Raspberry Pi 2 B + DS3231 (RTC)

source from http://thepihut.com/blogs/raspberry-pi-tutorials/17209332-adding-a-real-time-clock-to-your-raspberry-pi and https://nicegear.co.nz/blog/using-an-i2c-real-time-clock-rtc-with-a-raspberry-pi/ ... first enable i2c / spi

    sudo raspi-config
    #Advanced Options => enable Device Tree / SPI / I2C

fix boot

    sudo vim /boot/config.txt
    #Uncomment
    dtparam=i2c_arm=on
    dtparam=i2s=on
    dtparam=spi=on

fix modules

    sudo vim /etc/modules
    #fix to like
    snd-bcm2835
    i2c-bcm2708
    i2c-dev
    rtc-ds1307

fix init(hwclock -s = read from DS3231)

    sudo vim /etc/rc.local
    #add this before exit 0
    echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-1/new_device
    sudo hwclock -s
    date

check blacklist

    sudo vim /etc/modprobe.d/raspi-blacklist.conf
    #comment spi / i2c module #此處為黑名單，要加上註解，否則無法使用

init hwclock如果你的硬體本身是新的或是要重置（安裝NTPD）(hwclock -w = write to DS3231)

    #sudo service ntp stop
    sudo ntpd -gq #fix system clock to current time from NTP
    sudo hwclock -w #write to hwclock
    #sudo service ntp start

memo : load module if not found hwclock

    sudo modprobe rtc-ds1307
    sudo modprobe i2c-bcm2708