Why is Vtyp less than Vmin on a CMOS chip

cmosdigital-logiclogic-gateslogic-levelnxp

Regarding the chip 74HC08

On the table describing Static Characteristics (Section 9). Why is the typical VIH lower than the minimum value.
And vice versa for VIL

Best Answer

This question is all about the required input levels into a logic device

The minimum value for \$V_{IH}\$ is the guaranteed value for a "1". In other words, if you equal or exceed this level the input circuit will definitely recognize it as a logical 1. Typically it might work a bit lower than this.

As for \$V_{IL}\$, the maximum value is that input level that would be definitely recognized as a logical zero. Typically this may be a tad higher.

If you look at this picture it might help: -

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It describes inputs and outputs for a typical 5V logic scenario. Note how \$V_{OH}\$ (for an outputting device) is always bigger than \$V_{IH}\$ for a receiving device - this guarantees correct and unambiguous operation. This scenario would not be "spoiled" if \$V_{IL}\$ was a little bit bigger and \$V_{IH}\$ were a little bit smaller and, of course, this is where the typical values come in.