The load capacitor for LSE on the board Nucleo-H743ZI

[Jiaming]

Hello,

I bought one Nucleo-H743ZI2 board and found the two capacitors (C71 and C72) for X2 32.768KHz are not installed. May I know if I need to install them with 5.1pF if I want to use the X2? I went through the manual "UM2407 User manual for STM32H7 Nucleo-144 boards (MB1364)". But it seems not mention the two capacitors for the LSE clock. Thank you.

[AScha.3]

Hi,

The size of load capacitors depends on the value in ds, the crystal is tuned for.

So if this is a crystal for very small load capacitor, stray and pin capacity may be enough. The real test is: run the LSE as it is, measure frequency, but not at the crystal, by.routing it to MCO output pin and measuring there. If frequency is too high, add capacitors, increasing size, until matching the target frequency. This cap value then is optimum value for this type of crystal and thins board. When you build a new board or use other crystal, you have to do this tuning again.

And in Cube set the crystal drive level, if needed, to medium or high drive.

[STTwo-32 · Accepted solution]

Hello @Jiaming and welcome to the ST Community :smiling_face_with_smiling_eyes:.

The C71 and C72 are recommended to be installed in the Nucleo-H743ZI2 board to guaranty a good accuracy when using the LSE. You can find the recommended Capacitors in the bill of materials of the product. To understand more about the Oscillator design, I suggest you to take a look at the AN2867.

Best Regards.

STTwo-32