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I know that the inverter switches when it crosses switching threshold,but I have doubt that inverter switching should be considered when it is in Vil region or vih region.

for example , suppose Vm=0.45 Vil=0.35 vih=0.55

so when v input changes above vih or below vil then only switching should be considered and not when it crosses Vm...Is this thought process right?if yes then what is true meaning of Vm.

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In this mode of operation, it's best to think of an inverter as being a very high gain inverting amplifier. A slight movement around the Vm point will be amplified on the output. Indeed that is one application of inverters as analog amplifiers around a limited input range.

Whether or not it get's inverted is determined by whether or not teh follow on logic reads the input as having met their threshold.

So it's inaccurate to say it's indeterminate, perhaps transitioning is the better way of looking at it.

Here is a link to an old fairchild datasheet (warning *.pdf) that shows the curve.

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Reading across the lines, I assume that Vm is the switching threshold, and the voltages are given as a fraction of the supply voltage (Vcc).

These specifications say that any voltage below Vil will be considered as a Low, while voltages above Vih will be considered as High - the device will not switch while the input voltage is above Vih or below Vil.

If the input voltage is between Vil and Vih, the input state is indeterminate - the device may consider it as either High or Low. The Vm spec indicates that input voltages between Vil and Vm will probably be considered as Low, and voltages between Vm and Vih will probably be considered as High. I'd expect Vm to be stated as a typical (not guaranteed) value which may vary between devices.

I hope this answers your question - it would have helped if you had provided a pointer to a datasheet or whatever caused you to ask the question.

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The Vil and Vih specs are promises about every part you buy with that part number. The switching threshold is a characteristic of each individual part at a particular operating condition.

The switching threshold is promised to be above Vil and below Vih, but it is likely to vary from part to part, at different temperatures, or if Vdd changes.

It's even possible for the switching threshold to be different for low-to-high transitions than for high-to-low transitions. Schmitt trigger devices are deliberately designed this way.

In simulation, the model will generally have a "typical" switching threshold, somewhere between Vil and Vih, without variation between instances. This is one of the ways that simulations don't exactly reflect reality.

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