To save time, breadboard pins or just lack of knowledge people try to skip adding even one decoupling capacitor to a circuit. Either on IRC or in Forums you can almost always see it coming: “randomly, my circuit stops working” And then, “what do you mean a decoupling capacitor?” question.

While working on breadboard Arduino, I came across some unexpected measurements. Initially, the only capacitors on the breadboard were the two 22pF from crystal to ground and the capacitor connected to RESET for Auto-RESET.

Keep reading to find out what happen when I added a 100nF and a 1µF cap. A bunch of scope traces and surprising results follow.