Re: Missing Schmitt Gates??

On Tue, 04 Dec 2007 19:54:53 -0600, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 04 Dec 2007 16:47:14 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 04 Dec 2007 16:45:54 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Tue, 04 Dec 2007 15:40:47 -0800, D from BC
<myrealaddress@xxxxxxxxx> wrote:

On Tue, 04 Dec 2007 22:17:36 GMT, Rich Grise <rich@xxxxxxxxxxx> wrote:

On Mon, 03 Dec 2007 19:07:08 -0800, John Larkin wrote:
On Mon, 3 Dec 2007 17:57:28 -0800, "Joel Koltner"
"D from BC" <myrealaddress@xxxxxxxxx> wrote in message

A crystal needs a good linear amp.

Everything is linear if you look closely enough...

I am being a little obtuse here -- the kind of oscillator I was thinking
of was your canoncial microcontroller/FPGA clock that doesn't need to be
particularly accurate -- it's common to use 50 or even 100ppm rocks in
such systems; this is a completely different league of oscillator than
those you build for, e.g., fancy RF applications where you're after
2.5ppm or better.

I was never able to get the Schmitts to oscillate anywhere near the
supposed crystal frequency.


Maybe it's a little late in the thread to bring this up, but I'd
think that with the Schmitt characteristics of the input, the crystal
would have to be drastically overdriven, just to get the gate to
notice that there's a feedback signal.

But I wouldn't have any qualms about an HCU inverter or 3. ;-)

Cheers!
Rich

I think Ht for Logic with Schmitt inputs is about 1V @ 5V.

A crystal..well... isn't it just tiny jiggling piece of rock?
Ooops...I might be thinking piezo..
Damn..forgot all my crystal theory...cuts, shapes, modes and all that
jazz.
Anyways.. I can imagine that one has to be kind to a tiny piece of
crystal and not bash it with lots of drive.
However....depends on the precision required..
As someone posted, for clocking an uC or CPU ...who cares about some
drift..


D from BC

A crystal oscillator using an inverter with hysteresis WILL NOT
self-start.

...Jim Thompson


Of course it will self-start. It just won't run anywhere near the
crystal frequency!

---
Nope.

There's no guarantee that it'll self-start because you've only got
one delta V (on turn-on) to cause the crystal to ring, and if it
doesn't ring hard enough to get to the opposite switching threshold
it'll just sit there, squeezed.

The right way to do it is to use an inverter which can be biased so
that the input and the output are both at about Vcc/2 and then let
noise tickle the crystal until it takes off.

I assume there's a nfb bias resistor, like in a non-schmitt circuit.
The schmitt will tend to square-wave oscillate as if the crystal
weren't there, or at least as if the crystal were a capacitor. It's
unlikely that this oscillation will excite the crystal resonance; it's
usually a lot lower in frequency.

So it will just sit there, making a square wave.

It sounds as though some uPs may have a schmitt in the xo circuit, but
that's designed to work. Regular schmitt gates are not safe as xo's.

John



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Relevant Pages

  • Re: Missing Schmitt Gates??
    ... I am being a little obtuse here -- the kind of oscillator I was ... But I wouldn't have any qualms about an HCU inverter or 3. ... I think Ht for Logic with Schmitt inputs is about 1V @ 5V. ... Of course it will self-start. ...
    (sci.electronics.design)
  • Re: Missing Schmitt Gates??
    ... I am being a little obtuse here -- the kind of oscillator I was thinking ... I think Ht for Logic with Schmitt inputs is about 1V @ 5V. ... A crystal oscillator using an inverter with hysteresis WILL NOT ... Of course it will self-start. ...
    (sci.electronics.design)
  • Re: Missing Schmitt Gates??
    ... I am being a little obtuse here -- the kind of oscillator I was thinking ... I think Ht for Logic with Schmitt inputs is about 1V @ 5V. ... A crystal oscillator using an inverter with hysteresis WILL NOT ... Of course it will self-start. ...
    (sci.electronics.design)
  • Re: Missing Schmitt Gates??
    ... I am being a little obtuse here -- the kind of oscillator I was thinking ... I think Ht for Logic with Schmitt inputs is about 1V @ 5V. ... A crystal oscillator using an inverter with hysteresis WILL NOT ... amplifier is a Schmitt Trigger. ...
    (sci.electronics.design)
  • Re: Missing Schmitt Gates??
    ... I am being a little obtuse here -- the kind of oscillator I was thinking ... I think Ht for Logic with Schmitt inputs is about 1V @ 5V. ... A crystal oscillator using an inverter with hysteresis WILL NOT ... Of course it will self-start. ...
    (sci.electronics.design)