Reading a button

The same pin can be an input. Your program reads it and gets a 1 or a 0. The obvious wiring is a button between the pin and ground: pressed reads 0. The catch is what the pin reads when the button is not pressed.

An input connected to nothing is called floating. It has no defined level, so it picks up whatever is around: the wire acts as a small antenna, and touching the board changes the reading. A floating input reads random 0s and 1s. This is the number one beginner surprise.

The fix is a pull-up resistor: a resistor from the pin to the supply, usually 10 kOhm. With the button released, the resistor holds the pin high and it reads 1. Pressed, the button shorts the pin to ground and it reads 0. The resistor limits the current that flows while pressed. Nearly every MCU has an internal pull-up you can switch on in software, so the resistor is often one line of configuration rather than a part.

Notice the logic is inverted: pressed is 0. That is completely normal and it is called active low. LEDs are sometimes wired active low too, with the LED between the supply and the pin, so that writing 0 turns it on. Read the schematic, then write the code to match.

The second surprise: a mechanical button does not close cleanly. The metal contacts bounce open and closed for a few milliseconds before they settle, so a program that counts edges sees 3, 5 or 10 presses for one push. This is called contact bounce, and the usual fix is in software: after the first change, ignore the pin for 20 to 50 ms, or require the same reading several times in a row before accepting it.