stm32f7 RTC clock waveform is very small

The external RTC clock of stm32f765igt6 chip, the waveform of the clock is very small, RTC clock is very inaccurate. This is whether the crystal oscillator or capacitor selection problem, or our single-chip RTC drive problem;

For the external RTC clock of stm32f765igt6 chip, we use a 32.768KHz crystal oscillator of Epson, which is equipped with 10PF capacitor. Some of them will make the MCU unable to start, and reduce the capacitance value to 2pF, MCU can start, but we actually measured the waveform of the clock, and found that no matter what kind of configuration, the amplitude of the signal is very small, basically can not see the signal; at the same time, there is a problem to change the board with 2pF capacitor, RTC clock is very inaccurate, one night time, it will be 10 minutes;