After playing with cap values the crystal is running about 6.5ppm fast.
Plenty good for what it is. Of course this was with a sample size of 1.
Being a time-nut, I did try a few different cap values just to see if it
could be made any better. The Next closest cap combo gives 8.3ppm slow.
Putting a little heat on the crystal showed it tended to drop in
frequency, so 6.5ppm fast is probably better than 8.3ppm slow.
Looking at the slope/intercept of the values tried, the needed value
isn't easily achievable. Again, n=1, so sample to sample variation is
untested.
It looks like the 'ideal' cap value might be achievable, but that would
require a pretty big mismatch between C1/C2. More than 2:1, probably
closer to 3:1. I'm guessing that would be a 'no-no'?
Anyway, Thanks to all who replied!
Dan
On 4/2/2022 3:27 AM, [email protected] wrote:
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
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