On January 4, 2006 04:49 am, Jinx wrote:
> shows a trimmer on the drive (gate output) side

Now that I read your reply, other readers might think output xtal.....
I should have written output of "driver" too, because it is the output 
from the chip we should modify. the output side of the xtal is just too 
sensitive to disturbances, so you want that as stable as possible.

> I'm surprised (oh, really, why ? ;-)) ) that MChip didn't show
> a recommended configuration, given that they detail every
> other aspect of their oscillator sections

Trimmers are more of a radio issue since you tend to need a multiple of 
the frequency and changes are somewhat noticeable. It's why I tend to 
take 20ppm and numbers like that with a small grain of salt. Yes 
crystals are rather stable long term, but from one crystal to another, 
they are not exact frequency, so from an RF point of view, you tend to 
have to tune the circuit.
Microprocessors are (or were) generally expected to get their long-term 
timing from something else such as 60-hz line or an external time chip 
(sort of following the 1980 IBM PC type model.
Right now, ucontroller stuff is sort of trying to do the dual tasks of 
microprocessor stuff and also some stuff that tended to be on the rf 
side, so you are asking for something microcontroller manufacturers 
haven't really recognized as normal possibiliies yet.

By the way, 19MHz seems an odd frequency to use.
I don't know what you need it for, but since you mention seconds, I'm 
assuming it's time related, which is okay for the trimmer, especially 
since you see capacitor specifications all over the map ranging from 
10pf to 60pf, so a variable trimmer is not going to cause pain.
I would suggest looking at putting a small 10pf in parallel with your 
trimmer, and see if you can get one that tends to go the opposite 
direction with temperature to your trimmer/xtal/etc.
If you were using the ucontroller to generate the 19MHz for a radio 
signal (doubler, tripler, etc), I would suggest using an external 
oscillator instead since all the mathematical movements happening 
within the ucontroller is going to translate as noise on your rf (using 
an external oscillator circuit isolates the oscillator from your 
ucontroller currents/voltages/noises/etc).

Good luck on your design

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