STM32L071, LSE crystal and Driver Level

JJRR · ‎2020-06-04

Dear,

On MCU STM32L071 we use LSE crystal (32,768 khz) with param:

Cload = 12.5 pF
Cshunt = 1.2 pF
ESR = 50-70 kohm
Drivel Level = 1uW

and I am not sure how to set up LSEDRV bits in RCC register. I computed (according to AN2867) Gmcritical=2.06 uA/V

Do I understand correctly that I should set LSEDRV in MCU to 2.7 uA/V? Because when I set it lower than 2.06 uA/V then it would not work reliably?

But whe I set it higher than something than I could damage LSE - what is something?

Steps for setting Driver Level in L071CB are:

0.5
0.75
1.7
2.7

Thank you very much for help.

Jan.

Bouraoui Chemli · ‎2020-06-09

Hi @Community member

As mentioned in section 3.4 Oscillator transconductance of AN2867 Rev12, Gm_crit_max(MCU) must be grater then gmcrit(crystal); under this condition the crystal works fine.

According to your calculation you have to set the LSEDRV to high drive level as mentioned in table 45 of STM32L071 datasheet DS10690 Rev 7.

Best regards

Bouraoui