Understand one of the timing "timing" of RAM

Introduction

DDR,DDR2,DDR3 memory is categorized according to the maximum speed at which they can work and their timing. Timing are represented by a series of numbers such as 3-4-4-8,5-5-5-15,7-7-7-21 or 9-9-9-24, the lower the better. In this article, the meanings of these numbers are explained.

The memory of DDR,DDR2 and DDR3 complies with the DDRXXX/PCXXX classification specification.

The first set of digits, XXX, represents the highest clock frequency supported by the memory chip. For example, DDR400 memory chips work at up to 400MHz and ddr2-800 work at up to 800MHz. Note, however, that this is not the actual clock speed of the memory, and the clock speed of the DDR,DDR2,DDR3 memory is half the tag clock frequency. So the memory of the DDR400 goes in a total of 200MHz.

The second number represents the maximum transfer rate of memory, in megabytes (MB/s). DDR400 memory the highest transfer data speed is 3200MB/S, so its label is labeled PC3200.

The first classification ddrxxx mentioned above is the standard used to classify memory chips, and the second classification pcyyyy is the standard used to classify memory modules.

In the image above, you can see the pc3-10666 memory module, using the ddr3-1333 memory chip. Note the timing 7-7-7-18 and voltage 1.5v

The maximum data transfer rate for a memory module can be calculated according to the following formula:

Maximum theoretical transfer rate = clock frequency * Number of bits/8

Since the DIMM module transmits 64 bits at a time. The number of BITS is 64.

Then the formula above is simplified to:

Maximum theoretical transfer rate = clock frequency * 8

If the memory module is installed on a system with a lower clock frequency, the maximum data transfer rate that the memory module can achieve is lower than the theoretical maximum data transfer rate.

For example, join you to buy a bunch of ddr3-2133/pc3-17000 memory. Although they are ddr3-2133 by the toe end, it will not always automatically run in the system at 2133MHz. This is the maximum clock frequency that it supports, not the clock frequency at which it runs. If you put it in a normal PC that supports DDR3. It may run at 1333MHz. This is the highest DDR3 standard speed. Get the highest transfer aluminized for 10664mb/s "if running in dual-channel mode, you can reach 21328mb/s" so they are not automatically run at 21333MHz and will not automatically reach 17000mb/s.

So why would anyone buy these modules? There are people who buy them for overclocking. Because the manufacturers guarantee that these modules will run at up to 2133MHz. You'll know. You can increase the clock frequency of the memory bus to 1066MHz to achieve higher performance.

However, your motherboard must support such overclocking, so if you do not overclock, it is useless to purchase a memory module that is more frequent than the system supports.

timing Due to the existence of time series, two of the same theoretical maximum transfer rate of memory modules will have different performance, why so.
Timing is a delay between certain operations within the memory chip. Consider some of the most famous parameters, one of which is the CAS delay, or CL or access time. This parameter tells us how long it takes the memory module to return the data requested by the CPU. A memory module with CL of 9 will delay 9 clocks and return the requested data. A memory module with CL of 7 will delay 7 clocks and return data. The second memory module runs faster because it returns data more quickly.

As you can see in Figure 1. The CL value of the memory module in Figure one is 7.

Memory timings are given through a series of numbers, for example, 4-4-4-8,5-5-5-5-15,7-7-7-21 or 9-9-9-24. A number represents the number of clock cycles required for memory to perform an operation. The smaller the number, the faster the memory, the memory module in Figure 1 has a time series of 7-7-7-18, the memory module in the following diagram is the timing of 8-8-8-24,

The meanings of these operations are cl-trcd-trp-tras-cmd in order to understand them, remember that memory is organized into matrices, and data is stored at the junction of dry rows and columns.

Cl:cas Delay is the time it takes to start an answer from a command to memory. That is, the time from the processor to the memory to start the data back to the memory.

Trcd:ras to CAS delay, that is, the time to select CAs from the activation of a line command RAS to the column.

Trp:ras precharge time. Is the time from closing a line to the beginning of the visit to the Ling line.

TRAS; Active to Precharge time. Memory time to wait for the next memory request to be emitted.

CMD: The command rate, the time it takes from the chip being activated to the first command that can be emitted to memory. This usually takes one or two clock cycles.

Typically, you have two options to configure your PC to use memory standard timing, to set the memory configuration to auto in the BIOS, or to manually set up your PC to use lower memory timings to improve system performance, and note that not all motherboards support changing memory timings, and Some motherboards may not be able to run under very low memory timings.

When you want to overclock the memory, you may need to increase your memory timing, which makes the system more stable operation, there is actually a very interesting thing happened, by increasing memory timing, memory performance is even worse, even if it is running at higher clock frequency, due to the increase in memory timing introduced delay.

Here is another advantage of the memory module specially designed for overclocking, which ensures that the memory module can achieve the clock frequency labeled on the label, and that you can get the timing of the callout at the clock frequency.

For example, although you can use ddr3-1333/pc3-10600 to achieve 1600MHz "800mhz*2", under these modules, you may need to increase the memory timing. In the ddr3-1600/pc3-12800 memory module, the manufacturer ensures that you do not have to adjust the memory timing when you reach 1600MHz.

The meanings of each memory timing parameter are explained in detail below.

CAL delay "CL"

As mentioned earlier, CAS latency is the most famous memory parameter, which tells us how much memory needs to be delayed to return the data we request.

It is worth noting that the clock frequency mentioned here is the frequency at which the memory module is currently running "real clock rate". That is, half of rated clock rate, because DDR,DDR2, with DDR3 memory can transmit two data per clock cycle, their rate clock is twice times their real clock rate.

In the figure above, you can see the working process of CL, we give two examples, example of a memory module of the cl=7, a memory module of the cl=9. The blue command in the diagram is a read command.

The Cl=7 memory module has a 22.2% higher latency performance than the cl=9 memory module. Two modules run at the same clock frequency.

The calculation process: first ddr3-1333 memory for each clock cycle time is 1.5ns. Note that the real clock rate is used here, which is half the clock frequency labeled by the tag.

So ddr3-1333 memory requires a delay of 10.5ns for cl=7, and 13.5ns for cl=9.

SDRAM, DDR,DDR2, and DDR3 memory implement the burst mode, at which time the data stored at the next address can be transmitted in bursts.