Searching for an compatible Crystal
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2016-08-02 2:51 AM
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?
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2016-09-01 2:34 AM
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-To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.
