Risk of leaving CN13 (User USB) connected without powering the Nucleo

[DStu]

In the UM2407 Rev 5 (Nucleo-H753ZI user manual) on page 29/54, section 7.6.6 there is a warning:

USB Micro-AB connector (CN13) cannot power the Nucleo144 board. To avoid damaging the STM32H7, it is mandatory to power the Nucleo-144 before connecting a USB cable on CN13. Otherwise, there is a risk of current injection on STM32H7 I/Os.

However, in an informal empirical test it seems that nothing got damaged when I connected CN13 to a PC/Laptop without powering the Nucleo. So, how big is this "Risk" really and what are potential consequences? And is this risk related to the Nucleo only, or would the same risk apply when integrating a STMH753ZI directly on a PCB. 

Can anybody provide more detailed information?

Note: I do NOT want to power the Nucleo from CN13 but just leave the USB cable connected.

Thank you and best regards

Diego

[thismarkjohnson]

The risk is real but not guaranteed — connecting USB without board power can back-power the MCU through the USB data pins, causing latch-up or long-term I/O degradation. You got lucky, but repeated use can damage it over time. This isn’t just a Nucleo issue — the same risk exists on custom PCBs unless you add proper isolation or protection on the USB lines.