[time-nuts] Re: 32.768Khz Crystal Trimming

[Dan Kemppainen]

All,

Ahhh, Yes. Checking notes, during the initial build I slapped a pair of 
10pF caps down (all I had on hand) just to get the RTCC oscillators 
somewhat working. I can see now that is WAY under needed capacitance.

Measurements are taken right in the micro, so no external leads to 
influence the board. A serial debug line spits out the difference in 
elapsed time for the RTCC and time derived from the 10Mhz EXT REF 
(GPSDO). (No extra wires/leads etc.)

Honestly this RTCC doesn't need to be stellar. It's more of a holdover 
oscillator when the EXT CLK is lost. But, this is time-nuts and all, so 
might as well squeeze everything out of it! :)



Bernd,

Your post, and rule of thumb are what I was looking for. I will 
certainly archive this in my notes. This should be good enough to save 
destroying the board by soldering/de-soldering many 0603 SMT caps.


Thanks to all who replied. This should be enough to keep me busy over 
the weekend!

Dan



On 4/1/2022 12:26 PM, time-nuts-request at lists.febo.com wrote:
> Subject:
> [time-nuts] Re: 32.768Khz Crystal Trimming
> From:
> "Bernd Neubig" <BNeubig at t-online.de>

> 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