STM32F103: HSE Crystal Capacitors

Michael_CH · ‎2025-02-10

Hello

Which are the correct Capacitors to use for HSE on STM32F103 with the following Crystal?

Frequency: 8 MHz

Load capacitance CL: 20 pF

Thank you in advance for a quick hint.

Regards

Michael

Peter BENSCH · ‎2025-02-12

@Michael_CH

I have just taken the parameters of your crystal and calculated them according to the AN2867: with a gain margin of 58.451 you are considerably above the minimum of 5, so you can continue working with this crystal.

If you assume a pin capacitance of 4pF, your load capacitors should therefore be larger than 20pF and be around 32pF (2x32pF in parallel + 4pF pin capacitance = 20pF).

However, the layout is just as important as the capacitors, as I mentioned in a previous post.

Good luck!
/Peter

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Peter BENSCH · ‎2025-02-10

Welcome @Michael_CH, to the community!

You have provided too little information for a qualified answer. Essentially, however, you will find everything in detail in AN2867.

Hope that helps?

Regards
/Peter

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Michael_CH · ‎2025-02-10

I am not an engineer, but just a hobbyist trying to make my first schematic and a PCB.

I was looking the AN, but honestly that was too complex for me.

Therefore i hope some more experienced people can tell me the correct value for the 2 capacitors there.

Which information is missing?

Thank you.

Peter BENSCH · ‎2025-02-10

Firstly, the data sheet of the crystal used is important, from which you can see the key values: C0, CL and ESR.

Furthermore, the AN2867 mentioned above also contains important notes on layout from section 7 onwards, which you should read and try to understand.

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AScha.3 · ‎2025-02-10

So just look at the "blue pill" circuit, as an example:

or at circuit of STM nucleo boards:

So choosing 2x 20 pF should be ok; (out of the typical range 12...20 pF)

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Michael_CH · ‎2025-02-10

Crystal datasheet:

https://datasheet.lcsc.com/szlcsc/1903061615_Yangxing-Tech-X50328MSB2GI_C115962.pdf

CL i already provided: 20 pF

C0: 3 pF max.

ESR: 80Ω max.

Hope that help you to help me. :)

Andrew Neil · ‎2025-02-10

@Michael_CH wrote:
I am not an engineer, but just a hobbyist trying to make my first schematic and a PCB.

But designing an electronic is an engineering exercise!

Michael_CH · ‎2025-02-11

The 2x 20 pF on the nucleo board are for a 8 MHz crystal with 16 pF CL, if i understood that correctly.

Are this also safe values for my crystal?

CL: 20 pF

C0: 3 pF

ESR: 80 Ω

Thank you.

SofLit · ‎2025-02-11

Hello @Michael_CH ,

The recommended crystals for STM32 are listed in the application note AN2867 pointed out by @Peter BENSCH .

~~Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products:~~

~~Its shows the CL of the recommended crystals. I think you cannot exceed CL =12.5 pF.~~

EDIT: the table was a recommendation for the LSE.

The recommended crystals are provided in tables Table 8 and Table 9:

CL load max = 8pF.

I recommend you to find one of these crystals or find another one similar with the same characteristics

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Michael_CH · ‎2025-02-12

Hello @SofLit

Unfortunately i can't really change to another crystal with a lower CL, because they are not "Basic/Preferred Extended Parts" of JLCPCB and therefore would cause high addtional costs.

This will just be a hobby project, therefore it should be as cheap as possible.

If you would say: to use a CL 20 pF Crystal with an STM32F103 is impossible, well then be it.

But if it is just "not optimal, but should work in 95% of the cases if the right Capacitors are used" i would be happy.

@Peter BENSCHmentioned that a qualified answer is possible if i provide all details and the datasheet of the crystal.

I delivered that and really hope one of you experts can help a beginner with the calculation of the best values for the 2 Capacitors.

Thank you very much in advance.