An integrated circuit (IC) and a bus system are provided. The IC includes a first function module coupled to a first pin, a switch, a second function module coupled to a second pin, and a controller. The switch selectively couples the first pin to a pull-down resistor according to a control signal. When a specific condition is met, the controller turns on the switch to couple the first pin to a pull-down resistor, and detects a first input signal on the first pin, so as to determine whether there are slave devices coupled to a master device. When the controller determines that the IC is one of the slave devices or the single slave device connected to the master device, the controller turns off the switch to separate the first pin from the pull-down resistor, and assigns the second pin to the second function module.