Unfortunately it’s a hardware limitation. Unlike STM32F4 RTC peripheral [1], which provides subsecond resolution out of the box, as well as resets a subsecond counter when a new calendar time is written, STM32F2 [2] does not expose subsecond counter (prescaler) as a register and there is no way to reset it either (although I am not really sure that it does not reset it same as F4). Well, perhaps stopping/restarting the RTC peripheral completely might reset it, but it still sounds a bit useless, because the RTC setup time would still introduce ‘some’ latency.
The only viable option would be to have an external RTC IC if accurate subsecond calendar time is a requirement.
[1]
[2]
