Each of a plurality of master devices outputs a speed grade signal indicating a data transfer speed with a data transfer request. An arbiter arbitrates transfer requests and speed grade signals from the plurality of master devices. A clock enable generation circuit generates a clock enable signal with a varying ratio of a valid level according to the speed grade signal arbitrated by the arbiter. A slave device operates upon receiving a clock signal when the clock enable signal is at the valid level, and transfers data according to the transfer request arbitrated by the arbiter. Accordingly, the frequency of the clock signal which causes the slave device to operate may be changed for each transfer request, and a fine control of the power of the slave device may be easily performed. As a result, power consumption of the data processing system may be finely controlled.