AVR-Chat

On Wed, Feb 25, 2009 at 10:32 AM, Dennis Clark <dlc@frii.com> wrote:
>
>>
>> I know this sounds a simple "of course" type question but:
>>
>> I could have the controller output a 10uSec pulse and read it with a
>> logic analyzer to verify that the timing is correct.
>
>  If I were concerned I would make sure that the chip is "rail-to-rail"
> powering the oscillator and simply put a scope on the oscillator output.

I'm not sure what you meant here..

If you don't have the "rail to rail" oscillator selected, (CKOPT
fuse), then you are BEGGING INSISTENTLY for trouble.
Hanging a scope probe on either crystal lead, adds at least 10pF to
the loading, so it's entirely possible to have things working properly
BECAUSE the scope probe is there.

One "cheat" I've used to check frequency is to probe the crystal can,
since it's usually not grounded (though it should be) and it's
capacitance helps hide the probe's loading from the circuit.

Setting up an output that's derived from the clock via a hardware
timer is ok, but you have to be able to measure that with high
precision.

Still, this says nothing about drive level, or oscillation margin, and
since crystals are inherently hard to "pull" off frequency, it tells
you very little about the loading cap values.

One way to get at the crystal frequency without "touching" the
circuit, is to use a shortwave receiver with BFO.
The oscillator will radiate some, enough that you can receive it.  You
need to know how to set up the BFO, but you can practice on tuning in
WWV (or your local time standard broadcast) since their frequencies
are highly accurate.