A detecting circuit includes: a testing signal generator, arranged to selectively generate a testing signal having a first signal edge or a second signal edge to a connecting port; and a detector, arranged to detect a first detect signal on the connecting port after the testing signal having the first signal edge is coupled to the connecting port, and to detect a second detect signal on the connecting port after the testing signal having the second signal edge is coupled to the connecting port; wherein the detector is further arranged to determine if an external circuit element is coupled between the connecting port and a reference voltage according to the first detect signal and the second detect signal.