Compute Module • Re: How do you build a CM4 flashing circuit on a custom board?

CM4 only has a single USB port. There is a multiplexer that selects either the µUSB-B or the hub with the USB-A.

How come on the CM4 IO Board, there are 2 USB-A ports?
https://www.raspberrypi.com/products/co ... -io-board/

Actually, you're right. There are 2 ports, but they share traces. So, I assume the multiplexer would decide which port is active at what time. In fact, all 3 USB ports share traces.

Nevertheless, how would I trigger the multiplexer to select µUSB-B, instead of A? Which pin is that, and what are the signals?

See the schematics. U13 is the mux, ands the S pin is connected to VCC of the µB USB. Connect the µB to an PC, and the mux will switch to that port.

Thank you. Chat says U13 is the "FSUSB42MUX" model. Is that correct? Preferably, I'd use the same parts use on the PI. Is there a page you go to to find the specific component model labelled on a board?

Even when the current comes directly from the USB-A on a laptop via cable? It doesn't output a constant 5V?

Voltage is constant, but current varies, and therefore voltage drop on a resistor.

Note that only VBUS is 5V, D+ and D- have lower voltages depending on the trasnmission mode and speed. These don't get level-shifted.

Hmm, I see. But the CM4 has its own internal resistors to to handle its own current needs. So not something I have to worry about, right? As long as it gets 5V.

They chose 5V for VBUS to allow a certain voltage drop over the cables.

A 1.7V tolerance (given the CM4's default voltage reference for a high signal is 3.3V)? I think also because, higher voltage, lower current, and lower resistive losses.

Do you have any recommendations on understanding USB standards on an expert/engineer level? What I keep finding on the web is just "USB-C is more modern, symmetrical, supports faster transfer". I want to understand the pins in-depth.

https://usb.org/document-library/usb-ty ... release-24
This is the only source of truth - the official standard. What's in there is law.

Maybe you also need the power delivery standard.
https://usb.org/document-library/usb-power-delivery
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Dawg, That's 439 and 547 pages, respectively. I spent the past 3 hours just reading some of the other files that came with the .zip file. There is everything there. Stuff from material, shape, extraction force, design, mating cycles, durability, attenuation tolerance, data transfer algorithms, flammability, data flowcharts, compliance adoption forms". That's an entire scientific field right there.

You really delivered, thank you. I bought the USB micro-B ports, by the way. It's what I'll be using.

Statistics: Posted by Potato_Quality7 — Fri Aug 29, 2025 10:38 pm

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