Robert wrote :
> This seems to be one of those little spoken of issues. Hitex had a section
> on this in one of their white papers "Insider's Guide to Planning 166
> Family Design" (http://www.hitex.com/download.html)
>
> Robert
Some oscillators will have trouble driving a Xtal properly, some will drive
a Xtal
too hard.
Since the LPC2000 oscillator specs output caps as high as 56 pF, I doubt it
would
have drive issues. A crystal that is driven too hard will age much quicker,
and will
ultimately fail after a relatively short time.... A series resistor can fix
this. (also avoids 3OT)
In case startup and/or runtime is an issue, the Xtal does not necessarily
have to be
loaded with its optimal load capacitance. This mainly sets the proper
frequency within
say 50 PPM, but in an MCU that's typ. a peripheral issue.
The output caps can be decreased where a Xtal for example has a so-so
ESR.
This will provide a bit more drive, and then the input cap can be adjusted
accordingly
to get the suitable phase shift for reliable oscillation.
If the Xtal clock is actually cclk, then however the 2 caps need to stay ~
equal in value
to maintain a 50 % dutycycle, as otherwise it will affect R/W timing in the
CPU.
In that case the values can be dropped a bit to get a better startup and
sustained oscillation.
In a marginally high ESR scenario however, a good quality Xtal should be
used for production
though, for it's hard to characterize the oscillator in production over
temperature.
Always bear in mind that with thru-hole, HC49S (low profile) crystals
have a higher ESR
than their full profile HC49 buddies.
-- Kris