Computer memory kits and how their memory timings is one of the misconstrued elements of the PC enthusiast world.
Although there are several types of memory timings, in this article I will briefly explain what memory timings are what each timing is, and their use so it’s easier to choose which ones you should be using for your next build.
What Are Memory Timings?
Every memory kit has a series of latches that holds data until it is requested by the CPU. The time it takes to deliver this data is determined by several things, but in general every memory kit has a required latency time that is programmed into its SPD chip.
The latches are controlled by something called “memory timings” which basically govern how fast these latches can deliver the data they contain.
As a general guide, memory timings are the number of clock cycles required to execute certain commands. The timings will vary from kit to kit as each module has a different design and manufacturers make use of their own technologies.
The most commonly used memory timing settings include: tCL, tRCD, tRP, tRAS, Command Rate(or CL) and Data Transfer Rate(or DTR, also known as MT/s).
How Do Memory Timings Work?
There’s a lot of confusion about memory timings and how they work, so let me explain.
The most common memory timing settings are Command Rate (also known as CL), tRCD, tRP and tRAS.
Command Rate: for every read/write request sent by the CPU to the RAM, the memory controller will make X number of clock cycles before it starts to deliver data.
This timing setting works like a traffic light and enforces a set amount of time between one command and the next, which is why we refer to it as Command Rate.
tRAS: this timing dictates how many clock cycles can elapse while executing a certain command. This setting is also known as Row Active Time and sets the number of cycles a bank on the memory can remain active before it needs to go back to idle state, hence why we refer to this as tRAS or CAS before RAS.
tRP: this is a more complicated timing as it designates a certain amount of time after tRAS before another command can be executed. It’s also known as Row Precharge Time.
tRCD: this is shortest of all the timings, it designates the number of clock cycles that must elapse before a read or write command can be executed following an active to precharge transition. This timing is also referred to as RAS to CAS delay.
I could go on and on about the other timing settings, but this should give you a good idea of how memory timings work.
Which Memory Timings Should I Use?
There’s a lot of options as each memory timing settings depends on your computer and its capabilities.
As a general rule, however, the best-known memory parameter to use is CAS Latency (CL). It specifies how many clock cycles the memory will wait before returning the requested data.
A memory with CL = 7 will delay data delivery by seven clock cycles, whereas a memory with CL = 9 will delay data delivery by nine clock cycles.
For DDR2 memory, the CL settings are: 5, 6, 7 and 8.
For DDR3 memory, the CL settings are: 7 (1T), 8 (1T/2T) and 9 (2T).
The ideal scenario is to choose a memory with as low latency timings as possible without sacrificing any speed or bandwidth.
Here’s an example: if you want a memory that’s capable of running at 1333 MHz, has CL8 timings and uses 1.5V, you should look for a kit with the following specs:
1.5V CL 8 -1600C11T @ 1333MHz.
The reason why the system is engineered to allow such variation is due to the different manufacturers and their technologies. What i find shocking is that some people have a CL4 or CL5 memory running at 1600MHz or 1800 MHz with 1.65v, which I believe it’s total overkill.
So the best option is to do your own research, look for a reputable retailer and buy the best memory you can afford.
I have found this list of memory kits on Amazon as the best place to start as it is useful.
What Is CAS Latency (CL)?
CAS latency or CL is one of the most important timings to consider when purchasing a memory kit. This timing tells you how many clock cycles will result in any read command.
The lower the number, the faster the memory will be. If this is your first time buying a computer memory it’s always best to consult our Memory Hierarchy Chart for assistance.
How Do I Find The Right Memory For My Computer?
Oftentimes, Technology gurus recommend that you choose a higher frequency set of memory that has a lower CL rating or choose a lower frequency set of memory that has a higher CL rating.
For example, if you want to overclock your processor and can run it at 1600 MHz you should choose a kit with CL 9 (1333 MHz) or lower such as the 1333 MHz kit mentioned above. If you don’t plan on overclocking your system too much, then choosing a kit with CL 8 (1600 MHz) or higher would be the best option.
This is categorized as a “sweet spot” for memory overclocking and will provide you with optimal performance without sacrificing too much speed.
The other very important timing to consider is tRAS which tells you how long the memory can go before it needs to switch rows. However, this should only be considered if you plan on overclocking your memory as high as possible.
Memory timings are extremely important and should never be overlooked when choosing a computer memory kit to purchase. If you choose a kit with low CL ratings but slow frequencies, or higher CL ratings with lower frequencies, the performance of your system will be compromised.
The best option is to buy the highest frequency memory kit that has the lowest CL rating. If you plan on overclocking your system, then it’s also important to choose a memory kit with low tRAS ratings.
There are numerous choices out there and some people get overwhelmed when choosing their memory kit, but if you follow the aforementioned advice, you’ll be able to choose the best memory kit for your computer.