Latencies And Timings
From PEL Wiki
This is an excerpt from a memory technology article at xbitlabs.com
So, the weakest aspect of the DDR2 technology is high latency which led to DDR2-based systems having a smaller performance compared to DDR-based ones. And this was the problem the memory manufacturers interested in promoting DDR2 SDRAM in the market have been trying to solve. And they have made some success!
Early DDR2-533 SDRAM modules available at the time of the announcement of i925 and i915 chipsets had 4-4-4 timings [1]. Today, however, many memory manufacturers, especially the developers of advanced modules for PC enthusiasts like Corsair or OCZ, are offering DDR2 SDRAM capable of working at 533MHz with 3-3-3 timings. These modules are not overclocked, but fully comply with JEDEC’s appropriate standards (the official specification describes a DDR2-533 modification with 3-3-3 timings and a voltage increased to 1.9v).
Intel also approved of DDR2-533 SDRAM with 3-3-3 timings in LGA775 systems. The company has officially confirmed its new chipsets support such memory and even specifically emphasized that such memory would be the best choice for enthusiastic users. Intel’s reasoning is easy to grasp: theoretically, the reduction of the CAS latency of DDR2-533 to 3 cycles is a significant improvement of the parameters of DDR2 SDRAM:
| SDRAM Type | Timings[1] (cycles) | Latency (ns) | Bandwidth in dual-channel mode (GB/s) |
|---|---|---|---|
| DDR400 | 2.5–3–3 | 12.5 | 6.4 |
| DDR400 | 2–3–2 | 10 | 6.4 |
| DDR533 | 3–4–4 | 11.2 | 8.5 |
| DDR533 | 2.5–3–3 | 9.4 | 8.5 |
| DDR2-533 | 5–5–5 | 18.8 | 8.5 |
| DDR2-533 | 4–4–4 | 15 | 8.5 |
| DDR2-533 | 3–3–3 | 11.2 | 8.5 |
| DDR2-600 | 5–5–5 | 16.6 | 9.6 |
| DDR2-600 | 4–4–4 | 13.3 | 9.6 |
You can see that DDR2-533 SDRAM with 4-4-4 timings has a half worse latency compared to the widespread DDR400 SDRAM; the 30-percent growth of the bandwidth will hardly compensate such a worsening of the access time. The latency of DDR2-533 improves considerably at 3-3-3 timings, however, and is only 12 percent worse than the latency of 2-3-2 DDR400 SDRAM. As the result, considering that DDR2-533 SDRAM has a higher bandwidth, we can expect DDR2-based systems with reduced timings to be as fast as DDR1-based platforms.
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