Trigger digital interrupt on negative voltage
Simplest is to use a Schmitt Trigger. You can find the details at Wikipedia using that name. You also find schematics there for transistors and Op-Amps. (I would use an Op-Amp). You also find the formulas there. There was a remark about negative voltage but you don't need that. If you take a resistor divider from e.g. a 3.3V supply  and connect it to the sync, the voltage over the resistor will vary between 3.3 and about :-)  3.6V. For example, if you use two resistors in the ratio 3:7 your voltage swing will be ~200mV. Adjust your Schmitt trigger resistors to detect the difference between those voltages.