Rtc

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First, the i2c-bcm2708 i2c driver needs to be loaded. As far as I know, in the first few months of the raspberry pi being available, the module was blacklisted and didn't load automatically. Nowadays "raspi-config" will already give you the option to enable the driver.

What "raspi-config" does (but you can also do by hand) is remove the drivers from /etc/modprobe.d/raspi-blacklist.conf .

If the driver isn't loaded on your system start with:

sudo modprobe i2c-bcm2708

Then load the I2C and RTC drivers as root:

sudo -s
modprobe i2c-dev
modprobe i2c:mcp7941x
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-0/device/new_device  # For rev1 RPi
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-1/device/new_device  # For rev2 RPi

To write the system time to the RTC (you might need to run this command twice, when you use the RTC for the first time):

hwclock -w

Read out the RTC, and print the date and time to your console:

hwclock

Read out the RTC, and adjust system time:

hwclock -s

To automatically do this on startup, add the following lines to /etc/rc.local

modprobe i2c-dev
modprobe i2c:mcp7941x
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-0/device/new_device  # For rev1 RPi
echo mcp7941x 0x6f > /sys/class/i2c-dev/i2c-1/device/new_device  # For rev2 RPi
hwclock -s

Source: http://www.element14.com/community/message/63885