Memory Timings Explained

Author: djbbenn
Date: 2005-12-01 14:19:16

Introduction

Have you ever wondered what those numbers “2.5-3-3-8” or “2-2-2-5” and so on mean? Or have you seen words like “CAS” and “tRCD”? These are the timings for the memory, or the speed at which it processes certain commands. This whole settings area can be quite confusing and you have to have a certain knowledge of it, if you plan on overclocking. But some of you may just be plain curious as to what they are.

Rated Memory Timings.

  • CAS (tCL) Timing: CAS stands for Column Address Strobe or Column Address Select. It controls the amount of time in cycles between sending a reading command and the time to act on it. From the beginning of the CAS to the end of the CAS is the latency. The lower the time of these in cycles, the higher the memory performance.

  • e.g.: 2.5-3-3-8 The bold “2.5” is the CAS timing.


  • tRCD Timing: RAS to CAS Delay (Row Address Strobe/Select to Column Address Strobe/Select). Is the amount of time in cycles for issuing an active command and the read/write commands.

  • e.g.: 2.5-3-3-8 The bold “3” is the tRCD timing.


  • tRP Timing: Row Precharge Time. This is the minimum time between active commands and the read/writes of the next bank on the memory module.

  • e.g.: 2.5-3-3-8 The bold “3” is the tRP timing.


  • tRAS Timing: Min RAS Active Time. The amount of time between a row being activated by precharge and deactivated. A row cannot be deactivated until tRAS has completed. The lower this is, the faster the performance, but if it is set too low, it can cause data corruption by deactivating the row too soon.

  • tRAS = tCL + tRCD + tRP (+/- 1) so that it gives everything enought time before closing the bank.

    e.g.: 2.5-3-3-8 The bold “8” is the tRAS timing.
(The 2.5-3-3-8 figure is just an example for memory timings.)

These are the four timings that you would see when memory is being rated. It is in the order of CAS-tRCD-tRP-tRAS. The lower these timings, the higher the performance of the memory. Some motherboard manufactors (DFI for example) list the timings in their bios CAS-tRCD-tRAS-tRP.

Certain memories can take tighter (lower) timings at higher speeds. These are the more expensive memory modules out of the bunch. There are also other timings to consider in your BIOS. Not all boards will have options like these.

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