The NRST pin is bidirectional. It is handled by the on-chip Reset Controller (RSTC) and can be driven low to provide a reset signal to the external components or asserted low externally to reset the microcontroller. It resets the core and the peripherals, with the exception of the Backup area (RTC, RTT, Backup SRAM and Supply Controller). The NRST pin integrates a permanent pullup resistor to VDDIO of about 100 kΩ.
By default, the pin is configured as an input.