A system includes a memory controller and a plurality of semiconductor devices that are series-connected. Each of the devices has memory core for storing data. The memory controller provides a clock signal for synchronizing the operations of the devices. Each device includes a phase-locked loop (PLL) that is selectively enabled or disabled by a PLL enable signal. In each group, the PLLs of a selected number of devices are enabled by PLL enable signals and the other devices are disabled. The enabled PLL provides a plurality of reproduced clock signals with a phase shift of a multiple of 90° in response to an input clock signal. The data transfer is synchronized with at least one of the reproduced clock signals. In the devices of disabled PLLs, the data transfer is synchronized with the input clock signal. The enabled PLL and disabled PLL cause the devices to be the source and the common synchronous clocking, respectively. The devices can be grouped. The devices of one group can be structured by multiple chip packages.