These UARTs are found on systems such as the Raspberry Pi.

By default, the UART is used as the Linux console, this must be disabled to allow general purpose operation.

The following script run as root can solve this.

#!/bin/sh
C=`awk '{ for (i = 1; i <= NF; ++i) { if ($i !~ /ttyAMA0/) { printf $i" " } } printf "\n" }' /boot/cmdline.txt`
echo $C > /boot/cmdline.txt
sed -i 's/^T0:23:respawn:/#T0:23:respawn:/' /etc/inittab

High speed modes of operation are also not enabled by default and these should be enabled.

echo "init_uart_clock=64000000" >> /boot/config.txt

Reboot the RPi to apply all the settings.

Manual
Peripherals