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Michael_CH
Associate
Feb 10, 2025 4:22 PM UTC
Solved

STM32F103: HSE Crystal Capacitors

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

 

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Peter BENSCH
Peter BENSCHBest answer
ST Technical Moderator
Feb 12, 2025 2:08 PM UTC

@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 series + 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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22 replies

Peter BENSCH
Peter BENSCH
ST Technical Moderator
Feb 10, 2025 4:28 PM UTC

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_CHAuthor
    Associate
    Feb 10, 2025 4:53 PM UTC

    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
    Peter BENSCH
    ST Technical Moderator
    Feb 10, 2025 5:02 PM UTC

    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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    mƎALLEm
    mƎALLEm
    ST Technical Moderator
    Feb 12, 2025 2:35 PM UTC

    @Michael_CH ,

    Let's do a calculation based on what AN2867 says.

    The rule is to have gain margin > 5 (a condition that lets the oscillation to start).

    Where:

    SofLit_0-1739370602336.png

    gm is given by product in the table 6 in the application note:

    SofLit_1-1739370656521.png

    gm crit formula is provided in the 3.4 Oscillator transconductance section:

    SofLit_2-1739370714385.png

    In your case: 

    CL= 20 pF, C0 = 3 pF, ESR = 80Ω 

    -> gmcrit = 4 x 80 x ( 2 x pi x 8.10 6)² x (3.10-12 + 20.10-12)²  = 0,42 mA/V

    An example of calculation has been provided in the same application note:

    SofLit_3-1739370809772.png

    So in your case gain margin = 25 / 0,42 = 59 > 5

    Normally it should work.

    Hope that answers your question.

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      mƎALLEm
      mƎALLEm
      ST Technical Moderator
      Apr 17, 2025 6:42 PM UTC

      Hello,

      A new article "How to select a compatible crystal and load capacitors for STM32 with layout guidelines" has been published today and this thread has been linked to that article in the "Related links" section.

      Good reading.

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