The invention aims at providing a semiconductor device, a semiconductor system including same, and a semiconductor device control method enabling it to correctly judge that a module has been coupled to and decoupled from a communication bus. According to one embodiment, a host controller includes a variable resistance element and a control circuit that varies the resistance value of the variable resistance element so that the potential of a potential detecting line which is determined by the variable resistance element and a resistance element provided in each of modules will fall within a predefined range. It is thus possible to keep that potential varying to a certain extent or more due to coupling and decoupling of a module to/from the communication bus. It is therefore possible to correctly judge that a module has been coupled/decoupled to/from the communication bus.