I’ve connected the Arduino pro mini (328/5V) to my pcb. Of course it’s not directly soldered to the PCB but using a connector, so I can replace the parts that get bricked during development. I’ve downloaded the blink example using something like this. Directly after flashing it worked, but once I disconnected the flashing adapter it stopped. After remembering, that I’ve to short my optional filter in case it’s not assemble it works.