DDR2 SDRAM
DDR2 SDRAM is very similar to DDR SDRAM
but doubles the minimum read or write unit again, to 4 consecutive words. The bus protocol was also simplified to allow higher performance operation.
Typical DDR2 SDRAM clock rates are 200, 266, 333 or 400 MHz (periods of 5, 3.75, 3 and 2.5 ns)