STM32L412 External Crystal RTC inaccuracy of 24-30 Seconds in 24 Hours

[DBhut.1]

Hello,

We are using STM32L412 RTC with STM32CubeIDE.

Previously we are using

Crystal - 32.768 Khz SMD Crystal YXC YSX306GA 8.0 x 3.8 mm

load capacitance - 10pf 0805

In Previous Crystal we get about 6 seconds of RTC drift,

to solve this issue, study the application note AN2867 : https://www.st.com/resource/en/application_note/cd00221665-oscillator-design-guide-for-stm8afals-stm32-mcus-and-mpus-stmicroelectronics.pdf

and from this used below setup:

Crytsal - ABRACON ABS25-32.768KHZ-6-T Crystal, 32.768 kHz, SMD, 8mm x 3.8mm, 20 ppm

load capacitance - 6pF 0805

But In new setup we are getting 24-30 seconds of drifiting.

As we are following all guidlines mention in application notes, as compared to previous crystal, but getting more rtc drift (almost 4 time of previous rtc drift)

Please help us to solve this issue.

Thank You

[Piranha]

Read my comment in this topic:

https://community.st.com/s/question/0D53W00001JCWlASAX/stm32f103c8-production-board-schematic

With a load capacitance of 6 pF on a two layer board, a 6,8 pF capacitors are almost surely too small. The appropriate value should be 8-10 pF and I would start with a 10 pF.

[DBhut.1]

We finally solved this issue and concluded, using (Cl1*Cl2)/(Cl1+Cl2)=load_capacitance.

So our value for c1=c2=12pf.

Thank to all for your valuable suggestion and time.