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Physical / electrical concepts

Wire capacitance

Every wire / PCB trace has a tiny capacitance to ground. Usually negligible, but on long I²C buses it's the thing that limits speed.

How it looks

Drawing
A TRACK ABOVE GROUND, CUT ACROSSground planetracktogether they act like thetwo plates of a tiny capacitor
Any wire near ground stores a little charge, more for longer and wider wires.

How it works

  1. A track and the ground under it are two conductors separated by insulation: a very small capacitor.
  2. To change the wire's voltage, that capacitor must be charged or emptied first.
  3. Usually negligible; on long I2C buses with pull-ups, it is what limits speed (the RC time constant).

Where it fits

The same physics, made deliberate and controlled, is impedance for fast lines.

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