[time-nuts] Re: 32.768Khz Crystal Trimming

[Bernd Neubig]

Hi,

If you do not want to make it a time-nuts style research project, but just
look for a quick fix - here is a rule of thumb:
This kind of crystal usually has a trimming sensitivity of around -10
ppm/pF. This means, if you increase the value of both capacitors on either
side by 2 pF will increase the load capacitance by 1 pF and thus lower the
frequency by  about 10 ppm.
If you need to vary one cap slightly more than the other, (to get a finer
resolution), please use the one at the output side of the on-chip oscillator
stage.
Take care of the start-up margin (safety to get a reliable start-up after
power on). The larger the capacitors, the lower becomes the margin,
sometimes the margin drops rather quickly. Therefore, after having made the
changes, test the start-up behavior by switching on and off several times,
preferably with a slow voltage ramp.

Enjoy the crystal world (as I did for the last 45 years and still doing).

Bernd


-----Ursprüngliche Nachricht-----
Von: Dan Kemppainen [mailto:dan at irtelemetrics.com] 
Gesendet: Freitag, 1. April 2022 14:47
An: time-nuts at lists.febo.com
Betreff: [time-nuts] 32.768Khz Crystal Trimming

Hi,

I've got a 32.768Khz (USA number format) crystal on a RTCC oscillator of a
small micro, and it's running fast. Around 10 seconds per day or so. 
This is a bit more than an order of magnitude more than the datasheet
states.

The 9 seconds per day error should be a good measurement. The RTCC is
running a 1 second counter, and that's being compared to a 1 second counter
derived by clocking the micro from a 10Mhz EXT clock reference.
This is consistent between multiple copies of the board.

I'm assuming, the C1/C2 load capacitors to ground needs to be higher in
value to trim that oscillator closer to the correct frequency. Is this
correct? Any quick back of the napkin calculations how much additional load
capacitance would be needed?

For Ref, this is the crystal:
https://abracon.com/Resonators/ABS06.pdf
ABS06-32.768KHZ-1-T

Thanks,
Dan


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