Hi jergen,
Please correct me if I'm wrong:
You want to trigger INT0 of 8051 with only one square wave signal, first on leading edge, then for falling, again for leading and so on, following the square shape.
If yes, you can do that setting the 8051 trigger on falling edge and then apply your signal through a circuit built as follows:
- one NXOR gate with two inputs
- your signal straight to one input of the gate
- your signal through a resistor toward the second input of the gate
- last required item is a capacitor fitted with one pin toward ground and the second pin fitted to the above resistor lead that goes toward the second input of the gate
The resistor and capacitor builds up a small delay circuits (we can talk about the delay time if you'll mention the XTAL frequency and what 8051 you have).
Thus you get short falling pulses at the NXOR gate at each edge of your square wave signal.
Regards,
Silvio