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?
You must log in or # to comment.
In resistive impedance it’s a build up of the magnetic field, like a flywheel.
No that’s inductance not resistance. And for the flywheel analogy it’s inertance (inertia) as it’s kinetic energy.
Think of resistance as the portion of impedance that does not have a dependence on frequency, and dissipate energy in the form of heat. Capacitance and inductance together form the portion of impedance that is dependent on frequency. Further, capacitance is storage of potential energy and inductance is of kinetic energy. The charge is in motion in an inductor which created the magnetic field.
Capacitance also acts as a high pass filter (meaning it doesn’t react to / filters out high frequency inputs) and inductance as a low pass filter. There’s analogies to all this in mechanical and fluids as well. Capacitance’s high pass filtering can be thought of like a spring, and inductance like a mass or flywheel or volume of fluid in a pipe.
Floating implies not grounded, but not necessarily open circuit. Being open circuit could be thought of as having very high impedance (specifically resistance). A high enough voltage (force) would be required to cause a flow (current, arc) to close it. Floating implies it is closed circuit but not to a ground which is like a sink where the energy can dump into if no where else to go.
For chips (ICs) they often have high internal impedance so they don’t draw a lot of power through them but can still operate on the signal as intended.
Resistance is like shocks on a car… push hard to compress and it compresses faster. push less hard (voltage) and it doesn’t move as fast (current). Pull it (negative voltage) and it expands (current flowing the other way). Resistors resist (voltage against) flow (current).
Capacitors you sorta seem to get: current flowing in one direction through a capacitor builds up voltage the remains after the current stops… like the force in a spring builds up as it compresses and when the motion stops the force is still there.
What you seem to confuse with resistance is inductance, where the force (voltage) on an air hockey puck makes it speed up (current flow), and when the force stops pushing it it just keeps moving (current keeps flowing).
The general term for these voltage-current relationships is impedance, because in the general case where voltage or current is oscillating or rapidly switching on and off you get some effects that resemble resistance (voltage pushing back on current or vice versa).
Final concept is that any time you have something trying to force specific levels of current or voltage on a pin, the “setter” (whatever is doing the forcing, typically referred to as the “source”) has impedance and so does the “getter” (whatever is being forced, referred to as the “load”). If you have a fishing rod and you want the tip to move slowly, you can easily move it where you want it to go, but if you want to shake it fast it won’t move as far (the weight of the tip is like inductance resisting the motion with force/voltage).
So, a microchip pin might have high resistance to ground but also high capacitance to ground… and a quick pulse of voltage will immediately cause current to flow into the empty capacitor, and if the capacitance is big enough the voltage won’t change much, or will require more time to change. High capacitance has low impedance… it sucks up any available current as the desired change in voltage happens. interestingly, there are two options for making the pin voltage change faster… increase the current level being used by the source (by reducing impedance within the source so it can get out to the pin easier), or reducing the amount of current required to change the pin voltage by raising the impedance to ground inside the chip package (that is, reducing the capacitance inside the chip package).
When the source impedance is very very large, that is like having the signal generator probe laying on the bench instead of connected to the pin. When the source impedance is large and the internal pin impedance is large, then any stray electric or magnetic fields can push the pin voltage around easily. This is what they call floating… and if the microchip is reacting to those erratic voltage signals then the circuit as a whole will behave erratically as it tries to react to noisy input.
An output pin usually (but not always) has a lower source impedance than a tri-state input in its high impedance state. If you connect it to a floating input then the input stops floating and follows whatever the source is forcing it to.
An input pin usually has an input impedance similar to the source impedance of sources connected to it… this generally allows the input to be controlled most quickly. Inputs whose voltage doesn’t change quickly tend to be less useful than ones the do change quickly bandwidth and clock speeds can be faster.
If you try to connect microchips built with different technologies together (e.g. CMOS vs TTL) then they may not communicate quickly or with minimal wasted power because they have different typical impedances (and voltage levels).
Why do some ICs have tri state, low, high, and high impedance? Isn’t high impedance the same thing as floating?
It means that it will resist being changed by inputs. Yes, like a pin that’s floating relative to the chip.
If it is high impedance that means it had to be connected to something, right? Some kind of big capacitor or inductor in the chip?
No, it’s probably a transistor (active component) that switches to a highly resistive state, leaving the output pin effectively floating - that is, not connected via the chip. Impedance relates to how quickly the charge in that lump responds to voltage (or how quickly matter responds to force in a mechanical system). Not responding is very high, responding quickly is low.
Capacitors and inductors effect impedance, but they aren’t the only things that can do so, and in fact impedance tends to very with which frequency you’re measuring it at, so you can’t really say it has a certain exact value without context.
High - Connected to the high reference voltage.
Low - Connected to the low reference voltage.
High impedance - Not connected.
I thought transistors were just base, collector, and emitter.
How is it switched to be in a high impedance state?
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.
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.
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.
