AnsweredAssumed Answered

Searching for an compatible Crystal

Question asked by frank.matthias on Aug 2, 2016
Latest reply on Sep 1, 2016 by Amel N

I want to develop a PCB with a STM32F4 by my own. And I am searching for compatible Crystal.

During my research I looked at the values of the Crystal on the STM32F429I-EVAL Board and a now I’m a little confused.
Because according to my calculations (based on AppNote AN2867) should this crystal not be used.

Did I made a mistake at my calculations?

The values from JFVNY HC-49S-C16QSA-25M Datasheet:

       
  • Load Capacitance CL =16pF
  •    
  • Resonance Resistance Rr =ESR=30-180 Ohm <- I took the best “30 Ohm”
  •    
  • Shunt Capacitance C0 = 7pF
  •    
  • Frequency Range F = 25MHz

The value from STM32F429 Datasheet:

       
  • gm_crit_max = 1mA/V

In the AN2867 (Section 3.4) there is a calculation for the gain mcrit:

       
  • gmcrit = 4 x ESR × (2x π x F)2 × (C0 + CL)2

If I use this formula, I get the following result:

4 x 30 Ohm × (2 x π x 25MHz)2 × (7pF  + 16pF)2 = 1,57 mA/V

According to the AppNote AN2867 gmcrit must be smaller than gm_crit_max.

Did I made a mistake at my calculations?

Or is this Crystal not suitable for this application?

Outcomes