Hello,
From the UM2408 document, the HSE is connected to ST-Link MCO output which is supposed to be 8MHz.
When you select Nucleo-H755ZI-Q board, the HSE input is locked to 25MHz.
Is it a CubeMX bug or does the MCO clock really 25MHz ?
It's grayed out, which means HSE is not enabled.
Enable HSE in the RCC tab and you can set it to 8 MHz or whatever the ST-Link is outputting, which is configurable in the ST-Link firmware upgrade.
By default, it should be set to HSE Bypass at 8 MHz.
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