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, [email protected] wrote:
Subject:
[time-nuts] Re: 32.768Khz Crystal Trimming
From:
"Bernd Neubig" <[email protected]>
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
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