H733: MCO output active without GPIO init

[LCE]

Heyho,

so I was just checking the MCO1 output of my STM32H733ZGT6 and found some super jittery 64 MHz with full swing between 0V and 3.3V.

Then checked my code and found that I had not set the RCC->CFGR and not even initialized the GPIO.

When I actually set it up it does what it should, but why is there such an output signal without the GPIO having been configured?
This could be some serious EMI source or cause some other trouble if the GPIO is not yet configured, but connected elsewhere (I only have a serial R and a testpoint on my PCB).

[LCE]

No I put the GPIO init before any RCC / clock setup, setting the MCO pin to input with pull-up without any alternate function.

After power-up I still see this crappy 64 MHz on that pin shortly - which makes sense, but is still terrible.

What's wrong here?