fabritio,

your program should work.

There are a couple of things I would like to mention:

you should move the "PORTC=255"
to the end of your Interrupt Routine,
there is no need to write to PORTC every single time the mainloop is executed.

Due to the fact it this is "INTERRUPT ON CHANGE"you will see an interrupt on EVERY rising AND falling edge.
So if the execution time of the Interrupt routine is shorter than the incomming "burst" the routine will be executed more than once on a single burst.