Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!slxsys!stevem From: stevem@specialix.co.uk (Steven Murray) Newsgroups: sci.electronics Subject: Re: Crstal oscillator load capacitance Message-ID: <1990Dec27.101157.28247@specialix.co.uk> Date: 27 Dec 90 10:11:57 GMT References: <1990Dec21.193438.4172@mdivax1.uucp> Organization: Specialix International, London Lines: 43 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 I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null