Decoupling capacitor

A small capacitor from a supply pin to ground, fitted as close to that pin as the layout allows. It is a local charge reservoir. The regulator handles the average current a chip draws, and the capacitor handles the gulp the chip takes when its outputs all switch within a nanosecond of each other.

The load-bearing word is local. Copper has inductance, roughly 1nH per millimetre, and inductance resists a change in current. The relation is V = L ยท di/dt. Fifty millimetres of trace is about 50nH, so a demand that rises by 100mA over 10ns drops half a volt along the way. On a 3.3V part that is a 15% sag, arriving exactly when the chip is busiest and gone again in nanoseconds.

A regulator feeding a chip through a long inductive trace, with a 100nF capacitor from the rail to ground right at the chip's supply pin supplies the average about 50 nH of trace inductance regulator chip outputs switching supply pin 100 nF at the pin supplies the fast gulp ground

The failure this prevents is a quiet one, which is why decoupling-present is a warning rather than an error. A rail with no decoupling powers up, runs, and passes bring-up. It fails later and intermittently, as a spontaneous reset, a corrupted register read, an ADC noisier than it should be, or a link that drops once an hour, on three units out of ten and not the other seven. None of those symptoms points at a capacitor, and engineers lose weeks to them at the end of a project rather than the beginning.

A bulk capacitor is the same idea one level up. It is larger, one per rail rather than one per pin, and it covers slower swings such as a whole subsystem waking up. bulk-cap looks for it separately, because a rail can carry decoupling at every pin and still sag when something big turns on.

Two numbers matter once you have decided which capacitor you meant. The vendor states the capacitance it wants, typically 100nF at each supply pin, and that lives in the parameter layer because no netlist implies it. The voltage rating is the other one, and exceeding it has a mechanism behind it rather than a guideline. Ceramic capacitors fail short, so an overstressed decoupling cap becomes a dead short from the rail to ground. cap-voltage checks it with a derating factor and cites the datasheet row it compared against.

A netlist check can prove the capacitor is present. Only geometry can say whether it is close, and a capacitor on the correct net placed 20mm away does not decouple anything, because the loop it forms with the chip has enough inductance to defeat it at the frequencies it was fitted for.

Where the course teaches it: chapter 3 is the whole chapter, from the role through to the copper.