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.

  The option is there to NOT have that fuse programmed and your part will
work fine, usually, and on occasion go into the weeds for no apparent
reason.  I've spaced that fuse and scratched my head before.

> 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.

  Some parts have the option of spitting the clock out an IO pin (MEGA168
for instance) and you can use that to clock other parts in the circuit,
or to check the clock frequency if that is your desire.

DLC

> 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.
>
>
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-- 
Dennis Clark
TTT Enterprises