I found this
Changed the clock to 25MHz, recompiled, flashed and got a connection:
[17128.267372] usb 1-1.2: USB disconnect, device number 12
[17142.317964] usb 1-1.2: new full-speed USB device number 13 using xhci_hcd
[17142.429904] usb 1-1.2: New USB device found, idVendor=1d50, idProduct=614e, bcdDevice= 1.00
[17142.429936] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[17142.429953] usb 1-1.2: Product: stm32h743xx
[17142.429967] usb 1-1.2: Manufacturer: Klipper
[17142.429981] usb 1-1.2: SerialNumber: 120010801451373134393635
[17142.438210] cdc_acm 1-1.2:1.0: ttyACM1: USB ACM device
Summary
Checking the ST schematic
The 25MHz oscillator (X3) is optional - and not mounted on my board.
on another page, it’s an 8 MHz oscillator
X4 (a 25Mhz oscillator) is on my board - but it’s mounted to an ethernet IC.
Should I mount the optional a 25MHz oscillator + caps ? It seems to work fine without.