schmitt trigger

Archiver

Hello stampers

Little question of topic, But I want to use a inverter, Now I see
that there are inverters and inverters, type schmitt trigger. This is
also possible for AND ports, etc.

What's the difference betwen a normal port and a schmitt trigger port?

thanks pieter-jan

Archiver

A Schmitt trigger exhibits hysterisis. We like to think of signals as being
0V and then 5V all at one time. In real life, the voltages may not even go
to 0 and 5V. Also there will be high frequency noise and other garbage
superimposed on the signal.

So gates have to trigger at some threshold. Say that threshold is 1.6V (I
just pulled that out of the air). So as the signal goes from say .2 to 4.5V,
at some point it passes 1.6V and the gate output changes. What happens if
the voltage is changing slowly (say an RC charging). Because of the noise,
it might look like this:

.2
.22
.21
.33
.31
.45
.42
.60
.58
.75
......

The point is the voltage wiggles up and down while generally going up.

The Schmitt trigger doesn't have one threshold, it has two thresholds.
Suppose you start with the input voltage at .2V and the thresholds are 1V
and 4V. Since .2V<1V then the input is a 0. When the voltage rises above 4V,
the input is taken as a 1. However, the gate won't consider the input a 0
until it drops below 1V. This makes the little wiggles unimportant. Once the
gate "sees" a 1 it won't stop seeing it until the input voltage is much
lower. So even if it goes to 4.1V 3.9V 4.05V you still see a solid "1".

For most logic you don't need a Schmitt trigger. They are usually used where
you have some analog quantity like a resistor/capacitor circuit, or if you
are trying to make the gate oscillate. In a few cases, you'll want to use a
Schmitt trigger where you know you have a long run to "clean up" a noisy
signal.

Regards,

Al Williams
AWC
* Measure 8 pulse inputs at once: http://www.al-williams.com/awce/pak7.htm


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Original Message

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> From: diepenpj@h... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=7Pl5_4VHqGmAfvxn71zaOMBNh0SkrqPEYfDKbX6LiVxWiRK6gDVXdJPKOf6YnEo3YVVHcEAprgJHK267wQ]diepenpj@h...[/url
> Sent: Sunday, February 04, 2001 10:44 AM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]basicstamps] schmitt trigger
>
>
> Hello stampers
>
> Little question of topic, But I want to use a inverter, Now I see
> that there are inverters and inverters, type schmitt trigger. This is
> also possible for AND ports, etc.
>
> What's the difference betwen a normal port and a schmitt trigger port?
>
> thanks pieter-jan
>
>
>