STM32H5 32.6768khz and 24MHz load capacitor values

[AndyLee]

Hi, 

I’m using Nucleo-64 development board with STM32H533RE. On the board,

H533:

RTC crystal is Crystal, 32.768kHz, 9pF, +/-20ppm, 70KR, 1.6x1mm and load capacitors are 3.9pF each.

Crystal 24MHz, 6pF, +-20ppm, 80ohms max, SMD, and load capacitors are 6.8pF each. 

F723:

on the same board, the STM32F723IEK6, it is Crystal, 24MHz, 6pF, +/-20ppm, 80R, 2x1.6mm and load capacitors are 2.7pF each.

1. should the RTC load capacitor value close to 15pF each?
2. should be 24Mhz load capacitors close to 10pF each?
3. why STM32F7 and STM32H5 24MHz have different capacitor values?

Thank you for your concerns

[mƎALLEm]

Hello ​@AndyLee and welcome to the ST community,

Because each crystal have its own characteristics and HSE and LSE have different characteristics.

How to select a compatible crystal and load capacitors for STM32 with layout guidelines

AN2867 “Guidelines for oscillator design on STM8AF/AL/S and STM32 MCUs/MPUs”

 

[AndyLee]

yes, and, is it the only equation I should consider?

 

 

 

 

 

 

 

[mƎALLEm]

Yes but after confirming the gain margin of the oscillator / compatibility of the crystal with STM32 part number.

Please read How to select a compatible crystal and load capacitors for STM32 with layout guidelines / Sections: 2. Calculation to validate the crystal compatibility with STM32 /  4. Calculation of the external load capacitors value

Hope that helps.

 

 

[AndyLee]

oh sorry I had a mistake in typing the question. (I will amend it)

the 24Mhz crystal 6pF, from the equation, should be around 10pF each. But the EVB’s cap is just 6.8pF each. Totally understand the gain and other parameters will lead to adjustment. and that what I trying to understand more.

Can u help on the calculation steps, pls?

[mƎALLEm]

Can u help on the calculation steps, pls?

The calculation is provided in order in the article.

2. Calculation to validate the crystal compatibility with STM32 /

Example of calculation is provided in 3.1 Example of suitable crystal found after calculation

4. Calculation of the external load capacitors value.

Could you please have a look at these?

PS: download the attached calculator tool excel file STM32-Crystal-Calculator.xlsx and provide the values.

 

[AndyLee]

Yes, sure and I did. 

 

Just unsure why I have different results when trying to calculate the load capacitors used on STM EVB Nucleo-64 development board with STM32H533RE. 

 

Sorry for the rush and thank you for your reply. 

[MM..1]

Simply you require add PCB layout capacity based on layers, material, footprints ...