Re: Crystal problems/questions

From: analog (analog_at_ieee.org)
Date: 08/07/04 

Date: Sat, 07 Aug 2004 07:41:52 GMT

Brett wrote:

> I'm working on building a small prototype for a PCB we would like
> to have built. It is based on an ST7262 uP, and my first step has
> been to put this processor on a breadboard, hook up some of the
> basics like Vdd, reset, etc, and then attach the crystal. As far
> as I can tell on my first attempt, the crystal is not resonating,
> at least when I use an oscilloscope to probe it. I have a few
> ideas about what could be wrong, but nothing I'm really sure about.
> I would appreciate it if anyone could check over what I'm doing and
> let me know if it makes sense. My circuit looks like this:
>
> ------------OSCIN
> | |
> --- |
> 33pf--- ----
> | xtal
> GND ----
> | |
> --- |
> 33pf--- |
> | |
> ------------OSCOUT

Brett, just a quick shot in the dark, but you might try adding a
biasing resistor across the crystal. Depending on the micro's
internal circuitry, this might be necessary to enable the dc error
to drive the oscillator to within its linear range. Don't make it
too small or it will kill the oscillator's Q. Something in the
megaohm range should do the trick (try 4M7 or 10M). -- analog