Explain Like I'm Five

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Impedance, floating (self.explainlikeimfive)

submitted 4 months ago* (last edited 4 months ago) by half_built_pyramids to c/explainlikeimfive

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What's the difference between impedance and when a pin on a microchip is floating?

I get the basics of impedance. I'm capacitive impedance it's a build up of charge. Like air in a balloon. In resistive impedance it's a build up of the magnetic field, like a flywheel.

A floating pin isn't connected to anything reference voltage so it can fluctuate with surrounding interference or whatever.

Why do some ICs have tri state, low, high, and high impedance? Isn't high impedance the same thing as floating?

If it is high impedance that means it had to be connected to something, right? ~~Don't~~ Some kind of big capacitor or inductor in the chip?

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[–] [email protected] 1 points 3 months ago* (last edited 3 months ago) (1 children)

That's for a BJT or similar. There's other kinds of transistors, and CMOS digital logic is based on MOSFETs. In MOSFETs, it's gate, source and drain. If you apply the right voltage (negative for p-type, positive for n-type) to the gate of a MOSFET, the resistance between the source and drain skyrockets. It's like pinching off a hose. Ideally when fully closed it's like there's no connection at all. (And the gate shouldn't ever conduct - it just controls the channel between source and drain)

This is pretty much the whole principle behind CMOS, by the way. It's a bunch of hoses pinching each other on and off in such a pattern that it preforms logic. It's easy to manufacture on a chip for reasons I won't go into unless you really want.

[–] half_built_pyramids 1 points 3 months ago (1 children)

Thank you, I think I get it. I was only thinking in one type of transistor.

When the resistance goes high, why is that called high impedance, instead of something like high resistance?

And yes, please tell me about cmos manufacture stuff. Just watched a breaking taps video where he's trying to make his own die 1nm across with lithography. Cool shit, would love to hear any insight you want to share.

[–] [email protected] 2 points 3 months ago* (last edited 3 months ago)

When the resistance goes high, why is that called high impedance, instead of something like high resistance?

Disclaimer that I'm not actually an electrical engineer, but I'm pretty sure it's just convention. Positive charge is also an absence of electrons, just because Ben Franklin guessed wrong, and conventional current goes the opposite direction to the actual current. I probably would have called it "floating", "disconnected", "stopped" or "open", if it was up to me.

As for the manufacturing, making a MOSFET is as easy as taking a wafer of silicon, doping a couple of spots to be opposite to the bulk next to each other, and then oxidising a spot on top of the channel in between to form an insulating SiO2 gate barrier. This is good in terms of steps needed, chemical precision needed, and number of features required per transistor which translates to more transistors per area. CMOS allows an entire chip to be printed in place with barely any more steps, by using both N and P type MOSFETs in complement (Complementary Metal Oxide Semiconductor) to ensure that there's always a path available for current. Then, the only thing left to do is start building up the interconnecting wires over top of the semiconductor with vapour deposition or similar.

There's a video series where Sam Zeloof makes a MOSFET from scratch in his garage. Skip to the second video if you don't care about the theory so much. Wikipedia also has a nice illustration of the process of printing CMOS in more detail.