Thanks ,Oleg, Thanks, Steve. I did it. Everything works fine and smooth now. The problem was, that i wasn't familiar with 'movx'. And it's really simple - just like Steve said, my routines now look:

read_rtc:
clr CS_RTC
movx a,@r0 ;in my case r0
setb CS_RTC
ret

write_rtc:
clr CS_RTC
movx @r0,a
setb CS_RTC
ret

and everything works fine (timing is ok, RTC timings are all about 100ns). Thank you again. You were a great help for me