Why stm32h5-nucleo144-board USB need ST_LINK to plug-in, then USB can work?

[yingchuan]

I'm using  stm32h5-nucleo144-board to develop USB (HID or CDC_ACM) application, I start with STM32Cube_FW_H5_V1.3.0 code by directory STM32Cube_FW_H5_V1.3.0\Projects\NUCLEO-H563ZI\Applications\USBX\Ux_Device_HID_Standalone.

firstly, when I download the code into MCU and plug-in USB port(CN13) and change power supply to USB USER(JP2), it doesn't work. but when I plug-in stlink V3 USB(CN1), PC sussessful enum USB(STM32 HID Mouse Device) even without power change.  After this, I plug-out  stlink V3 USB(CN1), USB still here.

BUT without ST MCU(3V3_STLK) running, USB doesn't work. 

I have do many times try, and think that is because STLINK MCU do something to let STM32H5 USB to work when the time plug-in? THE only connect between STLINK MCU & STM32H5 is SWD and VCP port, could someone tell me the LINK-MCU role in this application?

[TDK]

The st-link chip doesn't configure the main chip, should work fine without it.

• Verify the board is powered.
• Verify NRST is high.
• Verify clock source being used by the chip is okay.

[AScha.3]

You have to set jumpers, to give st-link power, also if power from ext. or user usb coming. (1-2 , 9-10 jumpers)

-> Because st-link generates the mco -> cpu clk , + cpu reset .

Or modify board, add crystal , cut connections to st-link for nrst + mco.