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 (rajesh@comit.com)