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

[AScha.3]

Hi,

so take 20pF (or 16...22pF) - and it will work.

standard circuit:

In cpu F103 is everything, just connect the crystal + 2x 22pF .

[Peter BENSCH · Accepted solution]

@Michael_CH 

I have just taken the parameters of your crystal and calculated them according to the AN2867: with a gain margin of 58.451 you are considerably above the minimum of 5, so you can continue working with this crystal.

If you assume a pin capacitance of 4pF, your load capacitors should therefore be larger than 20pF and be around 32pF (2x32pF in parallel + 4pF pin capacitance = 20pF).

However, the layout is just as important as the capacitors, as I mentioned in a previous post.

Good luck!
/Peter

[SofLit]

@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.

[Michael_CH]

@Peter BENSCH 

I have to choose 30pF or 33pF.

I will select 33pF then for the 2 C, as it is more near your value.

Thank you very much!

About the layout: It sure not will be ideally, as my PCB will be 2 layers only.

I for sure try to put the crystal and the 2 Capacitors as close as possible to the OSC pins.

@SofLit  thanks for confirming that my crystal should work.

[SofLit]

As your question has been answered please Accept as solution the comment that answered your question (Not this comment).

Thanks.