Latencies

DDR2 memories work with higher latencies than DDR memories. In other words, they delay more clock cycles to deliver a requested data. Does this mean that DDR2 memories are slower than DDR memories? Not necessarily. As we said, they delay more clock cycles, but not necessarily more time.

If we compare a DDR memory to a DDR2 memory running under the same clock, the one with the lower latency will be the fastest. Thus, if you have a DDR400 CL3 memory and a DDR2-400 CL4 memory, your DDR400 will be faster.

Keep in mind that DDR2 memories have an additional parameter called AL (additional latency), which must be added to their nominal latency (CL) in order to get the total latency.

When comparing memories with different speeds, you need to consider the clock in your math.

On a DDR400 CL3 memory, this “3” means that the memory delays three clock cycles to start delivering the requested data. Since this memory runs at 200 MHz, each clock tick measures 5 ns (T= 1/f). Thus its latency if of 15 ns.

Now on a DDR2-533 CL3 AL0 memory, this “3” also means that the memory delays three clock cycles to start delivering the request data, but since this memory runs at 266 MHz, each clock tick measures 3.75 ns, so its latency is of 11.25 ns, making this memory faster to data delivery than our DDR400 CL3 memory. So a DDR2-533 CL4 and AL0 memory has the same latency as a DDR400 CL3. Notice that we are assuming the additional latency as zero, or we would need to take it into account, i.e., a DDR2 CL3 AL1 memory has in reality a latency of four clock cycles.

Some manufacturers announce their memory module latencies thru a series of four number, like “4-4-4-12” or “5-4-4-9” or “3-3-3-8”. The latency we’ve been talking about (CL) is the first number on the sequence. The additional latency (AL) is usually found on the memory module technical specs (usually a PDF file for downloading on the manufacturer website). You want to know what the other numbers mean, read our tutorial Understanding DDR Memories.

In order to make you calculations and comparisons easier, we prepared the following table containing the clock tick duration depending on the memory type. So, just get the number below depending on the memory type you want to compare and multiply it by the latency value in order to know the latency duration in nanoseconds, allowing you to compare latencies of memories with different clock speeds and to know which memory is faster.

Memory	Clock Tick Duration (each one)
DDR266	7.5 ns
DDR333	6 ns
DDR400 and DDR2-400	5 ns
DDR2-533	3.75 ns
DDR2-667	3 ns
DDR2-800	2.5 ns

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