Re: Crystal problems/questions

From: Active8 (reply2group_at_ndbbm.net)
Date: 08/07/04 

Date: Sat, 7 Aug 2004 09:21:01 -0400

On Sat, 07 Aug 2004 07:41:52 GMT, analog wrote:

> 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

I haven't had to do that, but also haven't used that uP, either.
Some probes will load it down and it'll stop. You also might need to
set a config fuse for the oscillator type. If you can set it for
internal clock and get an LED to flash, you have a clue. A known
good external oscillator is another option. Then you'll know what
you're dealing with. 

-- 
Best Regards,
Mike

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