STM32WB55RGV6TR external crystal

[RCali.1]

Hi guys. I am working on developing a product using STM32WB55RGV6TR. I referred to AN5042 and AN2867 to define the external crystal. Based on AN5042 if the used register setting for HSEGMC[2:0] is 0b111, the gm is 2.84mA/V. According to AN2867, the calculated gm gain should be greater than 5 to have a stable operation. Using AN5042 as reference, the crystal NX2016SA-32MHZ-EXS00A-CS06654 was used as reference design. However using the formula in AN2867 to get the gmcrit and gm gain, the calculated gm gain is less than 5 which means the crystal is not ideal for use. May I ask why this crystal was used as reference then if the gm gain >5 cannot not be satisfied? Can you advise me on this topic? Thank you.

[Bowman32]

Hi @RCali.1​ 

The AN2867 is not applicable for STM32WB HSE crystal as noted in page 29. Regarding your case, HSE crystal please refer to AN5042.