STM32G431RB PB4 GPIO Open Drain (no pullup/-down) only reaches 1.2 Volt on 10k 3V3
Hello Community,
given is this circuit:
Typically when I use external pullup circuitry for a signal I set the corresponding GPIO to "Open Drain" and no pullup/-down as the state of the line is defined externally.
Flash_CSn is directly connected to PB4 on an G431RB. It is a single, simple almost straight trace on the PCB, without any branches. There is nothing but EMV that could have an influence on that line, except the MCU and the Resistor (R2) + Supply Voltage. But in the Scope I see no evidence for a potential EMV issue, as the signal levels are pretty clean.
In this case the GPIO-Config is:
The issue is: if I do it this way the line level never reaches 3.3V as it should, but reaches max. 1.2 Volt.
If I change the GPIO Configuration to Push-Pull it works as it it's supposed to work.
I noticed this behaviour also on an G473RC. On this board I was able to disconnect the external ADC reference, which is generated at 2.5 Volt by a CJ431. When I disconnected the ADC Reference physically the Signal Levels of the Open Drain GIPOs became normal. When I reconnected the ADC Reference the levels of the Open Drain GPIOs are pulled down to 1.2 Volt again.
I measured the supply voltage of the MCU and the ADC Reference voltage, they are as they should be.
Edit: I also measured R2, it is 10 kOhm.
Edit 2: I change the title and added PB4, so others who stumble uppon this pitfall may find it easier.
Has anyone an idea what's going on here?
