> I think the one on the oscillator input pin (can't remember off the
> top of my head whether that's OSC1 or OSC2)

OSC1

> But why do you want to do this?
> Why not just buy a more accurate crystal

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. 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.
Or at least I could say I tried

> then hoping that nothing else changes, like the temperature or the
> age of the crystal  ?

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

> You can get crystals down to 20ppm or so

That's true. I didn't expect these to be quite so off. Others I've
had for timepieces (usually 3.2768MHz) have been pretty good,
but I need the 19.6608 for this. I can replace one of the SMT
caps with a trimmer quite painlessly. There's no need in this
product for a TXCO, and I do know what a trial they can be to
set up

Although, had a TOD been required at the start I would have
made other arrangements for that part of it to be accurate

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