Decoupling capacitor
also: bypass capacitor · bulk capacitance
Small capacitors placed right next to a chip's power pins to supply sudden current demands and keep the local supply voltage clean.
Digital chips draw current in sharp bursts as they switch. The power supply and board traces can't respond fast enough, so the voltage at the chip dips and noise spreads across the board. Decoupling capacitors act as small local reservoirs. A common starting point is a small ceramic capacitor (often 0.1 µF) at each power pin plus larger bulk capacitors nearby; chip datasheets usually give specific recommendations that take priority.
Placement matters more than value: a capacitor far from the pin, connected through long traces or vias, does little at high frequencies.
Where it bites: missing or badly placed decoupling causes random resets, noisy ADC readings, poor radio performance and EMC failures that are easily mistaken for firmware bugs.
Founder takeaway
Follow each chip's datasheet decoupling guidance and check capacitor placement in layout review, not just their presence in the schematic.
engineer review pending