A delay locked loop block receives external clocks to generate first internal clocks including a reference clock. An internal delay unit delays the first internal clocks to output second internal clocks, which are fed back to the delay locked loop block. The delay locked loop block adjusts delay time of the delay unit according to a phase difference between each second internal clock and the reference clock so that the second internal clocks are delay locked. A duty cycle correcting block corrects a duty cycle of each second internal clock and outputs a duty cycle corrected clock. An error determining unit compares a phase of each second internal clock with one another and, based on the comparison, feeds back a feedback clock including one of the duty cycle corrected clock or the second internal clock to the delay locked loop block.