Install Hardware

I used a PiFace Real Time Clock. After installing the CR1220 battery and correctly mounting on the board, see manual for that.

Install Software

Add the modules from the kernel

/etc/modules/

Add these modules so they're loaded on boot. You can modprobe each one individually if you don't want to reboot.

i2c_dev
i2c_bcm2708
i2c:mcp7941x

/media/mmcblk0p1/config.txt

You will need to mount this as read-write

mount -o remount,rw /media/mmcblk0p1

Add these two parameters to the bottom

dtparam=i2c1=on
dtparam=i2c_arm=on

mount -o remount,ro /media/mmcblk0p1

Then you will need to reboot.

Binding the hardware clock device

You will want to make sure the device is created when hwclock starts, if it isn't already created. To the bottom of /etc/conf.d/hwclock add this:

start_pre() {
    if [ -d "/sys/class/i2c-adapter/i2c-1/i2c-dev/i2c-1" ]; then
        einfo "Creating RTC device";
        echo mcp7941x 0x6f > /sys/class/i2c-adapter/i2c-1/new_device
    fi
}

For the time being this is necessary but in the future when issue 1032 you should be able to just add

dtparam=i2c-rtc,mcp7941x

to /media/mmcblk0p1/config.txt