| Overview
DDR (Double Data Rate) SDRAM is the next generation of the current SDRAM, which improves the RAM speed to at least 200 MHz. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling the output. DDR finds its foundations on the same design core of SDRAM, yet adds advances to enhance its speed capabilities.
There are two basic differences to the standard SDRAM: Firstly, DDR uses more advanced synchronization circuitry, which is not present on SDRAMs. Secondly, DDR uses a delay-locked loop (DLL) to provide a DataStrobe signal as the data becomes valid on the SDRAM pins. The controller can thus use this DataStrobe signal, one for every 16 outputs, to locate data more accurately and resynchronize incoming data from different dual in-line memory modules. DDR essentially doubles the memory speed from SDRAMs without increasing the clock frequency. Consequently, DDR allows data to be read on both the rising and the falling edge of the clock,
thus delivering twice the bandwidth of standard SDRAMs. In addition, DDR moves to a higher frequency by changing signaling from TTL/LVTTL to SSTL3.
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