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/

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.

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.

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

As for battery-powered solutions, I've heard the TPS630701 is a great option.

There are many.

For signals there are level shifters that convert digital signals with one voltage level into digital signals with another voltage level.

Any recommendations?

For unidirectional signals: 74LVC1T45/2T45/8T245.
For bidirectional signals TI TXB0102/04/08

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

Statistics: Posted by fchk — Fri Aug 29, 2025 4:53 pm

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