STM32F7 How to design external oscillator

[BSchn.2]

Hello,

I have been trying for a while now to properly design the external oscillator for the STM32F722RE MCU.

Until now, I don't find a clear answer with detail calcul.

To design the oscillator, I use 3 documentations:

• stm32f722re.pdf
• cd00221665.pdf (AN2867 - Oscillator design guide)
• The crystal specification documentation

As an example I would like to use the LFXTAL061975 from IQD crystal.

I will calculate the "gmcrit", the "extern capacity" and the "Rext".

gmcrit:

• Specification

• Calcul

• Conclusion

External Capacity:

• Specification

• Calcul

• Conclusion

Rext

• Specification

• Calcul

• Conclusion

As we can see the last control is not good.

After some research I find an answer on this forum:

https://community.st.com/s/question/0D50X00009XkXUJ/stm-not-respecting-recommended-design-guide-for-hse-crystal

The "STOne-32 (ST Employee)" user said that which capacity used is a special one and provides a link which highlight the specificity.

When you make the calculation for the external capacity "CLL":

CLL = 2(CL-CS) = 2(6.5-5) = 3pF

This result is not correct according to the "stm32f722re.pdf page 131" where is notified that the external capacity must be between 5pF and 25pF......

I would like to know how can I design an oscillator according to documentation ?

Is there any crystal that can be used as a public available component ?

If yes, how can I calculate external capacities ?

Is there a document which gives a complete electronic schema (as Nucleo board design) but with a complete detailed component list ?

Many thanks in advance.

[Bouraoui Chemli]

Hi @BSchn.2​ 

As first analyze, your calculation regarding gmcrit and external capacitors are correct. Since Gm_crit_max is provided in STM32F7 datasheet you do not have to calculate gain margin, just check if Gm_crit_max > gmcrit.

As mentioned in AN2867, REXT is used only If the power dissipated in the crystal is higher than the value specified by the crystal manufacturer. Thus to avoid overdriving the crystal. As I can see here, in your case there is no need to use REXT.

Bouraoui

[BSchn.2]

Hi @Bouraoui Chemli​ ,

First of all, thank you very much for your answer.

I am reassured that my crystal is good !

Ok, great, I can use this crystal for the MCU but maybe this componenent will not be available anymore or something else and I still don't know how to find or calculat it without asking this community. it's a pity.

How have you defined that this crystal do not need an Rext ? I don't know the power dissipated in the crystal and where I can find this information.

Maybe is there another list as the one for the LSR (AN2867 page 27-39 chap. 5.2) with a compatible HSR ?

Thank you very much.

[Bouraoui Chemli]

Hi @BSchn.2​ 

You can refer to AN2867 for the crystal choose, gmcrit and external capacitors calculation. Also you can open another question regarding any enquiry.

As mentioned in AN2876 "The high-speed oscillator (HSE) embedded into STM32 microcontrollers is compatible with almost all the resonators available on the market." So, there is no list like done for LSE.

Since the add of REXT will violate the gmcrit equation, I do not suggest to use it. Any way, if you will use another crystal, you can always make a footprint for REXT on your PCB and test.

Bouraoui

[BSchn.2]

Hi @Bouraoui Chemli​ 

Ok, I will try with my component and caluculation.

If it doesn't work I will post a another message again.

Thank you very much for your help.