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Capacitance Load for Crystals for the STM32 / NUCLEO - L452RE

Timvanl
Associate

‎2023-09-19 07:09 AM

I noticed this in the NUCLEO-L452RE schematics. The external capacitance added to the ABS25-32.768kHz crystal is 4.3pF. Could someone please explain the reason for this?

MicrosoftTeams-image (1).png

I've learned from the datasheet of the ABS25-32.768KHz crystal that a capacitance load of 12.5pF is required. By applying the formula provided in the datasheet and incorporating it into this equation, while assuming the use of two identical capacitors, the resulting value is as follows (taking into account that Cstray typically falls within the range of 2pF to 5pF):
2*(CL - Cstray) = 2*(12.5pF - 5pF) =15 pF.
2*(CL - Cstray) = 2*(12.5pF - 2pF) = 21 pF

So shouldn't the external capacitances for the 32.768kHz be between 15pF - 21 pF each?

Timvanl_1-1695131708092.png

 

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TDK
Guru

‎2023-09-19 07:15 AM

The load capacitance of the ABS25-32.768KHz-6-T is 6 pF. With that in mind, the calculations should be much closer. Does that clear it up?

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