TCWORD from pcDuino community posted a geeky way to add a RTC to pcDuino3.

First, he soldered on a 3V rechargeable  battery to pin (30) as AXP209 used on pcDuino3 has that pin for connecting a backup battery for the RTC. It will allow the internal RTC of the A20 to keep ticking.

Then he needs to solve the problem that the default value for the backup battery register (0x35) is set to disable charging of the battery. He needs to enable it by writing the value 0xA3 to this register.

The following commands will do that job:

sudo apt-get install i2c-tools
sudo i2cset -f -y 0 0x34 0x35 0xA3

After this, everything works as expected!