Adverse effects of noise are reduced. A photodetector circuit, a difference data generation circuit, and a data input selection circuit are included. The photodetector circuit has a function of generating an optical data signal. A first data signal and a second data signal is input to the difference data generation circuit and the difference data generation circuit has a function of generating difference data of data of the first data signal and data of the second data signal. The data input selection circuit has a function of determining that the data of optical data signal is regarded as data of the first data signal or data of the second data signal.