Last summary.
DS3231 is found by the sudo i2cdetect -y 1 . return 0x68
from registers od DS 3231 i can read data
Seconds 00h= 0b1010100
Minutes 01h= 0b110101
Hours 02h= 0b101
Day of week 03h= 0b1
Date 04h= 0b1
Month 05h= 0b1
Year 06h= 0b0
registered 0x00 , 0x01 , 0x2 changed in time , registered 0x03, 0x04, 0x05 , 0x06 not changed in time.
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As adviced Mr Lawson
i add into /boot/config.txt to last lines dtoverlay=i2c-rtc,ds3231 save and reboot.
after reboot i tried use the sudo hwclock -r , this command returned only
hwclock: Cannot access the Hardware Clock via any known method.
hwclock: Use the --debug option to see the details of our search for an access method.
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by the easy googling i found only advices as connect RTC to kernel before the 2015 , but as you know new version of kernell has totally another methods for run RTC
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Of course i give something wrong or miss very small but important step .
somebody help
P\S =>
pi@raspberrypi ~ $ sudo hwclock --debug
hwclock from util-linux 2.20.1
hwclock: Open of /dev/rtc failed: No such file or directory
No usable clock interface found.
hwclock: Cannot access the Hardware Clock via any known method.
pi@raspberrypi ~ $