There is one application note in Atmel which gives
simple method for zero crossing detector.
You just connect micro ground 0V with to Neutral
(return of 240V system)thru 1MEG resistor and P3.3 ( INT1) with another 1Meg resistor. Set INT1 with Falling Edge and not level
type. The internal protection diodes of micro take care of
the excess voltage. If not sure then connect anode of 1N4148
diode to P3.3 pin and Cathode to +5V. Similarly use other
1N4148 with anode to GND of +5V supply and cathode to P3.3pin.
Now you will get very precision interrupt when Mains voltage
is just falling to zero!
Presently what is your connection is absoultely wrong as
pointed by others. The emitter of OC should be connected to
0V and collector to P3.3.If no internal pullup used then use
external pull up of 10K ohm resistor. However this will be
incorrect by many degrees. The method described above gives
precison gives fraction of a degree!( By degree I mean there
are 360 degrees in one cycle)