Hi all 
I'm starting now (3 weeks) with an Arduino Mega.
I put a simple 555 variable oscillator on a breadboard. Now, how can I measure its frequency ?
Thanks.
Check out the Pulsein() command. If you measure the high duration portion of a square wave and then double that value and then divide it into 1 you should end up with the frequency value. That assumes a 50/50 square wave. It it's not a 50/50 ratio then you should measure both the high portion and low portion and add them together and then divide into one.
http://arduino.cc/en/Reference/PulseIn
At least I think thats right 
Lefty
Very kind of you, Lefty