STM32F103C8T6 dev board RTC AsynchPrediv = RTC_AUTO_1_SECOND not work correct

[NNam.18]

Here is my board

here my clock setting

I use CuceMxIDE, here my RTC setting

if

AsynchPrediv = RTC_AUTO_1_SECOND;

rtc take very long time to increase one second. I try some value, and choose 50 and seem the time count right

AsynchPrediv = 50;

but I'm not sure this value is correct and RTC count correctly, can not find any AN about STM32F1's RTC how to calculate this value.

So is it normal or my board get error? anyway to fix this?