External trigger so easy?

I just want to use the signal generator to trigger one IO pin of FPGA externally. On FPGA , this trigger sigal will be captured on the rising edge.

But, when I just connect the IO pin on FPGA to the cable which will be connected to the sigal generator or oscillograph, the trigger starts, it works so easy that I can't control it, what's interesting is that it works even when I toutch the pin.

Then I measure the voltage of my fingers which is about 500 mV.

FPGA Information:

LVTTL selected, 10 pf loaded, dual-in-line pin used as the input trigger.

Does anybody kind of use external trigger, and know why this happened?

John