[SI-LIST] Fwd: Re: Re: Crystal osciiator not oscillating

  • From: Eddy <eddyvk@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 23 May 2007 17:49:31 -0700 (PDT)

This is what I was thinking too. My first thought was
actually that the probe used to measure the signal
might be killing the oscillator. If the feedback
resistor is 1MOhm and you are loading it with a
100KOhm FET probe, then you pull the bias completely
out of its linear region where the AC gain exists. So
the oscillation stops....  The general advise is to
probe the inverter output where this sensitivity for
DC load does not exist. If you want to see the
waveform on the input, add a capacitor in series with
the probe tip to remove the DC load.

Eddy
 
 
> --- Andrew Ingraham <a.ingraham@xxxxxxxx> wrote:
> >
> > > 2)       By keeping the load capacitance value
> > same as 15pF,  a 100K
> > > externalresistor between the XTAL &EXTAL pin of
> > the microcontroller,
> > > population starts oscillations.
> > 
> > I don't know much about xtal oscillators, but I'm
> > wondering if there might
> > be a DC bias problem with the circuit.  With the
> > circuit not oscillating, is
> > the input (one of the pins) biased so that the
> > inverters are in their linear
> > (amplifying) region?  Perhaps the 100K resistor is
> > needed to help make that
> > happen.
> > 
> > Andy
> > 
> >
>
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