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Performance of DRAM memory speed and delay

It is an indisputable fact that it is directly related to . But these two very close terms though, as close as you think they are not.

Hearsay information about delays

As in all matters, about RAM memory, unfortunately, hearsay information.

  • Most users, CAS latency (known as the CL value) of the memory reflects actual performance thinks. However, engineers working in the field of semiconductors, CAS latency directly to memory performance. does not show sings.
  • Most users also increases with increasing frequency and the effect of frequency increase neutralize thinks. But DDR4-2400 MHz and CL17 with a memory DDR4-2666 MHz and CL19 when we compare a memory 2666 MHz frequency memory more performance.
  • If you are stuck between two memories with the same frequency CAS value. CAS value low memory would be the better choice.

Starting point of hearsay information definition of delay, so the delay redefine would be more accurate.

Delay paradox

The biggest reasons for memory latency misunderstanding are that of manufacturers and comparators CAS latency. The reason this causes misunderstanding is; CAS delay value, delay formula

a multiplier of . CAS latency values ​​are only clock cycle (clock cycle)

the number and it doesn’t give us the duration of this clock cycle. Therefore, it is wrong to look at the delay values ​​directly as a performance indicator.

As a result of the delay formula nanosecond how much of one module relative to the other by looking at the result whether it is stable you can understand. The nanosecond result as a result of the formula, in the documentation of the memory (datasheet). The information in the documentation generally looks like this:



Clock Cycle Time (nanosecond)

CAS delay

Delay resulting from the formula (nanoseconds)

SDR 100 8. 00 3 24. 00
SDR 133 7. 50 3 22. 50
DDR 333 6. 00 2. 5 15. 00
DDR 400 5. 00 3 15. 00
DDR2 667 3. 00 5 15. 00
DDR2 800 2. 50 6 15. 00
DDR3 1333 1. 50 9 13. 50
DDR3 1600 1. 25 11 13. 75
DDR4 1866 1. 07 13 13. 93
DDR4 2133 0. 94 15 14. 06
DDR4 2400 0. 83 17 14. 17
DDR4 2666 0. 75 19 14. 25
DDR4 2933 0. 68 21 14. 32
DDR4 3200 0. 62 22 13. 75

What is the correct definition of delay and delay formula?

Basically the delay is to the controller the moment the command is entered and data formation in memory

The elapsed time between . As soon as the memory controller sends a command to the memory, the data is processed by CAS and should turn into a clock cycle so by the controller the requested memory address is known and the command is processed.

With this information, you two factors sets:

  • Data how many clock cycles passes through; which we call CAS latency or CL,
  • Each clock cycle time; We measure this in nanoseconds.

And when I multiply these two values formula for the delay we get:

Delay (ns) = clock cycle time (ns) x number of clock cycles

During the development of memory technology while increasing speeds, CAS latency also increased we see. The reason for this is that as the frequencies increase clock cycle time naturally decreasing so the memory to more clock cycles because you can enter on paper CAS delay value increased.

What kind of memory should we choose in the light of this information?

When choosing memory to memory frequency, as well as to delay value you should pay attention. The right memory for you;

  • Your processor supports highest frequency speed,
  • As your budget allows with minimal delay is the memory that has.

Also, if you intend to upgrade memory and do not want to retire other memory, we recommend that the latency values ​​and voltages match exactly. Otherwise, compatibility problems on your system bring unlocked, blue screen

like bad surprises

You may encounter .


In this article, we talked about what memory latency means and how you can choose memory accordingly. You can use Technopat Social for your questions and express your opinions about this article in the comments section.

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