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?

Dear all,

Having a crystal compatible with our HSE is very important else there is a risk of no oscillation in worst case conditions.

That NDK crystal is specific for us (STMICROELECTRONICS ) and is not from their public website. Look to the exact reference at the end.

Cl =6.5pF

C0 = 1.3pF

Esr max = 50 Ohm

F=25MHz

Gm margin = 5/0.3 = 16 >>> 5.

Please do not confuse gm _crit and gm_crit_max there is a factor of 5 between them.

Cheers

STOne -32