There are different generations of SDRAM interfaces, such as Single Data Rate or SDR, Double Data Rate or DDR, and Double Data Rate 2 or DDR2. “Single Data Rate”, means fetching one data with each clock cycle. Each clock cycle has a rise and a falling edge and with SDR the data is fetched at the rising edge. “Double Data Rate”, achieves two data with each clock cycle, where one data is fetched from the rising edge and the second at the falling edge. Consider a microprocessor using a clock frequency of 166MHz; an SDR will result with 166 Mbps per I/O. With the same 166MHz frequency, using a DDR the result will be doubled, 333Mbps per I/O.