A detecting device for detecting an operating mode is disclosed. The detecting device includes a pulse generator and a hold-up unit. The pulse generator is disposed for issuing a one-shot pulse signal in response to each of button signals respectively. The hold-up unit is disposed for receiving the button signals to respectively generate delayed button signals by way of clock delay determined by a clock signal. The one-shot pulse signal and the delayed button signals are used to determine an operating mode of a system.