[time-nuts] Re: 32.768Khz Crystal Trimming

[John Lofgren]

Hi All,

Bob is right.  This is very much a cut-and-try thing for a couple of reasons.
1. Crystals with the same printed specs from different manufacturers respond differently.
2. Stray capacitance in the specific installation is part of the total load capacitance.  Every installation is different.

I've just run through this on a board for a customer.  It is setup with a Microchip RTC that likes crystals with a load capacitance somewhere in the 6 to 9 pF range.  Using 3 crystals with the same nominal specifications (7 pF, +/- 20 ppm tolerance) from the different manufacturers I would see frequency errors anywhere from -1.6 to -24.5 ppm just by changing the crystal.  On this particular board with this particular chip the 7 pF crystal with the lowest frequency error liked to have 12 pF load capacitors.  Assuming that the crystal actually works best at 7 pF and that the capacitors in the board really are 12 pF that would make the stray capacitance from the board and in the input/output pins of the chip ~1 pF.

The fact that you're seeing an error an order of magnitude off from where it should be means something is really mistuned.  I've seen this happen when the load capacitance is 2x or 3x off from where it should be.  The attached table is something I put together as a quick reference tool for myself to have at the bench when trimming capacitors.

If you're interested in digging deeper into the theory there's a pretty comprehensive application note from Microchip that might be of interest.
<https://ww1.microchip.com/downloads/en/Appnotes/AN2648-Selecting_Testing-32KHz-Crystal-Osc-for-AVR-MCUs-00002648B.pdf>
Microchip isn't the only one who publishes this type of application note.  ST also has some pretty good oscillator stuff available.

One other thing to note about tuning fork crystals is their temperature behavior.  It's terrible (see section 1.7 in the Microchip ap. note).  The curve is parabolic with the vertex nominally at 25C.  Once you start getting more than a few degrees away from that the slope really starts picking-up.  You don't need to get dramatically warmer or colder to start picking-up some noticeable time errors in a real-time clock.  These days a lot of the RTC chips have internal trim registers to allow the target system to adjust the crystal clock cycles per second so the RTC may be disciplined against an external time source, if available.

I'm not sure what your application looks like or what test equipment you may have available.  Depending on the specific circumstance trimming capacitors may be a matter of observing a buffered (possibly pre-scaled) version of the 32.768 kHz oscillator or measuring the RTC against a known time source.

-John


-----Original Message-----
From: Bob kb8tq <kb8tq at n1k.org>
Sent: Friday, April 1, 2022 8:01 AM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Subject: [time-nuts] Re: 32.768Khz Crystal Trimming

EXTERNAL EMAIL: Be careful with attachments and links.

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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