A semiconductor memory device includes input/output circuitry capable of operating in sync with an externally provided I/O clock signal. A data in buffer and a data out buffer provide for serial to parallel conversion of write data and, conversely, parallel to serial conversion of read data. The data buffers can be synchronized with the external I/O clock signal thereby decoupling their operation from the internal system clock signal. This strategy improves I/O bandwidth and further provides for matching different numbers of bit lines or word sizes as between the I/O data port and the memory array itself. An internal I/O clock generator can be provided for generating I/O clock signals, again without the limitation of synchronizing to the internal system clock signal.