Semiconductor device includes a pad for outputting a cyclic redundancy check (CRC) data for error detection and a signal outputting unit for outputting the CRC data or a data strobe signal, which is output together with data of being output in response to a read command, through the pad according to operation modes. Method for operating a semiconductor device provided a step of outputting a CRC data for error detection through a CRC data pad and a step of outputting a data strobe signal, which is output together with data output in response to a read command, through the CRC data pad according to an operation mode.