[time-nuts] 32.768Khz Crystal Trimming

[Dan Kemppainen]

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