How to calculate capacitor value for 32.768 kHz cystal (RTC for STM32F030R8T6)

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How to calculate capacitor value for 32.768 kHz cystal (RTC for STM32F030R8T6)

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PMath.4Best answer

Senior III

Crystals have a defined internal capacitance which you are trying to match. The two capacitors are effectively in series which halves their effective capacitance. So if you have a 10pF crystal then you need two 20pf capacitors.

EXCEPT

The circuit tracks also have capacitance and this should be taken into account to reduce slightly the required value of the capacitors. So in my example you might start with two 16pF or 2 18pF capacitors

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Associate III

From the reference design of the STM32F030R8T6 I see they are using two 10 pF capacitors for the LSE oscillator. Would that be enough information?

Javier1

Principal

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PMath.4Best answer

Senior III

Crystals have a defined internal capacitance which you are trying to match. The two capacitors are effectively in series which halves their effective capacitance. So if you have a 10pF crystal then you need two 20pf capacitors.

EXCEPT

The circuit tracks also have capacitance and this should be taken into account to reduce slightly the required value of the capacitors. So in my example you might start with two 16pF or 2 18pF capacitors

WPong.1Author

Associate II

Thank you. I just found this Infomation in "Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs"

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