[nunuvit]

This is a layer below the digital circuit abstraction in the territory of analog circuits and device physics.

The extra current is a physical thing and you need more material to temporarily withstand it, and more circuit to detect and control it. Since it's an uncontrolled switching event, it'll probably ring unless you add even more components to absorb and control that. Then that could exceed the physical limits and trigger a parasitic circuit that doesn't have an off switch, so you need yet another circuit to detect and shut it off somewhere else.

It can be a lot of work for the board designer to make it reliable and compatible, assuming the other chip it's talking to can also handle the extra current. It's cheaper and more reliable to type GPIO1DIR=OUT or whatever. Sort of like when you drive a car, it's easier to choose to drive in the correct lane than it is for the car to enforce it on you and protect you if you do it anyway.

[Dylan16807]

> The extra current is a physical thing and you need more material to temporarily withstand it, and more circuit to detect and control it.

Supposedly most of the chips can already temporarily withstand extra current. But the point of a current-limiting circuit is that you don't have extra.

> uncontrolled switching event

I'm not suggesting turning it off entirely, unless that's much much easier.

> it'll probably ring unless you add even more components to absorb and control that

If it fluctuates some when overloaded, that still sounds better than frying itself. But I'd expect an integrated implementation to keep pretty tight bounds.

> that could exceed the physical limits and trigger a parasitic circuit

What physical limits? You've lost me at this point.

> It can be a lot of work for the board designer

I was suggesting building it into the chip.

[nunuvit]

The current limiter is analog and dissipates a lot of heat compared to digital and also uses more passive devices, so it must be bigger.

Digital CMOS is triggered to switch between fully on and fully off. You can't really hold it in between. If you do, you get undefined behavior.

The ringing can have an initial spike that fries stuff.

CMOS can break down and the current will flow through a different path away from the gate where the gate can't turn it off. Called latch-up.

It is in the chip. The protection circuit can add a lot of parasitic elements to the pin interface that you have to account for when you design the board.

[Dylan16807]

> The current limiter is analog and dissipates a lot of heat compared to digital and also uses more passive devices, so it must be bigger.

Controlling the voltage put into the output transistor shouldn't use much power or output much heat, should it? The output transistor will heat up based on voltage loss, but it needs to be able to handle a notable amount of that even when it's not shorted.

> Digital CMOS is triggered to switch between fully on and fully off. You can't really hold it in between. If you do, you get undefined behavior.

The pins are already tri-state. The logic to output +V, or output 0V, or neither already exists. So it won't fight itself.

> The ringing can have an initial spike that fries stuff.

How can you make a transistor's output spike higher than it does with the existing digital drive method?

[nunuvit]

Turning off the transistor is only the last step and the previous steps like detection take up space. Digital CMOS is bi-state and the pin is tri-state, therefore you can conclude that there are additional components involved to achieve the third state. Spiking can be caused by suddenly shutting off current through a parasitic inductance because it sort of has inertia and can't stop immediately.

[Dylan16807]

> the previous steps like detection take up space

Yes but I'm missing why they would need significant amounts of space or power compared to the big transistor that's actually dealing with the current.

> Digital CMOS is bi-state and the pin is tri-state, therefore you can conclude that there are additional components involved to achieve the third state.

Yeah, so less to add and less to worry about compensating for because it's already handled.

> Spiking can be caused by suddenly shutting off current through a parasitic inductance because it sort of has inertia and can't stop immediately.

It already abruptly turns on and off. How does an extra trigger condition make that worse?

Or in other words, how are we not already in the worst case, with nowhere to go but up? (Since if we're just controlling the transistor better we won't be adding any more inductance than the pin already has.)

[nunuvit]

It depends on the design, but think of it this way. Digital if the smallest you can go. The protection circuit is not strictly digital, therefore it is bigger.

It's not already handled because you still need a circuit that detects the condition and switches to tri-state, if that's even how it's implemented.

Ringing and spikes come from electrical mismatch. If the protection changes the electrical properties of the pin, it may have to do more work to damp out the new mismatch. "Abrupt" isn't a single thing with a universal solution.

We're not just controlling transistors, but also sensing, shunting, clamping, damping, etc. And we're starting from the best case so we have nowhere to go but down.

You'll have to look up the rest yourself.