STM32H523 MCU unwanted dependency between PB14 and PC9 pins

[rbnt]

Hi everyone,
I am working on an STM32H523 MCU and I have recently observed some strange behavior regarding the PB14 and PC9 pins. I have the following setup:

• PB14 is configured as input (I use this for the TIM2_CH2 alternate function, but this does not affect the issue) and is externally pulled up using a 10kOhm resistor.
• The PC9 pin is configured as output.

When the PC9 pin is unset, the PB14 is 3.3V, as expected. But when setting the PC9 pin, the voltage on PB14 drops to only 1.13V.  These pins are otherwise not connected, so 2 seemingly unrelated pins seem to have some dependency within the stm32h523 chip. 

Another test was performed by configuring PB14 as output (without pullup). Now, when PC9 is unset and PB14 is set, the voltage on PB14 is 3.30V. When setting PC9, the PB14 voltage drops to 3.26V, which seems related...

To make sure it is not caused by our own hardware, we did the same test on a STM32H5 Nucleo-144 board (MB1404C), this development board uses the STM32H563 MCU, and we observed the exact same issue.

There seems to be nothing in the errata list regarding this issue.

Thanks in advance.

[STOne-32 · Accepted solution]

Dear @rbnt ,

Here you go with the FAQ

https://community.st.com/t5/stm32-mcus/faq-using-dead-battery-pins-in-an-stm32-with-an-integrated-ucpd/ta-p/695551

these pins have additional function of UCPD1 and should be handled in a way to not affect your application .

Hope it helps you ,

STOne-32