gpio let down 5 V pull-up

[dungeonlords789]

I use stm32f401's PB4 (NJTRST) pin. I attached 5 V pull-up resistor (5.1k) to PB4. While MCU is in reset mode the voltage on PB4 is 4.26 V. Only after starting the system, the voltage increases to 5 V. In my program I configure PB4 as input without pull-up or pull-down, just input. I couldn't find an explanation in AN4899.

Why voltage is 4.26 while reset? Is it because of schmitt trigger?

[Michal Dudka]

I gues that pin has during reset some JTAG function (NJRST), after startup is pin reconfigured from to desired mode...

[dungeonlords789]

I use SWD only.

[Michal Dudka]

It has nothing to do with SWD. Reference manual:

https://www.st.com/resource/en/reference_manual/dm00096844-stm32f401xb-c-and-stm32f401xd-e-advanced-arm-based-32-bit-mcus-stmicroelectronics.pdf

at page 148 in section 8.3.1 specify that

"During and just after reset ... PB4: NJTRST in pull-up"

Your pullup and internal JTAG pullup probably creating voltage divider... After startup, your code set pin from AF (JTAG function) to Input and deactivates that pullup.

[TDK]

Just to clarify, "reset mode" means VDD/VDDA is still powered, but NRST is held low?