can someone help me for Freertos givesemaphore when an external interrupt occured one of the int pins. (int0 perhaps) the task waiting for semaphore will toggle a led on an output pin.

The following page contains example code that shows how to give a semaphore from an interrupt.

http://www.freertos.org/a00124.html

However it is much more efficient to use a direct to task notification in place of a semaphore.

http://www.freertos.org/RTOS-task-notifications.html