Missing Schmitt Gates??

Oopss...let me rewrite that..

I wonder why these gates look like they're not available at Digik*y .. schmitt NOR schmitt XOR schmitt XNOR and schmitt AND

D from BC

Sorry, but Schmitt trigger inputs weren't created for those parts. BTW, recognizing Schmitt was a bloke, we capitalize his name, hence Schmitt inverter, not schmitt inverter.

You can build your own Schmitt trigger with two resistors:

You need a positive feedback for it, so maybe this is possible for converting an OR gate to one with a Schmitt trigger input, but for NAND you'll need an additional buffer, which can be converted to a Schmitt trigger buffer with two resistors per input.

But if you need more of such basic gates, maybe a CPLD is better. Nowadays parts like the MAX3000A are available for some $1, and they are more powerful than older and more expensive parts like the 16V8 GAL.

But we don't capitalize watt, ohm, volt, or any other SI unit named for a person, in deference to the person. Strange.

Nope. He also must have been involved in the good stuff:

If its any consolation you can get complete Schmitt micro controllers such as the MSP430.

Not the unit names, but the symbols:-

Unit symbols are printed in lower-case letters except that: (a) the symbol or the first letter of the symbol is an upper-case letter when the name of the unit is derived from the name of a person;

and

(b) the recommended symbol for the liter in the United States is L [see Table 6, footnote (b)].

Best regards, Spehro Pefhany

They're not part of the old CMOS line-up, but there are a few you've missed:

a) I think some of the 74hc buffers may have had Schmitt inputs (74hc244?). If not, Fairchild's TinyLogic buffers do: NC7WZ17. b) the single-gate programmable parts NC7SZ57, NC7SZ58 have Schmitt inputs. Make whatever you want! or c) make your own, as Frank suggested. That's what I did as an early user of 74hc, before Schmitt-trigger parts were available.

Cheers, James Arthur

So, you're saying that it should be called a "schmitt trigger", or abbreviated as St? ;-)

Adding external resistors to a gate (or a comparator) has problems. The positive feedback is delayed, so fast noise can sneak through before the feedback arrives. If you add an extra gate to adjust the polarity, that's even more delay so the problem gets worse.

If you just want hysteresis so that you can make an rc oscillator, it's OK, because the input is slow and controlled.

John

Neato... I didn't know about the single gate programmable parts!

And speedy too.. 2.8nS @5V

Thanks

D from BC

Oooh... ;( I didn't think of that.. It's just like using resistors to add hysteresis to comparators....

Cool...something new that I havn't tried yet... Hysteresising gates :)

Thanks D from BC

I think the SI folks are silent on the matter, but I'd call it a Schmitt trigger in deference to the late Dr. Schmitt:

Best regards, Spehro Pefhany

Ahhh... Maybe it should be the Mr. Schmitt inverter... :)

Thanks for comfirming those missing gates..

It's tough to find stuff that doesn't exist :)

D from BC

Cool. At 8.5 cents each in 1K they won't break the bank.

Best regards, Spehro Pefhany

I was just starting to do the analysis on that...then I saw your post..

(Another ball of paper hits the rim of the waste basket... ) :)

The NC7SZ57 universal configurable logic gate is looking waaayy better now.

I guess the built-in hysteresis in Schmitt gates is done differently compared to use of external resistors on a regular gate.

D from BC

That's probably because they're natural phenomena; all that the people did was come up with a way to measure them. The Schmitt trigger is a contrivance, in the same sort of sense as, say, the Wankel engine.

Cheers! Rich

Sure thing. There are plenty more -- I think the entire 74AUP1/2/3xxx- series' (TI, NXP) inputs are Schmitts too...

... ...

......

Ahh, here we go:

Cheers, James Arthur

Perhaps Schmitt was Schmitten down before he'd done these.

You can have hysteresis without feedback. In a cpld, you could use two inputs and externally bias the signal up/down, maybe +- a junction drop, to shift the logic thresholds. Internally, the two inputs would whack an r-s flipflop.

That's pretty much tease-immune. If you use two comparators and a couple of dacs, you get precise, programmable hysteresis.

John