Problem with voltage levels of GPIOs and UART pins on NUCLEO-F413ZH

[nileshb]

We are working on a project called 4G Testjig in which we have interfaced 4G module(EC25) with NUCLEO-F413ZH. We are powering the Nucleo board using micro-USB cable.

We are testing GPIOs and UART of 4G module (EC25) on NUCLEO-F413ZH board using GPIOs and UART pins available on STM32F413ZHT6U controller IC. 3 years ago, after flashing the firmware on Nucleo board, we observed that voltage levels on those pins were 3.3V. It has been working fine since then but currently the 4G Testjigs are not working properly. After debugging the Testjigs we observed that the voltage levels on those GPIOs and UART pins dropped down and are in the range of 0.7-1.2V. Also, the Nucleo board is not showing any debug logs in terminal. We have configured STM32F413ZHT6U for internal pull-ups on those GPIOs and UART lines. Even after isolating the Nucleo board with 4G module, those pins still show voltage between 0.7-1.2V. I tried resetting the Nucleo board and also gave power cycle, but the voltage levels remain same.

The 3.3V and 5V voltage rails also show the correct voltage values on the Nucleo board. After checking all options, we re-flashed the Nucleo board and the voltage levels of those GPIOs returned back to normal i.e. 3v3. The debug logs were also visible again after re-flashing. 

Kindly mention the probable root cause of this issue and also guide us on how we can solve this issue without the need of re-flashing the Nucleo board.

[Karl Yamashita]

So both RX and TX pins are 0.7-1.2V?

Have you removed the 4G module and measure to see if the TX pin goes up to 3.3V?

The TX pin is Push-Pull so it should be 3.3V.

The RX pin, if the internal pull-up is enabled, should also be 3.3V.

If neither pin goes to 3.3V, then there is probably an issue with the STM32. 

[TDK]

Can't see how reflashing is changing anything. Verify the contents of the flash haven't changed. Perhaps an issue with something else pulling down the pin, perhaps a power rail is insufficient.

> Even after isolating the Nucleo board with 4G module

Does this mean the nucleo is disconnected from the 4G module, or are they isolated from something else?

[Sco]

If you can try other IOs with the same actions you done?