[time-nuts] Re: 32.768Khz Crystal Trimming

[Bob kb8tq]

Hi

Given the (very normal) lack of information on the crystal, 
it becomes a “try it and see” sort of thing. They seem to 
want 12.5 pf as the load cap for the first one listed on the
data sheet. How far off of that is your circuit as it sits?

First step would be to take the “output” cap up one value
and see what happens. 

Bob

> On Apr 1, 2022, at 8:47 AM, Dan Kemppainen <dan at irtelemetrics.com> wrote:
> 
> 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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