SDRAM Latency - Measurements

Measurements

Lower latency results in better performance, although the difference will not be significant. RAM speeds are given by the four numbers above, commonly in the format "tCAS-tRCD-tRP-tRAS". For example, latency values given as 2.5-3-3-8 would indicate tCAS=2.5, tRCD=3, tRP=3, tRAS=8. (Note that 0.5 values of latency (such as 2.5) are only possible in double data rate (DDR) RAM, where two parts of each clock cycle are used)

Computer users don't need to worry about setting the SDRAM latency because the computer will auto-adjust the RAM timing based on the Serial Presence Detect (SPD) ROM inside the RAM packaging that defines the four timing values, decided by the RAM manufacturer. Although the SDRAM latency timing can often be adjusted manually, using lower latency settings than the module's rating (overclocking) may cause a computer to crash, due to memory read/write errors, or fail to boot.

Regardless of the said risk, power users typically perform overclocking because no manufacturer attempts to set the best settings in the SPD ROM due to economies of scale incurring from (among others) fabrication process variations - extra binning require extended testing and validation. Due to the loose settings of manufacturers, an SDRAM with SPD 6-6-6-18 and a command rate of 2T being able to support 5-5-4-14 timings with a command rate of 1T and function without instability, with all other settings being the same. The manufacturer may also only need to sell a specific, slower specification for compatibility purposes and may program higher grade modules with lower clock SPD timings due to being cheaper for them (and/or slower chips not being available anymore). Computer power users edit the bios, or use programs such as Memset, to reduce the default latencies or increase clock to the least that the modules can practically function, although they may then not work well up to the module's temperature rating or may require extra voltage to do so.