Missing Schmitt Gates??

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!

John

Well? It'll sit there and go "click-click" ;-)

...Jim Thompson

At what input voltage? Of course it's assumed you will also have a high-value bias resistor across the ST inverter.

The MCS48 used a ST in the clock oscillator IIRC. It would oscillate at some tens of kHz before the crystal got going. Maybe a "feature" kind of a limp-home thing if the crystal failed (usually, not always, open).

Best regards, Spehro Pefhany

The resistor and some stray capacitance will do that. It can't have a stable state with the resistor.

No, it's not clear just from the data sheet, however IDRC (I Did Recall Correctly-- hey, it happens..), if you look further back in your book to AP-155, _Oscillators for Microcontrollers_ (at least I think it's bound into that edition) and you'll find the following:-

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

I was referring to making an xo from a "standard" schmitt gate. Maybe it can be done, but it's very risky. There would be two oscillation modes, and all sorts of things could prevent the xo mode from kicking in.

John

Even a well-known uP manufacturer has used such a kluge in a vain attempt to avoid the components of a proper "pi" network... stupidity at its finest ;-)

...Jim Thompson

Rich Grise snipped-for-privacy@example.net posted to sci.electronics.design:

Good point, an ST might not start.

Jim Thompson snipped-for-privacy@My-Web-Site.com posted to sci.electronics.design:

Assuredly it will not self start normally. Maybe in one a trillion cases it will.

John Larkin snipped-for-privacy@highNOTlandTHIStechnologyPART.com posted to sci.electronics.design:

Maybe in 1 in a trillion cases.