Embedded and hardware engineering notes, measurements, and tools.https://100nf.com/https://100nf.com/articles/bring-up-a-board-that-does-nothing/https://100nf.com/articles/bring-up-a-board-that-does-nothing/A first-power-on procedure in the order that finds faults fastest: current before voltage, rails before reset, reset before clock, clock before code. Most dead boards fail in the first three steps.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/dc-bias-capacitance-loss/https://100nf.com/articles/dc-bias-capacitance-loss/Class 2 ceramics lose capacitance when you put voltage across them. Würth measured a 25 V X7R part losing 69 % of its value at its own rated voltage — and the datasheet headline never mentions it.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/ferrite-beads-are-not-resistors/https://100nf.com/articles/ferrite-beads-are-not-resistors/A bead is only useful in the middle of its impedance curve, the rated impedance is one point at 100 MHz, DC bias collapses it, and putting one in front of a capacitor can build a filter that amplifies the noise you were removing.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/reflections-you-can-see/https://100nf.com/articles/reflections-you-can-see/The rule is not length, it is rise time. TI's criterion is four times the one-way delay — below it a trace is a wire, above it every unterminated end is a mirror. With a lattice diagram for the ringing you are looking at.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/rs485-termination/https://100nf.com/articles/rs485-termination/Where the 120 Ω comes from, the loop-time test that decides whether you need it at all, why AC termination is a data-rate trade, and how failsafe bias resistors quietly eat a third of your node budget.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/the-via-is-the-decoupling/https://100nf.com/articles/the-via-is-the-decoupling/Würth built one board with the same ferrite and the same two 100 nF capacitors in every channel, and changed only the layout. The worst channel is more than 40 dB down on the next best. The capacitor was never the variable.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/theta-ja-is-not-your-board/https://100nf.com/articles/theta-ja-is-not-your-board/The thermal resistance on the datasheet was measured on a JEDEC board that is not the one you designed. Here is what the number actually contains, and the five terms you control.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/where-buck-ripple-comes-from/https://100nf.com/articles/where-buck-ripple-comes-from/The textbook formula ΔV = I/(8Cf) is the special case where ESR is zero. In the regime most real designs sit in, output ripple is I·ESR — independent of capacitance, duty cycle and switching frequency alike.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/why-100-nf/https://100nf.com/articles/why-100-nf/The value is inherited, not derived. What actually decides whether a decoupling capacitor works is its inductance — and above a few tens of megahertz your 100 nF is an inductor.Sun, 30 Aug 2026 00:00:00 GMThttps://100nf.com/articles/why-the-crystal-wont-start/https://100nf.com/articles/why-the-crystal-wont-start/A Pierce oscillator fails in a small number of ways: not enough gain for the crystal's ESR, too much drive, the wrong load capacitance, or a layout that adds stray capacitance where it does the most harm. Each has a test.Sun, 30 Aug 2026 00:00:00 GMT