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'[PIC]: What are the Schmitt trigger thresholds of '
2003\05\05@002852 by Tal

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Hello,

The 16F877 has several Schmitt trigger input. What is the guaranteed
minimal difference between the high and low thresholds of the ST inputs
?

All I found in the data sheet was a max input voltage that guarantees
'0' input and a minimal input voltage that guarantees '1' input but
nothing about the minimal difference between the two thresholds.

One of the PIC inputs are tied to an external comparator and I am
looking for way to drop the comparator feed the signal directly to the
PIC.

Thanks,

Tal

> {Original Message removed}

2003\05\05@004959 by Jinx

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> What are the Schmitt trigger thresholds of the PIC ?

You may need to characterise this for yourself with a pot and
a 5V supply. I did it once a couple of years ago (sorry, no
actual figures handy). The ST input thresholds were quite
repeatable

One thing though. Using a 10-turn pot to do that, I found that
a very slow voltage increase (equating to a very low frequency
wave) would be missed by the PIC and there was for example
no interrupt generated. I don't recall if I found a reason for it

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2003\05\05@020503 by Tal

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Thanks Jinx,

One more related question, is it safe to connect an analog input (e.g. a
sine wave) to a non schmitt trigger or does it significantly increase
the current consumption of the PIC due to non define logic states (or
any other negative affect) ?

My goal is to save an external comparator that converts the analog
signal to digital (the actual thresholds are not that important).

Thanks,

Tal

> {Original Message removed}

2003\05\05@025803 by Jinx

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> One more related question, is it safe to connect an analog input (e.g.
> a sine wave) to a non schmitt trigger or does it significantly increase
> the current consumption of the PIC due to non define logic states (or
> any other negative affect) ?

I couldn't swear to it, but I'd say linear-region current consumption in
digital or transistor devices would be of more concern in outputs
rather than inputs. For example, switching of FETs with PWM that's
got slow rise & fall times

From my previous post -

> > One thing though. Using a 10-turn pot to do that, I found
> > that a very slow voltage increase (equating to a very low frequency
> > wave) would be missed by the PIC and there was for example
> > no interrupt generated. I don't recall if I found a reason for it

I meant to add that I thought an ST should square up any waveform,
even a VLF. ISTR looking for a minimum dV/dT figure but don't think
anything came of it

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2003\05\05@053341 by Mike Singer

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Tal wrote:
> The 16F877 has several Schmitt trigger input.
> What is the guaranteed minimal difference between
> the high and low thresholds of the ST inputs ?
>
> All I found in the data sheet was a max input voltage
> that guarantees '0' input and a minimal input voltage
> that guarantees '1' input but nothing about the minimal
> difference between the two thresholds.

"guaranteed minimal difference between the high and low
thresholds" - why not the difference between these two
values?

Mike.

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2003\05\05@080527 by Olin Lathrop

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> I couldn't swear to it, but I'd say linear-region current consumption in
> digital or transistor devices would be of more concern in outputs
> rather than inputs. For example, switching of FETs with PWM that's
> got slow rise & fall times

Many digital inputs are intended for reasonably "fast" edges.  Holding the
input near the transition level can cause out of spec current draw and
even oscillation, making the rest of the circuit think the input is seeing
lots of fast edges.

Schmitt triggers are usually safe with slowly varying analog voltages, but
even then you need to check the spec sheet.


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2003\05\05@082846 by Jinx

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> Schmitt triggers are usually safe with slowly varying analog voltages,
> but even then you need to check the spec sheet

What's in a ST ? I've checked TTL, CMOS and PIC databooks and
had a look around the www but a ST is always shown simply as an
inverter with the // hysteresis symbol, yet how ordinary gates are put
together is well documented

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2003\05\05@090139 by Olin Lathrop

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> What's in a ST ? I've checked TTL, CMOS and PIC databooks and
> had a look around the www but a ST is always shown simply as an
> inverter with the // hysteresis symbol, yet how ordinary gates are put
> together is well documented

I don't know the exact circuits used in different gates, but basically a
schmitt trigger is a digital input with a little bit of positive DC
feedback.  You can do the same thing with an opamp, for example.  Signal
input + input via 100Kohm resistor, 1Mohm feedback resistor between output
and + input, - input 1/2 supply voltage.


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2003\05\05@093053 by Jinx

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> I don't know the exact circuits used in different gates, but
> basically a schmitt trigger is a digital input with a little bit
> of positive DC feedback. You can do the same thing with
> an opamp, for example

And similarly with a couple of transistors. Variations between
manufacturers can make it frustrating to repeatably produce a
ST or linear amp with a normal gate though

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2003\05\05@104942 by Spehro Pefhany

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At 09:00 AM 5/5/2003 -0400, you wrote:

>I don't know the exact circuits used in different gates, but basically a
>schmitt trigger is a digital input with a little bit of positive DC
>feedback.  You can do the same thing with an opamp, for example.  Signal
>input + input via 100Kohm resistor, 1Mohm feedback resistor between output
>and + input, - input 1/2 supply voltage.

The characteristics of CMOS ST gates are EXTREMELY loose- unsuitable for
many purposes, so that's often the best way to do it. With rail-to-rail
output from a comparator (sometimes an op-amp is suitable as such) feeding
into a CMOS input, you even get quite good accuracy.

Anyway, resistors of any kind are much more expensive than transistors
on chips, so they use nothing but transistors to make a ST gate.
Here's a data sheet that shows the internal schematic of an ST input:

http://cache.national.com/ds/CD/CD40106BC.pdf

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2003\05\05@123016 by Tal

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If the data sheet specifies for example that voltage above 4V is
considered high and voltage below 1V is considered low it the actual
thresholds can still be 2.4 and 2.6 or 1V and 4v. In the first case, the
range is 0.2V and in the second it is 3V and both cases complies with
the spec. This is why more information is needed in the data sheet.

Tal

> {Original Message removed}

2003\05\05@123024 by Tal

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One way to implement a schmit trigger is with two comparators and flip
flop. One comparator sets the FF when the input voltage is higher than
some threshold V2 and the other reset the FF when the voltage is lower
than V1 where V1<V2. I think the famous 555 timer works this way (the
thresholds are 1/3 and 2/3 VDD).

However, ST  are generally implemented using some active component with
gain and a positive feedback. For example, you can create a ST with a
CMOS buffer as follows:



  [in]-------[R1]----|>----+----[out]
                  |        |
                  +--[R2]--+

Where R1 << R2 (start for example with R1=10K and R2=100K and go from
there). The buffer can be any CMOS inverter such as a CD4xxxx buffer,
AND or OR or two inverters in series. The buffer acts as the gain
component (when in the linear region).

Hope this helps,

Tal



> {Original Message removed}

2003\05\05@123440 by Olin Lathrop

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> If the data sheet specifies for example that voltage above 4V is
> considered high and voltage below 1V is considered low it the actual
> thresholds can still be 2.4 and 2.6 or 1V and 4v. In the first case, the
> range is 0.2V and in the second it is 3V and both cases complies with
> the spec. This is why more information is needed in the data sheet.

I think the answer is you don't know except what is in the data sheet.
You will probably have to tolerate a wide hysterisis range unless you make
your own schmitt trigger circuit.


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2003\05\05@124703 by Spehro Pefhany

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At 09:18 AM 5/5/2003 -0700, you wrote:
{Quote hidden}

I would use the "UB" versions rather than the "B" versions for this circuit,
lest you see some nasty oscillation in the transition region.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
TakeThisOuTspeffEraseMEspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
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