Double Data Rate

DDR is a technique where two data are transmitted per clock cycle, instead of only one. With this technique the device achieves the double of the performance of a similar device running at the same clock rate but transferring just one data per clock cycle.

This technique is used by DDR-SDRAM memories and by the external bus of AMD CPUs (Duron, Athlon, Athlon XP, etc).

Since the device achieves a “doubled” performance usually the clock rate reported by the manufacturers isn’t the device’s real clock but its “performance” instead. For example, “400 MHz” DDR-SDRAM memories work at 200 MHz. Another example: Athlon XP’s external bus is said to be of 266 MHz, 333 MHz or 400 MHz while in fact it works at 133 MHz, 166 MHz and 200 MHz, respectively.

click to enlarge
Figure 1: How DDR technique works.

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