eli5: if input pins are in a high impedance state when no voltage is supplied, how is that equivalent to having a LARGE SERIES RESISTOR in front of the pin?


Wouldn’t the hypothetical large series resistor make it harder for current to flow through? Or is it because the pins can still change states even with the very little hypothetical current they get? I don’t know my own confusion lol.

In: 1

In a DC circuit the impedance is the same as resistance as there is no reactance component to the vector. So in the DC circuit you can replace it with an equivalent resistor to simplify calculations.

I’m assuming the question is about microcontroller input-configured pins?

Pins on a microcontroller that are configured in the high-impedance state can be pictured like a sea wall at a city’s ocean shore, stopping the ocean’s water at that point. High tide or low tide can be detected and measured from that wall, and that information can be sent to other places. Those places can act on that information in any way the controller chooses to.

When you want current to flow through a pin, such as to power an LED in an external circuit, you’d write your firmware program, to configure the pin in a low impedance state, to assert a voltage on the pin — allowing current to flow in or out…

…Like opening the sea wall around the city, to let a river flow to a low-tide ocean. Or letting a high-tide ocean flow inland into a low lagoon. It’s up to you to design an external circuit that restricts that current to stay within the “Max” current specs for that microcontroller.

A complete microcontroller data sheet will have electrical diagrams to show the electrical implementation of the Port pins.

>Wouldn’t the hypothetical large series resistor make it harder for current to flow through?

Yes. Which is exactly what you want.
Input pins measure the *voltage*, not the current.

You want to have as little current flow as possible, as that is just wasted energy and extra heat you have to deal with.