STM32F7 How to design external oscillator
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:
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.
