Reply by April 19, 20162016-04-19
```On Sunday, April 17, 2016 at 6:04:02 PM UTC-4, Warren wrote:
> I'm trying to put this in perspective, but I haven't found a satisfying answer.
>
> You have a 1/3 VDD and 2/3 VDD as described below for logic levels:
>
>   https://en.wikipedia.org/wiki/Logic_level (under Logic voltage levels).
>
> And yet the datasheet values are often values that represent 30%/70%. Then there is Don Lancaster's CMOS Cookbook, which speaks of 30% and 70% levels (page 19 of the 2nd edition).
>
> Obviously, the manufacturer's datasheet has the final say, not to mention that 1/3VDD and 30% VDD are close.
>
> But it strikes me strange that we have these two "rules" in use.  Can anyone expand on this?
>
> Warren

It's the same rule: 30% / 70% is 1/3VDD / 2/3VDD rounded to one significant digit. Anyone who makes use of the distinction is in trouble anyway...
```
Reply by April 17, 20162016-04-17
```On Sunday, April 17, 2016 at 6:29:52 PM UTC-4, John Larkin wrote:
> On Sun, 17 Apr 2016 15:03:58 -0700 (PDT), Warren <ve3wwg@gmail.com>
> wrote:
>
> >I'm trying to put this in perspective, but I haven't found a satisfying answer.
> >
> >You have a 1/3 VDD and 2/3 VDD as described below for logic levels:
> >
> >  https://en.wikipedia.org/wiki/Logic_level (under Logic voltage levels).
> >
> >And yet the datasheet values are often values that represent 30%/70%. Then there is Don Lancaster's CMOS Cookbook, which speaks of 30% and 70% levels (page 19 of the 2nd edition).
> >
> >Obviously, the manufacturer's datasheet has the final say, not to mention that 1/3VDD and 30% VDD are close.
> >
> >But it strikes me strange that we have these two "rules" in use.  Can anyone expand on this?
> >
> >Warren
>
> People are just estimating what would be reliable levels; there are no
> rules. Check actual data sheets.
>
> Most CMOS parts actually slice a bit below Vcc/2, 45% maybe. But they
> might draw a lot of power if an input is far from either rail. And
> might oscillate near the transition point. So people push the low/high
> specs close to the rails.
>
> HCT parts are deliberately asymmetric, so a 5 volt HCT part is "TTL"
> compatible, with input limits like 0.8 and 2.4 volts, something like
> that. Lots of sundry cmos parts have HCT-type inputs.
>
> And then there are the Schmitt parts.

I am aware of derivative families, particularly LVC and HCT. The need for level translation has had me studying datasheets lately, and consequently the treatment of VIL, VIH, VOL, VOH etc.

Warren
```
Reply by April 17, 20162016-04-17
```On Sun, 17 Apr 2016 15:03:58 -0700 (PDT), Warren <ve3wwg@gmail.com>
wrote:

>I'm trying to put this in perspective, but I haven't found a satisfying answer.
>
>You have a 1/3 VDD and 2/3 VDD as described below for logic levels:
>
>  https://en.wikipedia.org/wiki/Logic_level (under Logic voltage levels).
>
>And yet the datasheet values are often values that represent 30%/70%. Then there is Don Lancaster's CMOS Cookbook, which speaks of 30% and 70% levels (page 19 of the 2nd edition).
>
>Obviously, the manufacturer's datasheet has the final say, not to mention that 1/3VDD and 30% VDD are close.
>
>But it strikes me strange that we have these two "rules" in use.  Can anyone expand on this?
>
>Warren

People are just estimating what would be reliable levels; there are no
rules. Check actual data sheets.

Most CMOS parts actually slice a bit below Vcc/2, 45% maybe. But they
might draw a lot of power if an input is far from either rail. And
might oscillate near the transition point. So people push the low/high
specs close to the rails.

HCT parts are deliberately asymmetric, so a 5 volt HCT part is "TTL"
compatible, with input limits like 0.8 and 2.4 volts, something like
that. Lots of sundry cmos parts have HCT-type inputs.

And then there are the Schmitt parts.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
Reply by April 17, 20162016-04-17
```>But it strikes me strange that we have these two "rules" in use. &#2013266080;Can anyone expand on this?

The main point is that a great many noobs get misled by the "complementary" in the name 'CMOS'. They interpret it as meaning "symmetric", and so expect the logic threshold to be near V_DD/2, which it frequently isn't.

Cheers

Phil Hobbs
```
Reply by April 17, 20162016-04-17
```I'm trying to put this in perspective, but I haven't found a satisfying answer.

You have a 1/3 VDD and 2/3 VDD as described below for logic levels:

https://en.wikipedia.org/wiki/Logic_level (under Logic voltage levels).

And yet the datasheet values are often values that represent 30%/70%. Then there is Don Lancaster's CMOS Cookbook, which speaks of 30% and 70% levels (page 19 of the 2nd edition).

Obviously, the manufacturer's datasheet has the final say, not to mention that 1/3VDD and 30% VDD are close.

But it strikes me strange that we have these two "rules" in use.  Can anyone expand on this?

Warren

```