How to choose an external crystal for STM?

[SNorm.1]

The MCU of STM selects the external crystal, whether to choose a high frequency or a low one, what are the instructions?

For example, for stm32L486, the datasheet gives the maximum main frequency as 80MHz, so 4Mhz, 8Mhz, 16Mhz, 20Mhz, 40Mhz can be reached by multiplying the external crystal. So which frequency is the most suitable one to choose? What is the principle to choose? Are there any rules and regulations? For example, the consideration of EMI, power consumption, etc.

[Imen.D]

Hello @SNorm.1​ ,

I advise you to refer to this AN2867 Application note "Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs" and check the Table 7 for recommended crystal, that may help you on your needs.

Hope this helps you!

If you are satisfied of my answer, please mark it as "Best", so that this thread will be marked as solved.

Imen

[Javier1]

Your chip doesnt care about the XTAL frequency if its within specs. (never had a problem with 8 16 or 24Mhz)

In cubeMx , the xtal resonant freq your microcontroller is expecting can be programmed.

After that ,the microcontroller will get its internal working frequency trough plls and clock wizardy.

I allways choose the oneXTAL they use in the corresponding nucleo board.... because im lazy.

[SNorm.1]

Thank you for your responses! It was very helpful to me.

[Imen.D]

Hi @SNorm.1​ ,

Please select the post which is "Best Answer". This will help other users find this answer more quickly.

Imen

[S.Ma]

The choice of external clock also depends on its precision over the operating range. USB, Ethernet are more demanding. The frequency may alter power consumtion as well.