RTC 32.768kHz capacitance spec

Discussion about writing software for VS1005 and the VSOS Operating System. Also posts about VS1005-related hardware design and device drivers should be posted here.
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DavidMc
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Posts: 14
Joined: Mon 2018-12-03 22:54

RTC 32.768kHz capacitance spec

Post by DavidMc »

Hello!

I haven't found my inquiry in the forum, but if I missed it, please link.

There is presently a shortage of 32.768kHz clocks with 12.5pF capacitance spec in the 3215 SMD package. Our VS1005 design follows the breakout board, using the Fox FK135EIHM0.032768-T3 clock, with 18pF caps to DGND and no 1MΩ in parallel.

Can a 32.768kHz clock with 7pF capacitance spec be used with VS1005 RTC? If yes, would the external cap value (18pF) need to also be changed?

Thanks!
David
Hannu
Senior User
Posts: 176
Joined: Mon 2016-05-30 11:54

Re: RTC 32.768kHz capacitance spec

Post by Hannu »

Giving exact new value is hard because everything depends on everything.

However some guidelines...

The 18pF capacitors are on the safe side so that the crystal starts always. Also the board layout has some LRC features, which have some effects.
If quickly calculating, 18pF caps would be exact match for 9 pF crystal. So. If you have 7 pF crystal, 14 pF capacitors would be exact match. However there are other stray capacitances also affecting in traces and RTC pins have also input and output capacitances.

I would design with 10-12 pF caps and with the first prototypes, I would first try higher values to find where the crystal doesn't start (also in hot and cold) any more and then I would think over my values so that the clock starts always. Next problem could be find the correct capacitance so that RTC has correct time as the capacitance has effect on the frequency if a correct real time is critical feature.

Adafruit has nice intro post on this subject: https://blog.adafruit.com/2012/01/24/ch ... ur-design/

Last thing: If you probe with oscilloscope, remember that probe has capacitance too which affects your readings.
DavidMc
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Posts: 14
Joined: Mon 2018-12-03 22:54

Re: RTC 32.768kHz capacitance spec

Post by DavidMc »

Thank you Hannu! Just wanted to make sure there was no special requirement for VS1005, other than the typical crystal design criteria. Thank you for the helpful reference link, too!

Thanks!
David
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