Electronic – Should I connect the computer VBUS line to the 5V line of the board


I am doing a project that needs USB communication with my microcontroller.
On the board there is already a 5V power supply. I would like to know if it is correct to connect the VBUS line from the computer with the 5V line that already exists on my board (put L4). Or should I leave VBUS floating? (remove L4).

I have no experience with USB development, I know the need for U10, but how I should feed U10 pin 5 to me is still dubious.
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Best Answer

If your board has it's own power, the VBUS from upstream connector should NOT be used as power source and should NOT be connected to the +5V rail. It will cause unpredictable conflict between your on-board power source and USB host-supplied VBUS.

However, your device must have a circuitry (typically a GPIO input with proper level translation and additional ESD protection) to sense the VBUS presence. This function is defined in Section of USB 2.0 Specifications,

The voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the USB cable such that when VBUS is removed, the pull-up resistor does not supply current on the data line to which it is attached.

and further explained in Section 7.2.1, p.171:

They [devices] may not provide power to the pull-up resistor on D+/D- unless VBUS is present (see Section 7.1.5). When VBUS is removed, the device must remove power from the D+/D- pull-up resistor within 10 seconds.

If you have a concern about pin5 on ESD protector U10, then it would be the best to keep this pin at 3.3V form some internal rail, it will provide somewhat better level of protection.

There are more answers on this topic, like here.