Abstract: A semiconductor device for generating a transmission clock in a sink without a reference clock and a method of transmitting data from the sink to a source by use of the generated transmission clock are provided. The sink may include: a receiver configured to generate a digital control oscillator code by using a phase difference between a reception clock of a data signal received from a source and a recovered clock and configured to recover data from the data signal by using the recovered clock recovered by the generated digital control oscillator code; and a transmitter configured to generate a transmission clock by the digital control oscillator code having the recovered clock locked to the reception clock and configured to transmit return data to the source by using the transmission clock when a return data request identifier is received from the source.