A serial NOR memory device receives serial input data using a single data rate (SDR) mode and transmits serial output data using a double data rate (DDR) mode. In some embodiments, a serial NOR memory device includes an input-output circuit including a transceiver coupled to receive a clock signal and serial input data and to provide serial output data. The transceiver is configured to receive serial input data using the single data rate mode and is configured to transmit serial output data using the double data rate mode.