A semiconductor memory device has an operation mode in which a read/write operation is performed in response to a command supplied externally in synchronization with a clock, and a power-down mode in which no external read/write command is accepted. The semiconductor memory device performs a refresh operation in response to an externally supplied signal during the power-down mode. A memory system has a plurality of the semiconductor devices and a memory controller. The memory controller outputs a control signal during the power-down mode, and the plurality of semiconductor devices perform a refresh operation in response to the control signal during the power-down mode.