A memory system is described, where a plurality of memory modules is connected to a memory controller. The power status of each of the memory modules is controlled, depending on the functions being performed by the memory module. When no read or write operation is being performed on a particular memory module, at least a portion of the circuitry may be operated in a lower power mode. A memory circuit associated with the memory module may be placed in a low power mode by disabling a clock. The memory circuit data integrity may be secured by issuing refresh commands while when the memory circuit is in the lower power mode, by enabling the clock, issuing the refresh command, and disabling the clock after completion of the refresh operation.