A clock system is provided capable of using an external system clock for driving at least one charge circuit of a semiconductor memory unit for restoring and refreshing a data array of the memory unit. The clock system, in one embodiment, includes a plurality of control circuits each having a clock select circuit which has as an input the external system clock, an internal clock generator circuit for generating an internal system clock, and a multiplexer. The multiplexer has as inputs an output of the clock select circuit, i.e., the external system clock, and an output of the internal clock generator circuit, i.e., the internal system clock. The multiplexer outputs either the external system clock or the internal system clock to the at least one charge circuit according to at least one control signal transmitted by a central processing unit to the clock select circuit.