GPIO can be used to switch something on/off or read in some digital sensor signal, like motion detector. 
In previous files was described how to use Arduino and USBserial communication for GPIO. 



From open WRT website GPIO fro WRT160NL 4 outputs and 2 inputs:
pin 	function

6 	LED_WLAN
8 	LED_WPS_BLUE
9 	LED_WPS_AMBER
14	LED_POWER

7 	BTN_WPS
21	BTN_RESET 


GPIO was supported
opkg list_installed|grep gpio

kmod-input-gpio-buttons - 2.6.39.4-1
kmod-input-gpio-keys-polled - 2.6.39.4-1
kmod-leds-gpio - 2.6.39.4-1

-----------------------------------------------------------------
LED control works nicely
cd /sys/devices/platform/leds-gpio/leds

wrt160nl:amber:wps   wrt160nl:blue:wlan
wrt160nl:blue:power  wrt160nl:blue:wps


cd wrt160nl:amber:wps
echo "1" > brightness
echo "0" > brightness

echo "1" > /sys/devices/platform/leds-gpio/leds/wrt160nl:amber:wps/brightness


cat trigger
[none] timer default-on netdev phy0rx phy0tx phy0assoc phy0radio phy0tpt usbdev

blinking LED 
echo timer > trigger
echo 250 >delay_on
echo 550 >delay_off

-----------------------------------------------------------------
GPIO Inputs 
First tried buttons in directory

cd /sys/devices/platform/gpio-keys-polled 

but could not find anything that contains button state.

Next tried classical Linux method on pin 7 (WPS button according to OpenWRT webpage) 

cd /sys/class/gpio
echo 7 > /sys/class/gpio/export
does not work, probably button is in use by OpenWRT



Half a day later found solution in a Polish forum:
http://openwrt.tytanos.pl/wrt160nl/GPIO/GPIO_wrt160nl.jpg

GPIO 13 is RX pin nr 3 on the five pin connector

echo 13 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio13/direction
echo 0 > /sys/devices/virtual/gpio/gpio13/value
echo 1 > /sys/devices/virtual/gpio/gpio13/value


GPIO 0 is LED4 that has not been soledered in.

echo 0 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio0/direction
echo 0 > /sys/devices/virtual/gpio/gpio0/value
echo 1 > /sys/devices/virtual/gpio/gpio0/value

Works!

Now tested GPIO 13 as input
echo 13 > /sys/class/gpio/export
echo in > /sys/class/gpio/gpio13/direction
cat /sys/devices/virtual/gpio/gpio13/value
does not work, always gives value 1, probably because of serial port driver

echo 0 > /sys/class/gpio/export
echo in > /sys/class/gpio/gpio0/direction
cat /sys/devices/virtual/gpio/gpio0/value
works nicely.

So I soldered a PIR motion detector board output to place of LED4.