Searching for an compatible Crystal

mfrank9 · ‎2016-08-02

Posted on August 02, 2016 at 11:51

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 ? x F)2 � (C0 + CL)2

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

4 x 30 Ohm � (2 x ? 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?

#crystal

Amel NASRI · ‎2016-09-01

Posted on September 01, 2016 at 11:34

Hi frank.matthias,

Characteristics of compatible crystals depending on selected device are provided in AN2867.

The calculation you made is correct, but I am not sure about C0 value: 7pF is the maximum value. The crystal selected in our eval board should have a lower Shunt Capacitance.

-Mayla-

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