Load capacitance (AN2867)

[Bhat]

I am working in small startup in Bangalore. I had designed the board using the STM32F103R8T6 controller now i am facing oscillator issue, oscillator is not working as i expected.

thereafter i googled it for oscillator design guide and i found AN2867. i was reading AN2867 everything explained neatly. in Load capacitance section (page number 12/54) they explained the CL value by taking one example.

CL=15pF, Cs=5pF,

then CL=10pF,

then CL1=CL2=20pF.

at last how CL1 and CL2 become 20pF?

1) if CL value is doubled then why it is doubled?

2)if CL value is multiplied by 2 then why it is multiplied by 2?

3) Load capacitance value should be mentioned by crystal manufacturer, then how CL should become change than the original value?

4) Does designer should use manufacture specified value or should we go through this AN2867 ?

kindly answer my question.

i am attaching some images with this.

[Piranha]

CL must be taken from crystal's datasheet. Do you understand the word "example"?

When C1=C2, the equation simplifies... It's basic math. The same, but simplified: https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/

[Bhat]

Dear piranha,

I understood the word meaning.

can you just go through my 4th question, there i have asked which value capacitor should designer use? either manufacture specified or calculated value?

[Bouraoui Chemli · Accepted solution]

Hello @Bhat​ 

Would you please provide as the datasheet of your crystal and the value of external capacitors CL1, CL2?

You have to choose CL1 = CL2 in order to simplify CL calculation. You can always change CL1 and CL2 value to slightly affect the LSE frequency to tune it into desired value. But keep in mind that CL1 and CL2 are used to tune the desired value of CL to reach the value specified by the crystal manufacturer.

Please refer to AN2867 to find in table 7 a list of compatible crystals with STM32 MCUs.

And you will find in section 7, tips for improving oscillator stability and PCB design guidelines.

Link to AN2867 : https://www.st.com/content/ccc/resource/technical/document/application_note/c6/eb/5e/11/e3/69/43/eb/CD00221665.pdf/files/CD00221665.pdf/jcr:content/translations/en.CD00221665.pdf

Best regards

Bouraoui

[Bhat]

Dear Bouraoui,

Thank you for your replay.

I had gone through the AN2867, i calulated the CL value.

As per crystal datasheet CL value is 18pF.

After reading the AN2867, CL= 18pF, Cs= 5pF,

Then CL - CS =18pF - 5pF = 13pF.

Now Cl1 = CL2 = 26pF.

Here my question is now which capacitor value should i use?, one which datasheet recommends (18pF) or one which I got values from the as per AN2867 application note (26pF).

Kindly find the attached crystals datasheet link and CL1 and CL2 capacitor datasheet link.

1) https://abracon.com/Resonators/abls.pdf

2)https://content.kemet.com/datasheets/KEM_C1003_C0G_SMD.pdf

[Bouraoui Chemli]

Dear @Bhat​ 

You have to know; in crystal datasheet you find load capacitance (CL) that gives the nominal oscillation frequency. In order to obtain the adequate frequency, the oscillator circuit has to show the same CL to the crystal. That is why, the CL value is determining by the external capacitors (CL1, CL2) and the stray capacitance of the printed circuit board and connections (Cs).

This lead you to the equation in AN2867:  CL(crystal)  = ((CL1CL2) / (CL1+CL2)) +Cs 

 I confirm your calculation of CL1 = CL2 = 26pF. 

Best regards

Bouraoui

[Bhat]

Dear Bouraoui,

Thank you for your reply.

from this conversation I understood that now i should use 26pF, as my CL1 and CL2 to crystal to get desired frequency.