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On www.JEDEC.org they say: The per-pin data rate for DDR4 is specified as 1.6 giga transfers per second to an initial maximum objective of 3.2 giga transfers per second. JEDEC Standard No. 79-4A (DDR4 specifications) says: An application should use the next smaller JEDEC standard tCK(avg) value (1.5, 1.25, 1.071, 0.938 or 0.833 ns). tCK=1.5ns means 667MHz clock frequency and 1333MT data rate.
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