The way I've been measuring is to measure the time - let it run for 8
hours, and compare it to a better standard. I use my PC's clock, since I
use NTP to sync the clock to the atomic standard, so the PC clock is
accurate over the course of several hours. I don't have access to a freq
counter, unfortunately. I could try using the PIC's clock (20 MHz xtal) to
measure the period of the watch xtal, but I don't know how accurate that is
either!

Unfortunately, the osc is running slow, so a trimmer will make things
worse, not better. I can try making the caps in the osc circuit smaller and
see what happens there.

Making one adjustment every 8 hours is a slow way to do things. Maybe I
should just compensate in the code. Just add 1 second every once in a
while. Sort of a software trimmer capacitor. That's how I handle analog
calibration - rather than trim the gain of an op amp, I just use a
precision resistor as close as I can get, and calibrate and compensate in
the code.

Larry

At 01:39 PM 12/17/2002 +0000, you wrote:
>These xtals often need tuning by tweaking the capacitance.
>remember also that they are designed to run over a fairly narrow temp
>range (i.e. strapped to people) and accuracy will fall off outside
>that range.
>All the watch modules I've ever taken apart have had a trimmer.
>Obviously you can't trim by measuring the xtal freq as the  measuring
>probe will change the capacitance - you need to generate a signal from
>the PIC to measure, but remember that you will get jitter from the
>PIC's main clock sampling the 32K xtal, so you need to measure over a
>reasonably long period - probably several seconds at least..
>
>On Tue, 17 Dec 2002 08:17:47 -0500, you wrote:
>
> >Any idea what sort of accuracy I should get from a cheap watch xtal in a
> >PIC oscillator?
> >
> >I'm using one as the T1 osc on a '877. I let the timer free-run (I don't
> >preset it - it rolls over in 2 seconds), and have an interrupt routine that
> >keeps track of time-of-day. When I compare the time of day from this with
> >the actual time, the PIC has lost about 6 seconds in 8 hours. Not very good.
> >
> >Larry
> >
> >Larry Bradley
> >Orleans (Ottawa), Ontario, CANADA
>
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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