Connection in STM32L433 crystal LSE 32kHz

[AStol.2]

Hello,

I am developing the PCB for the STM32L433 board, I come across a question about the 32kHz crystal. I will use the ABS07-LR-32.768KHz (CL = 6pF), it is among those recommended by the uC datasheet.

My question is, since the capacity of the crystal matches the stray capacity by the PCB is it necessary to add the external capacitors? In that case, what value would these have?

I add a photo of the EVB to hear any suggestions.

Thank you so much!

[AStol.2]

...

[Bouraoui Chemli]

Hi @AStol.2​ 

I confirm the chosen crystal is compatible with STM32L4.

If you are sure that your PCB provide enough capacitor then you can try without external capacitor.

For more details you can refer to Application note AN2867 "Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs"

Bouraoui

[waclawek.jan]

As Bouraoui said above, start with reading AN2867.

> matches the stray capacity by the PCB

This, together with the capacitance of the "pins" (which includes internal structures of given package) is not something which can be easily and precisely determined.

I'd recommend, unless you are very tight on space, to put the pads on. You can always leave them unpopulated, after initial development.

JW

PS. The forum "software" (salesforce) is a pile of crap. To add pictures, you have to click on the "picture" icon on the bottom of editor, and upload picture as a file. The Copy/Paste feature does not work, even if it appears to.

[Tesla DeLorean]

That's using a 12pF crystal, for the 6pf one perhaps 2.7 - 3.3pF

Provide the footprints, evaluate start time / on-frequency to decide what you want on the BOM