STM32F103: HSE Crystal Capacitors

[Michael_CH]

Hello

 

Which are the correct Capacitors to use for HSE on STM32F103 with the following Crystal?

Frequency: 8 MHz

Load capacitance CL: 20 pF

 

Thank you in advance for a quick hint.

Regards

Michael

 

[Peter BENSCH]

Welcome @Michael_CH, to the community!

You have provided too little information for a qualified answer. Essentially, however, you will find everything in detail in AN2867.

Hope that helps?

Regards
/Peter

[Michael_CH]

I am not an engineer, but just a hobbyist trying to make my first schematic and a PCB.

I was looking the AN, but honestly that was too complex for me.

 

Therefore i hope some more experienced people can tell me the correct value for the 2 capacitors there. 

 

Which information is missing?

Thank you. 

 

[Peter BENSCH]

Firstly, the data sheet of the crystal used is important, from which you can see the key values: C0, CL and ESR.

Furthermore, the AN2867 mentioned above also contains important notes on layout from section 7 onwards, which you should read and try to understand.

[mƎALLEm]

@Michael_CH ,

Let's do a calculation based on what AN2867 says.

The rule is to have gain margin > 5 (a condition that lets the oscillation to start).

Where:

gm is given by product in the table 6 in the application note:

gm crit formula is provided in the 3.4 Oscillator transconductance section:

In your case: 

CL= 20 pF, C0 = 3 pF, ESR = 80Ω 

-> gmcrit = 4 x 80 x ( 2 x pi x 8.10 6)² x (3.10-12 + 20.10-12)²  = 0,42 mA/V

An example of calculation has been provided in the same application note:

So in your case gain margin = 25 / 0,42 = 59 > 5

Normally it should work.

Hope that answers your question.

[mƎALLEm]

Hello,

A new article "How to select a compatible crystal and load capacitors for STM32 with layout guidelines" has been published today and this thread has been linked to that article in the "Related links" section.

Good reading.