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

# 4XXX CMOS Levels: VIL/VIH (1/3..2/3 vs 30%..70% VDD)

Started by ●April 17, 2016

Reply by ●April 17, 20162016-04-17

>But it strikes me strange that we have these two "rules" in use. �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

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? > >WarrenPeople 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

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 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? > > WarrenIt'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...