🚀 go-pugleaf

Barry Moss writes, concerning load capaitors on crystal oscillators:

>When designing a parallel mode oscillator like those used on most
>microcontrollers (80C31, 68HC11, etc.), it is necessary to specifiy
>a parallel crystal with a given load capacitance.  I am trying to 
>come up with a formula for determining the load capacitance.


Mr Moss -

I can't really answer your question - I do not know why
22pf - 33pf capacitors are the ones that most often get
used in this situation - but I do know that they are often
the most suitable.

A year or so ago I had to design a built-in modem for an 80C31
based product, and this required that we have the 80C31 oscillator
be spot on in terms of frequency - the 80C31 oscillator is not
really rated in terms of accuracy, and for best results with a
switched capacitor modem, you want to be on frequency.  (There
were good economic reasons for using the 80C31's oscillator).

Being a bit pragmatic I gave up on the books, got a load of 
different manufacturers 80C31's, crystals, a good frequency 
counter, some freeze spray and a heat gun, and lots of caps.

In this particular situation 22pf on one side and 39pf on the
other gave the best, stable, trimmed frequency.  I had decided
that I would be happy if I could consistently get 50ppm 
frequency accuracy in production, with maybe a few units out 
to 100ppm.  I was getting 25ppm on the lab bench, so left it
at that.

Regards,
Steven Murray

-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
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