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