I would like to know Load capacitance requirement of RTC crystal - 32.768 KHz

[RV.4]

We have selected FC-12M 32.7680KA-A3 for our new project and we wre in proto 0 , as per datasheet & previous design , load capacitance selected as 11 pF ( 2 X 22 pF) but RTC init is failed in STM32.

we tried multiple value and its init is successful at 2.2 pF.

Can you please explain for below question

1. why init is failed at 22 pF
2. why init passed only at 2.2 pF
3. how its working in previous project
4. How to ensure this 2.2pF will work in Proto 1 design

[Peter BENSCH]

Welcome, @RV.4​, to the community!

Well, in order to be able to help you, one would first have to know which STM32 it is and possibly to which mode its LSE has been set.

Since the LSE oscillator has an extremely high impedance and therefore reacts very sensitively, it also makes sense to take a look at all layers of the layout, whereby the immediate vicinity of the oscillator is sufficient, but where all passing and possibly interfering tracks are also recognisable.

Can you provide this information here, taking care not to include any sensitive information, as the community is a public forum that can be viewed by any registered user worldwide?

Regards

/Peter

[PatrickF]

Hi @RV.4​ 

Please have a look to AN2867: Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs

Using a crystal specify at 12.5pF load seems quite high for an efficient startup on low power system (requires high-drive LSE setting).

Better to target a crystal specify at 6 to 9pF load and use it with Medium or Medium-high drive.

Obvsiouly, board capacitor should be adapted to crystal and PCB characteristics (all formulas are in AN2867).

Regards,

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