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What are Memory Timings?

When the topic of memory performance comes up, most people usually think of a memory module speed. Module speed is a measure of the ability to transfer data, like: DDR2 800MHz, DDR3 1600MHz, and DDR4 2400MHz (or MT/s). Timings, however, determine how fast your memory can respond to requests for performing actions.

If we think of memory as a race car, the module speed (MH/z) would be like the raw horse power of the engine, and the timings would be the driver of the car. As the driver of the car gets better at handling turns and responding to obstacles on the race course, the car will perform faster, so much so, that you can have a car with lower horsepower outperform one with more horsepower, if the driver (timings) are faster in the lower horsepower car than the car with more horsepower.

When we look at timings of memory, they are typically displayed in a numerical format; 9-9-9-24 is as an example of a generic DDR3 memory timing. Below is a table that displays some standard timings for different types of DDR memory.

Generation

CL

tRCD

tRP

tRAS

DDR2

5

5

5

15

DDR3

9

9

9

24

DDR4

16

16

16

N/A

Timings are most commonly broken down to the four values: CAS Latency (CL), Row Column Delay (tRCD), Row Precharge Time (tRP), and Row Active Time (tRAS). If you noticed the table above has the tRAS missing for DDR4, this is because this value has been merged into another number with the new memory technology, so it is no longer relevant.

CL

tRCD

tRP

tRAS

This is the time it takes for a memory module to have data ready upon request of the memory controller

The time it takes to read memory after the memory is ready

The time it takes for memory to have a new row ready for using data

Minimum time required for a row to be active to ensure data can be accessed from it

The most widely recognized timing for memory would be CAS Latency. This value is typically synonymous with performance. However, it can be  misleading at times. Most would think that the lower the CAS Latency the better, as this value refers to your memory's ability to quickly respond to new information.  This isn't completely accurate as newer memory types typically have much higher CAS latency times than their older counterparts. 

Why do new memory types have slower latency times? Along with different timings, there is an attribute called Clock Cycle Time. This is a measurement reflective of how quickly the memory can be ready for a new set of commands. New memory types, like DDR4, have significantly faster Clock Cycle Times than older memory. As the chart below illustrates, this effectively means the True Latency (real speed) is much faster. If you would to know more about speed vs latency, check out this in-depth article.

Technology

Module Speed (MT/s)

Clock Cycle Time (ns)

CAS Latency (CL)

True Latency (ns)

SDR

10E

8.00

3

24.00

SDR

133

7.50

3

22.50

DDR

335

6.00

2.5

15.00

DDR

40B

5.00

3

15.00

DDR2

667

3.00

5    

15.00

DDR2

800

2.50

6

15.00

DDR3

1333

1.50

9

13.5

DDR3

160B

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

18

13.50

In most cases you shouldn't worry about your memory timings. As long as you purchase memory that the Crucial® Advisor™ tool or System Scanner tool  says is compatible with your computer, you can be assured that you have memory that is capable of running in your system. The only exception to this rule is when purchasing high performance Ballistix® parts for custom built systems. Some CPUs are limited with the memory speed and latencies they will support, so it’s always a good idea to check the max memory speed your CPU will support, before paring it with any higher-end memory.  If you have any further questions be sure to reach out to Crucial support.

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