Hi,

I'm designing a count down timer to start nordic ski racers at set interval=
s
over the period of an 8 hour day.  I wondering what is the best way to get
an accurate time over this period with little drift.  What I want to do is
synchronize the device to the rest of the timing system at the beginning of
the day and let it run without having to resync it later.  The accuracy tha=
t
I would like to achieve is 0.5 sec over 8 hours or roughly 17 ppm over a
temperature range of -20 C to +40 C.

The PIC device will be battery powered, have a 4 by 7 segment multiplexed
LED, voltage regulator, buzzer, a button or two and a few leds.  Not that
complicated.  It is a one off device for my own use.  I'm going to use a 1
sec pulse from a GPS to check calibration.

The question is the best way to get an accurate clock over this temperature
range.  I'm looking for something in the 4 to 10 Mhz range.  In past PIC
projects I've used in internal oscillator and don't have any experience wit=
h
crystals.

This is what I've considered so far.

My initial though was that I would have to use a temperature compensated
canned oscillator to get the accuracy over the range because a standard
crystal has a tolerance of +- 30 ppm and a stability of +- 50 ppm.

Then I thought that I could use a temperature sensor costing $0.30 or so
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3DMCP9701T=
-E/TTCT-ND
and then use a standard crystal and do the temperature correction.

Next I found some better crystals with +-10 ppm tolerance and +-10 ppm
stability for about 80 cents
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3D887-1265=
-1-ND
The problem I see here is what caps should I use?  My understanding is that
if I'm off on the cap value, I will also be off on the frequency.  As well,
the caps will have a temperature coefficient so the load capacitance with
change with temperature.  Any thoughts about the dos and don'ts about this.

Lastly, if I have a temperature sensor, could I just use the internal
oscillator to correct for temperature changes?  I would like this as it is
easy.  What I'm wondering is how repeatable it would be with temperature.
The internal osc will have a range of about +-5% over this temperature.  It
seems like a bit of a stretch to bring the accuracy from 50,000 ppm down to
17 ppm even with knowing the temperature.  Any thoughts?

Thanks,

Gordon Williams


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