Turning off the UART functioning as a serial console
UPDATE – You can now disable serial via the GUI so the below will be redundant for many users
(This is based on Clayton Smiths nifty guide).
Backup the /boot/cmdline.txt file before you edit it just in case of screw-ups:
sudo cp /boot/cmdline.txt /boot/cmdline_backup.txt
Edit the file:
sudo nano /boot/cmdline.txt
Running "2014-09-09-wheezy-raspbian.img" the file contained the following:
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
Delete any parameters involving the serial port "ttyAMA0", which in this example is:
dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
Press CTRL+X to exit and save .
You also need to edit this file <<This is no longer present with Raspbian Jessie:
sudo nano /etc/inittab
Search for the serial port usage by using CTRL+W and typing:
This should find the line of the file specifying the serial port (if there is one). Use "#" at the start of the line to comment it out. Then press CTRL+X to save and exit.
Enter "sudo reboot" to restart the pi and now the UART will be available for another process to use it, or the pins can simply be used as general IO.