See "[solved]Due interrupt not working" in this forum. There is sample code for capturing external interrupts
Thank you jstampfl.
I assume that you mentioned this post http://forum.arduino.cc/index.php?topic=201810.0
. The code can't help me because the external signal that I was talking about is precise 13MHz TTL. High level Arduino IDE is very slow for my requirements. So I think I need an assembly code for doing this correctly.
Actually what I'm trying to do is, creating a 10us, 1kHz tunable puls generator synchronized with the main 13MHz TTL external
clock. So I'm stuck here.