[quote user="Masayoshi SAKAMOTO"]First, I use XCAP setting as 6pF, then the clock late about 5sec/day.
So I change XCAP setting as 1pF, then the clock gains about 5sec/day.[/quote]
So you say you know how to change XCAP sucessfully. Then why you ask? :)
[quote user="Masayoshi SAKAMOTO"]But Is it able to change XCAP setting while running the oscillator ?[/quote]
Yes, you can do that, everything will be fine.
BTW there's other way to "trim" RTC oscillator - by adding/substracting it's counter.