How to properly implement external 32.768 kHz crystal for accurate RTC on STM32F407VGT6?

[rohithinthingstech]

Hi STM Community,

I am using a custom board based on the STM32F407VGT6 MCU. Currently, I’m not getting accurate RTC time when using the internal LSI clock (the drift is too high for my application). So I’ve decided to use an external 32.768 kHz crystal oscillator connected to PC14 (OSC32_IN) and PC15 (OSC32_OUT) as recommended.

Could you please clarify:

1.  What are the exact hardware requirements (crystal specs, load capacitors, recommended ESR, drive level)?

2.  How should I calculate and place the load capacitors to match the crystal?

3.  Are there any layout best practices (trace length, shielding, keep-out) to get stable LSE operation?

4.  Any recommended crystal part numbers that work well with STM32F4?

5.  Any common issues or troubleshooting tips if the LSE does not start?

[mƎALLEm]

Hello,

You can refer to this article: How to select a compatible crystal and load capacitors for STM32 with layout guidelines

or for more details refer to the AN2867 "Guidelines for oscillator design on STM8AF/AL/S and STM32 MCUs/MPUs"

Also read this article on How to calibrate the STM32's real-time clock (RTC)