Using the 32Khz as RTC clock while power is off

[engidea]

Good morning Olimex

I am using you excellent A20 SOM for one embedded product
Things are generally good, I have a gentoo distribution on it (no systemd) and no issues while on power.

I have wired the usual CR32 battery to the AXP_BACKUP pin (7-6) of the "additional" connector of the A20-SOM

Test have been made checking with hwclock -r if the clock is maintained while poweroff with or without the battery and with the battery the clock is kept and also advances.

However, it seems that there is quite a difference in speed between different boards, resulting in clock skew quite big (minutes in a day)

The question is:
1) Is the AXP_BACKUP pin the correct one to supply to power a "backup" RTC ?
2) How can I check the actual frequency of the 32768Hz clock ?
(Normally one cannot put a scope on the pins since the extra capacitance change the measure a lot)
3) Is there any way to "adjust" the 32768Hz if it is found to be quite wrong ?

Thanks for any possible help

Damiano