Its more simpler. Dunno why no one gives the right advice. Stefan, if you read the data sheet, and the ports section in particular, it says that writing binary 1 to a port pin configures it as an input. At reset, all ports are by default written with all 1's. So even a reset to the chip doesnot bother you that the pins might be in driven state. They are tristated. If you look at the port structure, the port driver transistor is either pulled up by resistor OR by pull-up FETs. When you write 1 to a port, it turns the main transistor off. The pull-up transistor OR resistor is conducting, but since the main transistor is off, it DOESNOT conduct. Hence even if you pull your output low or high, it doesnot matter as it is protected by the internal pull-up. Hope this solves your problem!