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Fallingelectron , 10-11-2025, 10:46 AM

Hi, i hope everyone is doing fine. i need help with the crystal selection for this MCU.
Keypoints to keep in mind are:
-Not using thIe RF capability of MCU
-its going to be a four layer PCB design
-I dont know what my PCBs stray Cap would be
also i don't understand how to choose a stack up for this PCB

QDrives , 10-13-2025, 10:19 PM

Crystal or (crystal) oscillator? Two different things.

Fallingelectron , 10-14-2025, 04:49 AM

@QDrives what is this?

Fallingelectron , 10-14-2025, 07:03 AM

this is what the datasheet says

QDrives , 10-14-2025, 01:00 PM

The datasheet you show is of a crystal.
PCB stray capacitance is usually in the range of 5pF.
As for the stack-up: one side components or both? Many traces or not high density?

Fallingelectron , 10-18-2025, 10:15 AM

one sided componets, i dont think it will have many components

QDrives , 10-18-2025, 04:33 PM

The best stack-up for that would be sig - Gnd - Sig - Gnd. You do need to pour copper on the signal layers to balance the copper, which should not be done without consideration.
Alternatively, Sig - Gnd - Gnd - Sig makes things easier. Here it is more important to add a Gnd via for signal vias.

Fallingelectron , 11-04-2025, 06:05 AM

Hi, this is the layout that i have created, what can i improve and do you see any problems with it. the PCB thickness is 0.8mm and 50 ohm stack up

Fallingelectron , 11-04-2025, 06:06 AM

3D views

QDrives , 11-05-2025, 01:31 AM

This is impossible to produce. Keep in mind that a mill is something like 0.6mm wide at the smallest and often they want 2 or 2.4mm for the board edges.
Also make sure you do replace the 'acute angles'.

Fallingelectron , 11-07-2025, 05:10 AM

green boxes are the accute angles?

QDrives , 11-07-2025, 03:03 PM

No, the green boxes show the board outline that goes to infinitely small.
The green arrow in this picture point to some of the acute angles.

Fallingelectron , 11-10-2025, 07:51 AM

here is the adjusted top layer and the board outline does this make sense now ?

Fallingelectron , 11-11-2025, 04:15 AM

i have another question

Fallingelectron , 11-11-2025, 04:16 AM

this is the crystal that i am using it has a frequency of 40 mhz but the data sheet doesnt clearly mention what is the Cload of the crystal

Fallingelectron , 11-11-2025, 04:18 AM

this what it says on LCSC website

Fallingelectron , 11-11-2025, 04:18 AM

9pF i used that to calculate the caps

Shiro , 11-11-2025, 07:24 AM

I might be wrong but I think datasheet is for the different range of the crystals. For the 40Mhz, I think you should take Cl value as 9pF or 10pF. But 9pF would be better since JLSC says 9pF as the Cl

QDrives , 11-11-2025, 08:47 PM

The datasheet is a general for the family. There are different frequencies and load capacitances.
I cannot see a part number 'key', but I hope for you that LSCS has got it correct with the part number, frequency and capacitance.