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Understanding, choosing and overclocking DDR3

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huddy
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Understanding, choosing and overclocking DDR3

Post #1 by huddy » Fri Jun 28, 2013 10:42 am

The following guide describes what DDR3 you should be buying and how to get the best from it.

Introduction to DDR3

It’s quite clear from the naming convention, that DDR3 provides higher clock speeds than its predecessor. The higher frequencies of DDR3 mean that the system bus can run much faster increasing overall performance of your PC. However, it’s slightly foolish to believe that DDR3 is just a speed bump variant of DDR2 and can only achieve the higher frequencies at the cost latency. Although this is partially true, it’s not the full picture. New technology and fabrication of the modules have provided the ability to extend the modules beyond DDR2, which mechanically is at its peak.

The higher frequencies of DDR3 also provide plenty of headroom for some fantastic overclocking results as well as set profiles for those less experienced. However, it requires a new DIMM socket and less power than its DDR2 sibling so you may need to invest in a new system to upgrade and although DDR3 theoretically pushes more data than DDR2, it may not result in the performance leap you are looking for.

Therefore, simply buying the fastest DDR3 modules may disappoint if chosen or used incorrectly. As with my DDR2 guide, I’ll hopefully make the choice clearer to understand.

Understanding DDR3

As with DDR2, DDR3 is an extension to the SDRAM family and continuous to transfer data on both the rising and falling edge of each clock cycle so it carries out twice as many operations per clock cycle (compared to SDRAM which only carries out one operation per cycle). Therefore, although the memory continues to run at the designated clock speed, the I/O bus effectively operates at twice the speed. For example, while the memory speed runs at is 200MHz the effective speed is doubled at 400MHz. This method is known as Double Pumping and DDR (Double Data Rate) gets it name from this method.

Image
4GB of DDR3 1600 MHz or PC3-12800, which seems to be best choice for both Intel and AMD platforms.

How does DDR3 differ from DDR and DDR2?

Firstly, DDR3 modules have half the core memory speed of DDR2 and four times that of standard DDR modules. DDR3-800 for example, has a memory clock speed of 100MHz whereas DDR2-800 has a memory clock speed of 200MHz yet the effective memory transfer speed remains the same (800MHZ in this case) because DDR3 transfers the data across the bus at twice the speed of DDR2 and four times the speed of standard DDR, where core memory and transfer bus speeds operate at the same speed. This is done by increasing the memory’s prefetch buffer, the method in which information is gathered before it’s needed.

Increased prefetch

The prefetch buffer of DDR3 has been increased to 8 bits wide as opposed to 4 bits wide of DDR2 and 2 bits wide of standard DDR to produce the same effective transfer speed and bandwidth. Yet it can achieve this with a lower clock speed and therefore less power consumption.

For example, DDR3 only needs a memory clock speed of 100MHz to produce a memory transfer speed of 800MHz whereas DDR2 requires a 200MHz memory clock to achieve the same transfer speed.

This in turn provides plenty of headroom for faster memory and greater bandwidths. The offset for this is higher latency timings but as you’ll see later in this article the increased frequency more than compensates for this.

Energy efficiency

As mentioned above, the refined fabrication process has allowed for a more efficient integrated circuit (IC) module requiring less power. DDR3 adheres to the JEDEC JESD 79-3B standard of 1.575v but must withstand 1.975v. Since DDR2 requires 2.2v to run at 1066 MHz that a 32% saving making DDR3 more energy efficient. It can do this by partially refreshing the memory cells rather than refreshing all the memory.
Fly-by
As I said in the introduction DDR3 isn’t just a DDR2 with higher stock frequencies. DDR2 has hit its physical limitation where data the data integrity and their topology become a problem at higher speeds where signals could no longer remain balanced across the memory platform (“T branch”).

To resolve this issue, DDR3 sees the introduction of “fly-by” data bus technology which uses a single direct link between all DRAM components thus allowing the system to respond quicker by tuning each signal. It removes the mechanical line balancing limitations of DDR2 and replaces it with a signal time delay generated by the memory controller.

Higher frequencies

The faster frequencies of DDR3 allow for higher system processor clock and system bus speeds thus increasing overall system performance.

Capacities increased

Chip capacities from 512Mb to 8 GB enabling maximum memory module size of 16Gb.

Reset Pin

Introduction of Asynchronous reset pin to complete a device reset without interfering with the PC operation.
The new improved technology has meant a big change in the layout and architecture of motherboards which has seen motherboard prices increase significantly.

Identifying DDR3

Although DDR3 shares the same DIMM profile and overall appearance of DDR and DDR2, that’s where the similarity ends. DDR3 DIMMs have 240 pins and are not compatible with DDR or DDR2 slots. They have a difference key notch so you can’t accidently install the wrong modules.

Image
From left to right, SDRAM, DDR, DDR2 and far right DDR3 - Notice none of the alignment notches are the same.

RAM is normally identified in two ways and either is correct:

DDR3-xxxxx is the standard name where xxxxx denoted the quoted transfer speed (effective)
PC3-xxxx is the module name where xxxxx denoted the theoretical bandwidth which is rounded up or down.
So, for example PC3-6400 is sometimes referred to as DDR3-800 and visa versa. Both are correct.

The following table shows the available DDR3 models with their names, speeds and their bandwidths:

Image

There are various interim variants of the above table.

Understanding the bandwidths

With data transferred at 64 bits per module DDR3 gives a transfer rate (memory clock rate) × 4 (for bus clock multiplier) × 2 (for data rate) × 64 (number of bits transferred) / 8 (number of bits/byte).

