>> That is, from the very Schmitt Trigger idea it is guaranteed
>> that the level below 2.7V won't be considered logic "1".
>
> No, read the datasheet again. =A0The signal is guaranteed to
> be interpreted as high if it exceeds 80% of Vdd.

I did read the datasheet before. It says min logical high is 80% of
Vdd. I did find no clause that it guarantees any level higher to be
interpreted as high; nor find I a clause that levels below 80% of Vdd
are guaranteed not to be interpreted as high.

I used to think that since the feature the Schmitt Trigger is all
about is its hysteresis, and no any other voltages are mentioned, then
that MIN/MAX values are about the hysteresis, that is, the signal
within the "dead range" won't affect theTrigger state. Perhaps I was
wrong, I'll re-check it later.

If hysteresis is unspecified or even is absent, - this won't be a
problem for SDO or SS lines, but for slowly rising/falling SCK (due to
divider's resistors) this may cause "level bouncing" near the point
when logical "0" level starts to be interpreted as high. A significant
load on 5V LDO at the moment would cause (a divider's voltage/input
signal) to "fall back" slightly and  the logic "1" would get back to
"0" for a moment.  Some desing arrangements should be taken to avoid
the effect.

-- =

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