Crystal oscillator selection for 170MHz | STM32G484RET

[Matthew47]

Hi,

I am designing a STM32G484RET into an application, and I wanted to confirm that my oscillator design will theoretically operate before having it manufactured.

Target clock speed: 170MHz

Crystal chosen: https://www.digikey.com/en/products/detail/ecs-inc/ECS-240-10-30B-CKM-TR/8023196

Closest available capacitors: https://www.digikey.com/en/products/detail/kemet/C0805C330F5GACTU/2212505

Resources used for design: AN2867

CL = (C1 * C2) / (C1 + C2) + Cstray

-Matthew

[mƎALLEm]

Hello @Matthew47 and welcome to the ST community;

As you referred to the application note AN2867, what is the calculation you did and what was the result so we could verify?

The most important thing to start with is the "Oscillator transconductance":

Read the AN2867 / section 3.4 Oscillator transconductance.

There is a calculation example provided there:

[Matthew47]

Hi,

I used the following setup in Excel to calculate the values. I assumed C_stray to ~3pF, and then pulled the 20pF C_L from the crystal data sheet.

-Matthew

[mƎALLEm]

Before going ahead with CL what calculation you did for gm/gm_crit?

[mƎALLEm]

And for the CL calculation see this thread.

[STOne-32 · Accepted solution]

Dear @Matthew47 ,

I'm not sure that the part selected in the schematics has 20pF of CL ..     it should be 10pF instead as mentioned in the datasheet.  

ESR Max is 30Ohm for 24MHz and here is a Bonus from our Partner ECS :

Microsoft Power BI

Hope it helps you

Ciao

STOne-32.

[Matthew47]

Good catch! Looks like my capacitors should be about 10pF