For example, a system with a bus speed of 133MHz requires memory to run at 533MHz or 1066MHz effective. (133MHz x 4 = 533MHz) x 2 = 1066MHz
DDR3-1066 runs at a frequency of 8500MHz (1066MHz x 8 bits). Thus DDR3-1066 is also referred to its bandwidth. In this case PC3-8500.

Selecting the right DDR3 RAM for your system

Choosing DDR3 really depends if you intend to overclock your CPU and or bus speeds or not. If you aren’t thinking of overclocking then the selection is quite simple. PC3-10600 which runs at 1333 MHz (or DDR3-1333) is more than adequate for both Intel and AMD systems.

Today’s platforms allow memory to run much faster than the bus speed by default which means you can install faster memory for an increased performance boost with no overclocking. This is called Asynchronous operation. It’s not a new thing by any means but it appears that the days of Synchronous memory are long gone.

Not all has changed though. The speed of the RAM is still determined by the system bus speed (Intel’s Base Clock or AMD HyperTransport) but uses a memory multiplier or memory straps to increase the memory speed whilst maintaining the bus speed.
When matching the correct DDR3 to your CPU, you need to choose RAM that operates to at least the CPU bus speed.

AMD

An AMD Phenom CPU for example has a default HyperTransport (HTT) frequency of 200MHz and a memory ratio (or strap) of 6x. This results in a memory frequency of 1,200MHz requiring at least DDR-1333.

Intel

The Intel equivalent of the HTT is Base Clock (BCLK) which uses various multipliers depending on the CPU you are buying. A CPU that has a BCLK speed of 133.3MHZ with a default memory strap of 10 will result in a memory frequency of 1,333MHz, Spot on DDR-1333.
As you can see in both of the examples above, more so the Intel, that there is little headroom for increasing the bus or memory speeds beyond the default specifications.

For example, let’s say you want to increase the speed of your Core i7 920 to 3.6 GHz which currently runs at 2.67 GHz. This will require a BCLK of 180 MHz since 3.6 GHz / 20 (CPU multiplier) = 180 GHz. With a memory lowered to 8, the memory frequency would run at 1440 MHz requiring at least DDR3-1600 or PC3-12800.

If you have any intensions to overclock or use the automatic overclocking features such as Intel's Turboboost, then PC3-12800 appears to be the best bet.

Voltages

Most modern processors, such as core i3/i5 and i7 CPUs, come with integrated memory controllers as opposed to older processors where the controller was located on the motherboard. This makes them susceptible to damage when running with earlier versions of DDR3 which ran at 1.9v. As a result, Intel imposed a cap of 1.65v for the more sensitive motherboards to prevent damage to the CPU. The more dynamic AMD processors seem to be happy with either.

Latencies

The different revisions of RAM have seen an increase on the latency i.e. the time it takes memory to refresh the data column and rows. However, as you cannot extend the RAM frequency without increasing the memory latencies. The higher frequency of DDR3 more than compensates for the higher latency of which DDR3. Therefore, the issue whether latencies are indeed important anymore remain debatable since the I/O bus clock are much shorter and the time taken is similar.

As far as timings are concerned, the JEDEC specifications for DDR3 are 7-7-7-20 for DDR3-1066 and 7-7-7-24 for DDR3-1333

Dual Channel and Triple Channel Memory

Dual Channel memory and triple channel memory theoretically doubles or trebles the data bandwidth respectively. It does this by spreading data across two or three modules in an alternating pattern meaning that each module is accessed sequentially (Interleaving). So DDR3-1066 in dual channel mode effectively produces 17.06GB/s of bandwidth (2 x 8530GB/s) and triple channel effectively produces 25.6GB/s of bandwidth (3 x 8530GB/s).

Dual channel memory is activated when each dual-channel slot is occupied using identical capacity and speed RAM. This means you will need a dual-channel memory kit which provides modules in even multiples, for example, 4 x 1GB or 2 x 2GB and so on.

Likewise, triple channel memory is activated when all three-channel slots are slots are occupied using identical capacity and speed RAM. This means you will need a triple channel memory kit which provides modules in multiples of 3GBs. 3 x 1GB or 3 x 2GB modules and so on.

Presently, at time of writing, only the LGA-1366 platform such as the Intel Bloomfield Cores (i7-920 and i930) and X58 chipset support triple channel memory.
It is of some debate whether the additional bandwidth is of any benefit to gaming or the average PC user as it only seems to favour heavy core duty processing such as video encoding. This is something you need to consider when buying your new system because the outlay for the platform let alone the additional memory costs may be expensive for little if any notable gain.

XMP

XMP (eXteme memory Profile) is an expansion to DDR3 and provides the BIOS with multiple optimal settings in different profiles, unlike SPD (Serial Presence Detect), which normally provides a single setting.

XMP is basically an “Automatic Overclocking” tool designed to optimize your system memory and allows novices to squeeze extra performance from their Intel based systems. Each profile holds values such as DRAM data rate, basic timings, memory bus voltage etc via 2-3 switches in BIOS. You can use an XMP profile for gaming, video encoding or just simple web-browsing.

You can read more here http://www.intel.com/Consumer/Game/extreme-memory.htm

It's quite common for users of XMXMP memory not to see their system running at the quoted specs. For example, Corsair XMS3 PC3-12800C should run at 1600MHz but shows as 1066MHZ. Enabling XMP in the BIOS will invoke the correct settings.

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