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For example, 0 is ground or 0 volts and 1 is VDD or 5 volts

what happens if the voltage is 4.99 volts? or 3.2 volts?

Is it 0 or a 1??!?

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  • \$\begingroup\$ The word for the red region on the right is "indeterminate", not "intermediate". The effect is not determined, i.e., what happens is not part of the component's specification. Don't do that. \$\endgroup\$ – Pete Becker Sep 7 '15 at 21:59
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    \$\begingroup\$ As your drawing shows, a logic "0" is not just 0.00 Volts - it is a range of voltage somewhere near Ground, and a logic "1" is a range of voltage somewhere near Vcc. The actual range will be shown in the part's datasheet. The device may do what it wishes with voltages in the red region, so, for reliable operation, we make sure signals don't remain in that region. \$\endgroup\$ – Peter Bennett Sep 7 '15 at 23:09
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It is undetermined. It could sometimes be seen as a '0' other times as a '1'. Different devices might act differently, it might vary with temperature or anything. It may even oscillate between 0 and 1.

It can't be relied upon so don't expect anything at that point. It should be passed through as quickly as possible - a few nanoseconds so as to minimize the time that the logic level is uncertain.

If it is above or below that region the device will work as specified.

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I'm assuming you're talking about discrete digital logic, such as the 7400 or 4000 series of IC's. When an input is lower than VIHmin or higher than VILmax, the device is in an "undefined state" and may* consume much more power than usual (possibly leading to the death of the device.) This is because in this state, some circuit elements inside the chip are all "on" and create essentially a dead short between Vcc and Vss. So this condition must be avoided on most digital IC's.

*Depends on the exact IC in question. Some are "worse" about this than others.

(In highschool electronics class, we designed an electronic "coin toss" using a 7400 series counter and other gates. Many students left input pins unconnected, which is a big no-no but we didn't know any better. Most devices failed, due to excessive power consumption, leading to overheat condition, which was due to several inputs being in a "floating" or "undefined" state.) Always tie inputs either high or low on unused digital inputs.

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As long as it's less than Vilmax (and higher than 0) it's a 0, as long as it's higher than Vihmin (and lower than Vcc) it's a 1.

If it's between Vilmax and Vihmin then it's indeterminate. If it's a bit higher than Vcc or a bit lower than 0 it's still a 1 or a 0 respectively (practical, not textbook answer).

For example, for a CMOS part 74HC00 with 5V Vcc, Vihmin is 3.5V and Vilmax is 1.5V.

So 4.99V is surely a 1 and 3.2V is indeterminate (in practice the latter is usually going to be a 1 because the typical transition is at Vcc/2 and 3.2V is much more than 2.5V- the typical range at room temperature is 2.33~2.67V- but it's almost always best not to depend on this sort of thing).

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If the input voltage is above VIHmin, according to the figure, it is defined as a logical '1'. This would probably apply to the 4.99 V case. (Since the figure lacks any numerical values for the voltages, we can't really know, of course.)

3.2 V would probably fall in the indeterminate region, and the state of the circuit would be undefined, ie it could be interpreted as either a '0' or a '1' depending on tolerances etc.

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If the voltage is between Vil and Vih or Vol and Voh, it is said to be in intermediate stage. In this state, the voltage remains in ambiguous state. The result will not be stable.

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