Hi Wenlong,
The following commands should set up the correct pin multiplexing to enable serial mode on Pins 0 an 1. The key part is the echo of 'mode1' into 'current_pinmux', as that sets the pins into UART mode, otherwise that may be in GPIO mode.
echo 214 > /sys/class/gpio/export 2>&1
echo high > /sys/class/gpio/gpio214/direction
echo low > /sys/class/gpio/gpio214/direction
echo 131 > /sys/class/gpio/export 2>&1
echo mode1 > /sys/kernel/debug/gpio_debug/gpio131/current_pinmux
echo 249 > /sys/class/gpio/export 2>&1
echo high > /sys/class/gpio/gpio249/direction
echo 1 > /sys/class/gpio/gpio249/value
echo 217 > /sys/class/gpio/export 2>&1
echo high > /sys/class/gpio/gpio217/direction
echo 1 > /sys/class/gpio/gpio217/value
echo out > /sys/class/gpio/gpio131/direction
echo 0 > /sys/class/gpio/gpio131/value
echo 130 > /sys/class/gpio/export 2>&1
echo mode1 > /sys/kernel/debug/gpio_debug/gpio130/current_pinmux
echo 248 > /sys/class/gpio/export 2>&1
echo low > /sys/class/gpio/gpio248/direction
echo 0 > /sys/class/gpio/gpio248/value
echo 216 > /sys/class/gpio/export 2>&1
echo in > /sys/class/gpio/gpio216/direction
echo in > /sys/class/gpio/gpio130/direction
echo high > /sys/class/gpio/gpio214/direction
Pins 0 and 1 on the Arduino baseboard are at /dev/ttyMFD1, and a getty can be enabled to give a longin prompt on this port by editing /etc/inittab and adding the following line:
T1:23:respawn:/sbin/getty -L ttyMFD1 115200 vt100
Then, using a USB to serial cable, you can connect to the board and log in using minicom or putty, e.g.:
minicom -b 115200 -D /dev/ttyUSBx
Of course, you don't have to set up a getty, if you simply want to talk to another device serially.
Hope this helps,
Dave.