user8033: I was thinking of using multiple analog light sensors (variable resistance by light), but basically I only need to know if there is "a lot of light" or "not a lot of light". And would like to use interrupts.
Since I'm using multiple analog inputs, I can't set the ADC module to interrupt on a specific voltage, for each of the (5?) sensors?
I'd like to do this withouth using a external comparator.
I've checked the ATMega328p datasheet (going to use Arduino Nano probably).

The fun thing, however, is that on 5V a high signal is >2.6V and a low signal is <2.2V
What does this mean for the grey area? Would my inputs be glitching randomly? Would they maintain high, until below 2.2V and maintain low until above 2.6V? Or can the switching point be any where between that (and is it different per chip or pin?
EDIT: I've checked, my voltage divider circuit has 0.6V for "nothing" and 3V for "interrupted", so it should work neverthless. Though, I'm still interested in how the grey threshold area behaves.

user7856: The titles of the graphs are actually saying what happens: the hysteresis.
In other words:

if the input level is high (greater than 2.6V), it will be considered high through the hysteresis area (until it goes below 2.2V)
if the input level is low (below 2.2V), it will be considered low through the hysteresis area (until it goes above 2.6V)

According to the description you gave, this should work for your case.
In case it doesn't, you have to condition it until the values you want to consider 0 and 1 are respectively below 2.2V and above 2.6V, with a safety margin, for good measure (say 30%).
The hysteresis is meant to prevent noise from causing unwanted spurious transitions: without safety margin, it should be greater than 0.4V.
You can dimension the safety margin to meet your needs.
But, for the sake of completeness, I'll say that it might be preferable to have a timer to trigger periodic sampling with the ADC.
If you know the maximum frequency at which every input can vary, you can sample it at twice that frequency and be sure that you will not miss anything. Again, this is theory, I would probably make it 4 times, to allow for unforeseen variations.
This works as long as the conversion time of the ADC is shorter than the maximum combined frequency at which you need to read the values.
The GPIOs, of course, are faster, but might require additional circuitry for the conditioning I mentioned earlier.

8033: <p>I was thinking of using multiple analog light sensors (variable resistance by light), but basically I only need to know if there is "a lot of light" or "not a lot of light". And would like to use interrupts.</p>

<p>Since I'm using multiple analog inputs, I can't set the ADC module to interrupt on a specific voltage, for each of the (5?) sensors?
I'd like to do this withouth using a external comparator.</p>

<p>I've checked the ATMega328p datasheet (going to use Arduino Nano probably).</p>

<p><a href="https://i.stack.imgur.com/Dqnbp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Dqnbp.png" alt="enter image description here"></a></p>

<p>The fun thing, however, is that on 5V a high signal is >2.6V and a low signal is &lt;2.2V</p>

<p>What does this mean for the grey area? Would my inputs be glitching randomly? Would they maintain high, until below 2.2V and maintain low until above 2.6V? Or can the switching point be any where between that (and is it different per chip or pin?</p>

<p><em>EDIT: I've checked, my voltage divider circuit has 0.6V for "nothing" and 3V for "interrupted", so it should work neverthless. Though, I'm still interested in how the grey threshold area behaves.</em></p>
 7856: <p>The titles of the graphs are actually saying what happens: the hysteresis.</p>

<p>In other words:</p>

<ul>
<li><p>if the input level is high (greater than 2.6V), it will be considered high through the hysteresis area (until it goes below 2.2V)</p></li>
<li><p>if the input level is low (below 2.2V), it will be considered low through the hysteresis area (until it goes above 2.6V)</p></li>
</ul>

<p>According to the description you gave, this should work for your case.
In case it doesn't, you have to condition it until the values you want to consider 0 and 1 are respectively below 2.2V and above 2.6V, with a safety margin, for good measure (say 30%).</p>

<p>The hysteresis is meant to prevent noise from causing unwanted spurious transitions: without safety margin, it should be greater than 0.4V.</p>

<p>You can dimension the safety margin to meet your needs.</p>

<p>But, for the sake of completeness, I'll say that it might be preferable to have a timer to trigger periodic sampling with the ADC.</p>

<p>If you know the maximum frequency at which every input can vary, you can sample it at twice that frequency and be sure that you will not miss anything. Again, this is theory, I would probably make it 4 times, to allow for unforeseen variations.</p>

<p>This works as long as the conversion time of the ADC is shorter than the maximum combined frequency at which you need to read the values.</p>

<p>The GPIOs, of course, are faster, but might require additional circuitry for the conditioning I mentioned earlier.</p>
