I/O Pin status during startup

[Jerson]

Hi
To be accurate, during power on, these I/Os turn on and off (and then stay off) very quickly (like a flash). The idea is to stop that high-low behavior.

Simple answer- you cannot do that.

You should design your project with active low outputs to account for the High-Z input state at power on. Almost every microcontroller, I know, behaves similarly unless there is special hardware built in to retain a certain state at power on. Till such time as your program code runs, the microcontroller IO pins are kept in a High-Z state to prevent unintentional damage to itself.