Thanks, that's a nice summary.
But I don't see much sense in simultaneously pulling the pin up with the internal resistor and pulling it down externally. Shouldn't this lead to a continous current from the pin to ground while the button is not pressed?
I think the intention was to make sure you -aren't- enabling the pullup, for the very reasons you gave.
I guessed so, but it only worked the other way around, strangely enough.
I recently started with the Arduino (UNO R3) after I got an ebook at a reduced price. In this book there is an example of a button connected to an input pin of the arduino, pulled to ground by a 10k resistor with a 100nF capacitor in parallel to that resistor to cover up button bouncing.
By the sound of it that's a bad circuit, its a crowbar on the supply voltage. There needs
to be a RC divider to form a low-pass filter, and R and C in parallel isnt a divider.
That's the closest explanation to the strange behaviour so far, I think. I already thought for myself that this setup might lead to a current straight from +5V to ground for short periods of time, but as a beginner I assumed just assumed the example in the book would work correctly.
Thanks for the answers everyone. I hope you had a good start into the new year.