Question
g_m_crit of a crystal for the LSE oscillator
The app note AN2867 has a table of recommended crystals for the LSE oscillator (Table 7). There’s a line for Abracon ABS07 32.768kHz crystal (datasheet), and it’s recommended for the G4 family. The gmcrit = 2.1631 μA/V in the table.
Section 3.4 in the app note AN2867 asks for a gain margin gm / gmcrit greater than 5. The gm for STM32G474 is 0.5 μA/V for low drive capability and 2.7 μA/V for high drive capability (table 42 in the STM32G474 datasheet).
The 2.7 / 2.1631 is noticeably less than 5. At the same time, this crystal is recommended for STM32G4. How does the ABS07 32.768kHz crystal meet the gain margin recommendation? I’m asking because I’d like to understand the gain margin requirement.
Best regards,
Nick