Articles: Memory

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DDR3 SDRAM: What’s New?

The title of this section of our review suggests that DDR3 SDRAM is not very much different from the previous generation DDR memory in terms of its design and working principles. In fact, it is true: DDR3 SDRAM is a sort of third reincarnation of DDR SDRAM principles. Therefore, we have every right to compare DDR3 and DDR2 SDRAM side by side here. Moreover, this comparison will hardly take a lot of time.

The frequencies of DDR3 memory could be raised beyond those of DDR2 due to doubling of the data prefetch that was moved from the info storage device to the input/output buffer. While DDR2 SDRAM uses 4-bit samples, DDR3 SDRAM uses 8-bit prefetch also known as 8n-prefetch. In other words, DDR3 SDRAM technology implies doubling of the internal bus width between the actual DRAM core and the input/output buffer. As a result, the increase in the efficient data transfer rate provided by DDR3 SDRAM doesn’t require faster operation of the memory core. Only external buffers start working faster. As for the core frequency of the memory chips, it appears 8 times lower than that of the external memory bus and DDR3 buffers (this frequency was 4 times lower than that of the external bus by DDR2).

So, DDR3 memory can almost immediately hit higher actual frequencies than DDR2 SDRAM, without any modifications or improvements of the semiconductor manufacturing process. However, the above described technique also has another side to it: unfortunately, it increases not only memory bandwidth, but also memory latencies. As a result, we shouldn’t always expect DDR3 SDRAM to work faster than DDR2 SDRAM, even if it operates at higher frequencies than DDR2.

The final DDR3 SDRAM specification released by JEDEC recently describes a few modifications of this memory with frequencies from 800 to 1600MHz. The table below shows the major specifications of the memory modifications listed in the spec:

Considering that the latency of widely spread DDR2-800 SDRAM with 4-4-4 timings equals 10ns, we can really question the efficiency of DDR3 SDRAM at this time. It turns out that the new DDR3 can only win due to higher bandwidth that should make up for worse latency values. Unfortunately, the transition to DDR3 SDRAM is a forced measure to some extent. DDR2 has already exhausted its frequency potential completely. Although we can still push it to 1066MHz with some allowances, further frequency increase lowers the production yields dramatically thus increasing the price of the DDR2 SDRAM modules. That is why JEDEC didn’t standardize DDR2 SDRAM with working frequencies exceeding 800MHz, supporting the transition to DDR3 technology.

However, DDR3 SDRAM offers a few other useful improvements that will encourage not only the manufacturers but also the end users to make up their minds in favor of the new technology. Among these advantages I would like to first of all mention lower voltage of the DDR3 SDRAM modules that dropped down to 1.5V. It is 20% lower than the voltage of DDR2 SDRAM modules, which eventually results into almost 30% reduction in power consumption compared with DDR2 memory working at the same clock speeds. More advanced memory chips manufacturing technologies also contribute to this positive effect.

The BGA chip packaging also underwent a few modifications, and now it features more contact pins. This simplifies the chip mounting procedure and increases mechanical robustness of the ready solutions as well as improves signal quality at high frequencies.

They have also introduced some improvements to the DDR3 SDRAM signal protocol, as the memory bus frequency increased significantly. Now they use fly-by topology with on-module signal termination to transfer addresses, management and stabilization commands. It means that the signals are sent to all chips of the memory modules one by one, and not altogether at the same time.

As a result, the data reading/writing algorithms have also changed. DDR3 controller had to successfully recognize and process time shifts on data receipt from the chips generated by fly-by architecture used for commands transfer. This technique is known as read/write leveling.

Taking into account different voltage and protocols of DDR2 and DDR3 SDRAM, these two memory types are logically incompatible with one another. Although DDR2 and DDR3 memory modules both have 240 pins, they require different DIMM slots: the “notch” location has changed. That is why you will not be able to install DDR3 SDRAM DIMMs into DDR2 slots and the other way around.

Top - DDR3 SDRAM, bottom - DDR2 SDRAM

Yellow and orange slots – for DDR2 SDRAM, 
green - for

I would like to point out one more thing about DDR3 SDRAM. The industry is not only shifting to DDR3 at this time, but they are also beginning to use 1 and 2Gbit memory chips. So, we can expect that 2GB and even 4GB memory modules to become very widely spread in future.

As for the life span of the DDr3 SDRAM in general, analysts believe that the next generation DDR4 SDRAM should come to replace it in 2011 or so.

So, DDR3 SDRAM should last for a little over 3 years in the high-performance market segment, i.e. pretty much the same as DDR2 SDRAM did.

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