Abstract: A reference sync signal for synchronizing with peripheral equipment and an internal clock for an internal operation of a processor system are generated from a reference clock inside the processor system. A base clock is used to generate an internal clock in which a duty of each clock cycle changes. A circuit includes a synchronous counter using a reference sync signal as a reset signal and deciding a count number in accordance with a frequency ratio set in advance, and generates an access timing signal to the peripheral equipment. A conversion circuit synchronously gains access to an external bus operating at a different frequency by using the access timing signal as an enable signal of a latch of an external interface.