Hi,


here is another idea. If Crystals are defined as Xppm, then how about
increasing your crystal frequency to, lets say 8MHz, then this will
decreas your error by 8M/32K = 250 (assuming both crystals have the same
tolerance).

Chaipi

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! Chaipi Wijnbergen                                                           !
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On Wed, 22 May 1996, Keith Dowsett wrote:

> >I ran it last night and the next morning it was late with about 1/4
> >second.
>
>
> I've just sat down with my calculator. It looks like  1/4 second in 12 hours
> (43200 seconds) is just short of 6ppm deviation which could well be within
> the tolerance of your crystal. I have a catalog in front of me which quotes
> +/- 20ppm for their 32 kHz crystal. One ppm corresponds to 0.6 seconds a
> week and even that is quite hard to achieve by adjusting the oscillator.
>
> If you need real longterm accuracy you might have to use an eeprom location
> to store corrections for each individual crystal. A correction of +/-16
> seconds every ten days should allow you to get within 1ppm over a +/- 15ppm
> range. You will also need an accurate clock to calibrate them against,
> probably using a thermostated oscillator.
>
> There are two furtber issues you will need to address. Firstly temperature
> coefficient, and secondly ageing.  The 32kHz crystal I was looking at has
> quite a small temperature correction, less than .05ppm/degree  This is
> unusually good because the crystal is made for use in timing applications.
> Ageing is another cause of errors, and one which is hard to quantify, look
> _hard_ at the manufacturers figures for ageing because several ppm drift is
> not uncommon.
>
> I hope this is some help,
>
> Keith.
> ==========================================================
> Keith Dowsett         "Variables won't; constants aren't."
>
> E-mail:      kdowsett@rpms.ac.uk
> Phone:       0181-740-3162
> Fax:         0181-743-3987
>
> Snail mail:  MRC Clinical Sciences Centre, Cyclotron Unit.
>                Hammersmith Hospital. London W12 0NN.
>