A communication system has a master device and a slave device clocked by the system clock generated by the master device for data transfer between them. When no data transfer takes place between them for a predetermined period, the slave device goes into a sleep mode. The slave device in the sleep mode can return to the active mode when it receives a system clock. Thus, the slave device can be placed in the sleep mode and return to the active mode without any additional signal lines. The communication system can save a large amount of power during the sleep.