from 'hwclock -c'

ds3231 module on a raspberry pi2:

you want to use this to set it up on i2c:
echo ds3231 0x68 > /sys/class/i2c-adapter/i2c-1/new_device

using echo ds1307 (with the 3231 module):
"220, 300000001, whatever it takes":
hw-time      system-time         freq-offset-ppm   tick
1444579083   1444579082.997622
1444579095   1444582702.481239         300623635   3006236
1444579106   1444582713.481490         156847125   1568471
1444579117   1444582724.481620         106102471   1061025

using echo ds3231 -
"very stable":
hw-time      system-time         freq-offset-ppm   tick
1444584144   1444584144.494349
1444584156   1444584156.494203               -12     -0
1444584167   1444584167.494742                17      0
1444584178   1444584178.494432                 2      0
1444584189   1444584189.494868                12      0
1444584200   1444584200.495405                19      0
1444584211   1444584211.494968                 9      0