[SI-LIST] Re: Fwd: Re: Re: Crystal osciiator not oscillating
- From: "Oscar Fallah" <oscarfallah@xxxxxxxxx>
- To: art_porter@xxxxxxxxxxx
- Date: Sat, 26 May 2007 11:40:06 -0700
Hello Shivdas,
Have you tried a "known good" crystal from a working board in the circuit
that is not working?
You could have a bad batch of parts, a counterfeit device, too old of a date
code, or one that might have been damaged in the soldering process. Also,
if crystal oscillators are not kept in a controlled environment, they could
degrade.
Regards,
Oscar
On 5/24/07, art_porter@xxxxxxxxxxx <art_porter@xxxxxxxxxxx> wrote:
>
> Probing crystal oscillators right across the crystal is a tough =
> challenge. Any high-resistance scope probe has too much capacitance. =
> Even a tiny amount of capacitance will kill the oscillator. And active =
> probes with lower capacitance have 100K or lower resistance, which =
> changes the bias, which also kills the oscillator.=20
>
> Art Porter
> Agilent Technologies
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
> On Behalf Of Eddy
> Sent: Wednesday, May 23, 2007 6:50 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Fwd: Re: Re: Crystal osciiator not oscillating
>
>
> 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
> =20
> =20
> > --- 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.
> > >=20
> > > 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.
> > >=20
> > > Andy
> > >=20
> > >
> >
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