Quoting Electron <electron2k4@infinito.it>:
> Hello,
> is it normal that an oscilloscope probe put on a oscillating xtal
> doesn't stop it from oscillating (yes, it is normal, it has got 1M
> impedance and few pF after all).
>
> BUT
>
> if the xtal was stopped (the chip it's connected to was in power
> down mode) and I power it up WHILE the oscilloscope probe touches
> the xtal terminal, the oscillator will never start up. Now, is THIS
> normal?
>
> It seems a xtal has a very hard time starting up, but when started,
> it will work robustly. I didn't know this, I haven't read it anywhere
> in the past, and am wondering if it's the side effect of some other
> problem I haven't identified.
>
> Did you experience anything similar too?

Yes, and it makes sense to me.

The oscillations start small, and gradually increase in amplitude.  
That is why most uCs wait a few clock cycles, before executing the  
first instruction -- to let the oscillator reach the full amplitude  
and stabilize.

Additional load at full amplitude  doesn't have a significant impact  
on performance, but additional load at the beginning doesn't give the  
oscillator the chance to start up.

Try connecting the probes through a resistor. You may find that even a  
10K resistor connected in series may allow your circuit to start up  
properly.

Vitaliy



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