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STM not respecting recommended design guide for HSE crystal

Question asked by Maz D on Aug 16, 2017
Latest reply on Aug 17, 2017 by STOne -32

I am trying to choose a crystal for the STM32F7. The only problem being is that I cant find a component that respects the recommended values found in the following arm design guide. Even STM for it's STM32F779I-EVAL board does not respect the gain_margin.

 

 

If I have understood correctly there is one main rule governing the choice for a crystal so that the oscillator starts and reaches a stable phase.

gainmargin = gm / gmcrit

 

gm is the oscillator transconductance specified in the STM32F779 datasheet.

  • Gm_crit_max = 0.001

gmcrit is computed from oscillation-loop passive components parameters.

  • ESR is the equivalent series resistance 
  • C0 is the crystal shunt capacitance
  • CL is the crystal nominal load capacitance.
  • F is the crystal nominal oscillation frequency

The one used for the eval board is this one : NX3225HA

But I found gainmargin = 1.674730 when it's supposed to be >5

 

Does anyone know why?

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