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'Schmitt Triggers etc..'
1996\12\17@135658 by Shawn Ellis

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Ok, stupid question time..  Could someone please define for me once and for
all the meaning of the "Schmitt Trigger" and how it differs from TTL?  Also
the Schottkey, but I think this is just a low power TTL...

1996\12\17@140916 by Tony Matthews

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Shawn Ellis wrote:
>
> Ok, stupid question time..  Could someone please define for me once and for
> all the meaning of the "Schmitt Trigger" and how it differs from TTL?  Also
> the Schottkey, but I think this is just a low power TTL...
The schmitt trigger is simply a buffer or an inverter with hysteresis
at the input.A rising signal will trigger a change in output when the
voltage reaches say 66% but if the voltage then starts to decrease the
output will not change state until the voltage drops to say 33% this
difference in trigger levels is the hysteresis and is useful for
squaring
slowly rising signals and removing noise from signal lines which is why
it
is used for the data & clock pins on the 16c84 in programming mode for
instance.
       regards Tony M.
       just this guy ya know

1996\12\17@145627 by Rajesh Bawankule

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Hi All
>
> Shawn Ellis wrote:
> >
> > Ok, stupid question time..  Could someone please define for me once and for
> > all the meaning of the "Schmitt Trigger" and how it differs from TTL?  Also
> > the Schottkey, but I think this is just a low power TTL...

> The schmitt trigger is simply a buffer or an inverter with hysteresis
> at the input.A rising signal will trigger a change in output when the
> voltage reaches say 66% but if the voltage then starts to decrease the
> output will not change state until the voltage drops to say 33% this
> difference in trigger levels is the hysteresis and is useful for
> squaring
> slowly rising signals and removing noise from signal lines which is why
> it
> is used for the data & clock pins on the 16c84 in programming mode for
> instance.
>         regards Tony M.

       I think following figure will make whole thing clear.
       It is a characteristic of Scmitt trigger.
       (plot of Output voltage versus Input voltage.)
       figures are arbitrary and not to scale.)

Output voltage
       ^
       |
       |
   5V  |               ------<-----------<--->  Logic "1"
       |               |           |
       |               |           |
       |               |           |
       |              \/           ^
       |               |           |
       |transition     |           |
       |from "1" to "0 |           | <- transition from "0" to "1"
       |            -->|           |
       |               |           |
       |               |           |
       |              \/           ^
       |               |           |
       |               |           |
       |               |           |
  0.4V |------<-->----------->-----|   Logic "0"
       -------------------------------------------> input voltage
       0               1.5v        2.5 V          5V

                       |___  ______|
                           \/
                       Hysteris

       Arrows indicate travel of plot. When Input increases till 2.5V
       Output remains at logic "0" then goes to logic "1".
       When input decreases below 2.5 V it won't come to logic "0"
       (unlike comparator) It will come to  "0" only when input
       reaches 1.5V.

       Hope this helps.
       Enjoy
       Rajesh Bawankule (spam_OUTrajeshTakeThisOuTspamcomit.com)




1996\12\17@183307 by djmullen

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Shawn Ellis wrote:
>
> Ok, stupid question time..  Could someone please define for me once and for
> all the meaning of the "Schmitt Trigger" and how it differs from TTL?  Also
> the Schottkey, but I think this is just a low power TTL...

Schmitt trigger circuits are designed to convert slowly varying analog
inputs into fast switching digital outputs that can be fed into standard
logic gates.  Digital gates want their inputs to switch instantly from
low to high and vice versa.  They tend to do strange and nasty things,
such as oscillate or burn up, when their input voltages rise to the
point where the output starts to switch and stay there.

Typically, the output of a schmitt trigger stays low as it's input rises
above zero until the input reaches a threshold voltage that is usually
somewhere around half of the power supply voltage.  At this point, the
output instantly (a few nanoseconds) changes from low to high.  The
output then stays high until the input voltage goes a bit lower than the
original threshold voltage.

The difference between the input voltage that switches the output high
and the lower voltage that switches it low again is called hysteresis.
It keeps the schmitt trigger's output from switching rapidly up and down
due to noise and ripple on the input.

Schmitt triggers with inverting outputs are also available, of course.

Dave, N9LTD

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