DDR SDRAM Explained

DRAM is asynchronous, meaning it operates independently of the CPU and the system bus’ clock cycle. SDRAM is synchronous, meaning its operation is dependent on the data bus’ clock cycle (as is the CPU). This synchronicity translates to faster data transfers between memory and the CPU.

SDR (Single Data Rate) SDRAM sends one bit per clock cycle. DDR (Double Data Rate) SDRAM sends two bits per clock cycle (on both the rising and the falling edges of each clock cycle). DDR2 achieves speeds twice as fast as DDR by using an internal clock that runs at double the speed.

DDRclockSignal

The maximum transfer rate for a memory module can be calculated with the following formula:

Maximum Transfer Rate = (clock rate in MHz) x (number of bits) / 8 (the reason for dividing by 8 is to convert bits to Bytes)

Since DIMM modules transfer 64 bits at a time, “number of bits” will always be 64. Since 64 / 8 = 8, we can simplify this formula to:

Maximum Theoretical Transfer Rate = clock rate x 8

Example

DDR2-400 / PC2-3200 Memory Module

Has a 400MHz clock cycle (real clock = 200MHz, doubled), so it can send/receive 400M bits per second

To determine Bandwidth (Transfer Rate), multiply:

400M (Hz) x 8 (bits) = 3200 MB/s

Sources

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