.

This poses the question whether a USB termination and/or external Pulldowns are needed and how it should look like on the CM4 to properly function. Not only the Pulldowns, also serial termination is suggested sometimes. I did not find any clues to this on in the datasheet of the CM4. Most resources I found on the general topic of USB termination suggested implementing termination according to the data sheet. Especially the series termination seems to variant to the internals of the used device.

I want to mention that the the problem only happens with specific downstream devices and under certain circumstances. Namely the usage of USB Modem Dongles and mostly in the case of bad reception.

My guess is that the omitted use of the termination is often masked by the usage of a USB hub that implements this feature, by downstream devices doing so, very short connections that are beneficial to data signal integrity or operating the device under good conditions.

Long story short, please help me with the following question: Is external termination and/or Pullup/down resistors needed when using the CM4 USB port and how should it look like?

FYI: for the description of our initial problem, see here and here. I felt that the question for gereral proper USB termination on a CM4 is different than "Why is my USB device not working" therefore the new thread. Please correct me on this if necessary.

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