[wtallis]

Transistors that can handle a lot of current or voltage will need to be much larger than most of the logic transistors on a chip, so even though they may appear to be few in number on the schematic, they can add up to significant die area for a small part like a microcontroller.

[Dylan16807]

They need to control the transistor that actually does the output, so they shouldn't need to be very big themselves.

[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.