A memory controller is connected with a memory. The memory controller includes a clock signal pin and plural command pins. The clock signal pin is connected with the memory for transmitting a clock signal to the memory. The plural command pins are connected with the memory for transmitting a command signal to the memory. The command signal contains an entering self-refresh command and an entering power down command. The memory enters a self-refresh state when the entering self-refresh command is executed. The memory enters a power down state when the entering power down command is executed.