> do not apply a noisy signal to an RPi input and expect reliable results.
No signals, nor ground nor power are completely free of noise.
> Hysteresis protects inputs from a slow monotonic transition, not a noisy one.
One of the key protections that a Schmitt input provides is against noise during what would otherwise be the near-metastable input voltage range. And one particular variety of that noise to be avoided is noise on the power/ground caused by the current spike of the slow transition-in-progress of this particular input and its downstream stages shorting the power supply. This supply noise couples back to the input and results in oscillation and a burst of transitions at the output. And that's not the only source of noise.
> All digital systems are susceptible to metastability. If an input isn't held at a defined logic level for the specified length of time before the sample clock edge...
Right -- one might distinguish two parts to the metastability problem: (a) a slowly transitioning input that passes through the the metastable range of a subsequent (non-clocked) stage, and (b) an input that transitions slowly or rapidly, at a time that fails to meet setup and hold for a clocked stage. A Schmitt input helps explicitly the (a) part of the problem, and for slow signals greatly improves the statistics for the (b) part of the problem, but doesn't eliminated it. For GPIOs which have no accompanying clock, one would hope that the SoC contains synchronizer stages to address the type (b) problem. (Various ATMEGA and ARM literature indeed show such synchronizers on GPIOs and interrupt inputs).
> If you have a noisy signal, by definition that's analogue
At some level, all signals are analog!
>and you need to make it non-noisy before applying it to a digital input. You must not rely on a little bit of input hysteresis to eliminate noise.
Yet all signals include noise. And consequently Schmitt trigger ICs tout their noise immunity as a feature. See for example https://assets.nexperia.com/documents/d ... 4LVT14.pdf "it has a greater noise margin than conventional inverters"... "High noise immunity".
> There is no need for the hysteresis on the RPi inputs to be specified,
For what it's worth, I've accepted that RPi foundation doesn't have that spec, and there's no benefit to pressing the point.
> you must supply input signals that meet the logic high and low voltage specifications.
> If your actual signals don't match the specifications, you need to provide conditioning before they get to the RPi.
Sure, regardless of hysteresis, the basic logic level specs have to be met, and RPi foundation published those specs. Yay!