[Diy_efi] CMOS Levels

[Jeffrey Engel]

Bruce,

We're getting off into the weeds here.  The original
RCA 1800-series chips did have an odd set of logic
levels.  I believe Larry Ing's assertion that the
thresholds were 10 and 70 percent is correct for the
chips in question.

The HC and the HCT series chips have the thesholds you
describe.

I suggested using HC or HCT logic to drive the 1843
because TTL doesn't (via specs) meet the higher '1'
threshold the 1800-series CMOS needs, while the HC or
HCT has rail-to-rail output and meets those specs
easily.  It turns out that lightly-loaded TTL _will_
meet the higher '1' threshold, so my suggestion turned
out to be more superstition than fact.  Sorry.

Jeff Engel
--- bcroe at juno.com wrote:
> That is not right.  The threshold is not 10 and 70%,
> although 
> the OUTPUT drive of some chips might be.  The
> threshold 
> depends on the type chip.  An HC chip INPUT
> threshold is 
> close to 50%, but an HCT chip expects a minimum
> swing 
> of 0.8V to 2V.  Check the specific type to be sure.
> 
> Bruce Roe
> 
> On Tue, 17 Jun 03 Larry Ing <ling at calweb.com>
> writes:
> > Hi Jeff,
> > 
> > I was reading your post on the CMOS logic and you
> don't 
> > have it quite correct.  The logic thresholds for
> CMOS is 
> > 10% and 70% of the supply rails, not 50%.  That
> implies
> > that with a 5v supply, the '0' potential is 0.5v
> and 
> > the '1' potential is 3.5 volts.


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