Jinx wrote:
> I bought a load of 19.660800MHz for a product in which a small
> deviation wouldn't be noticeable. Basically it has several modes,
> from minutes measured in 100ths/sec or 10ths/sec to a few hours
> in Hh:Mm or Mm:Ss. The customer has asked for a reasonably
> stable time-of-day function to be added, which is no great bother,
> except the crystals are around 130ppm fast (~19.663400MHz)
> which will make a TOD about 11s fast per day.

So why not calibrate in software?  Measure the real frequency at production
time, burn some sort of calibration factor into EEPROM or program memory if
need be, then have the firware adjust accordingly.

> If I can get that down to under 20ppm (say 1s/day or better) at
> an average ambient (in this country probably 15C-20C) that would
> make me happy.

20ppm is asking quite a lot, especially for crystals that are off by 130ppm
in the first place.  Are you sure your crystals are specified for parallel
resonant application?  Actually none of that would matter if you calibrate
to whatever frequency you ended up with at production time.  That would be
more stable too as you don't have to worry about a trimmer changing over
time or via idle fingers that think they are "fixing" something.  Basically
analog trimming sucks.

> I think it's a pretty safe bet the crystal will age

That was rather the point.

> I didn't expect these to be quite so off. Others I've
> had for timepieces (usually 3.2768MHz) have been pretty good,

The 32768Hz watch crystals are usually the most accurate for the money since
they go into applications where a few seconds per day matters.  You could
run a timer 1 oscillator perhaps and do the long term timing from that.

> but I need the 19.6608 for this.

Do you really need exactly 19.6608MHz, or any value 20MHz or a bit below as
long as you know what it is?


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