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Goodbye DDR, Hello Serial Memory: DDR4 is the last of the popular DDR line of memories

by Xilinx Employee on ‎07-22-2014 03:34 PM (49,239 Views)

By Tamara I. Schmitz, Director of Memory and Power, Technical Marketing, Xilinx



A seismic shift is shaking up the memory landscape. The cause for this shift is the fact that the line of incredibly popular DDR memories, a fundamental buffer used by 90 percent of Xilinx customers, will end with DDR4. This is not cause for immediate panic—DDR3 has a comfortable address on the majority of system boards (see the figure below showing the RAM usage by Xilinx MIG—Memory Interface Generator—users) and DDR4, though ramping slowly, will replace some of those sockets and serve them for years to come. Still, with the knowledge that DDR4 has no natural successor, customers are eyeing the next crop of memories and mulling over trade-offs such as bandwidth, capacity or power reductions. The likely successor is LPDDR3/4, with certain application spaces preferring serial DRAM solutions such as Hybrid Memory Cube (HMC).



 Percentage of MIG Users.png



To get a handle on these important changes in memories, let’s look first at market trends that are affecting these devices and the limitations that are forcing the end of the DDR empire. Then let’s consider the new class of DDR alternatives, from LPDDR to serial memory, a new concept about which designers will want to stay informed.





Usually when customers are designing for their next generation of products, they look to the next generation of the same memory to give more capacity, speed and throughput. DDR3 enjoys almost 70 percent of the total DRAM market today. Its rise to dominance was assured in the steep 40 percent uptick in adoption between 2009 and 2010. DDR4 has been slower in adoption, partly because of the incursions of Mobile DRAM, also known as LPDDR. DDR4 simply doesn’t have as many sockets to claim if LPDDR is meeting the needs of the wireless market.



DRAM Market Share Trends.png 



Looking at the above graph, DDR4 is indeed picking up momentum, because it does have advantages—namely, a lower supply voltage, which saves power, and higher speed. So it will eventually take over for DDR3 in almost every market, eventually driven by the PC space. Despite the fact that PCs no longer drive >70 percent of DRAM consumption, they are still the largest commodity-device segment. For now, according to memory vendors, DDR4 usage is localized more to the server space rather than in personal electronics segments. Still, DDR4 is an excellent choice for many designs. It is a well-known memory type and will be available for a very long time—in particular because there is no successor.


Will DDR4 completely replace DDR3? Probably not in all cases. Trends show that the server market is adopting DDR4 while the lower cost of DDR3 continues for now to make it the predominant choice in the personal-computing segment. There is no doubt that consumer appetite will continue to grow for more speed as well as more memory capacity, and eventually PCs will migrate to DDR4 down the road.


If there is no DDR5, then what other options are available? The most likely choice to replace DDR3 and DDR4 is LPDDR4. The LP stands for “low power.” Low-Power DDR4 is actually a type of double-data-rate memory that has been optimized for the wireless market. Advantages of LPDDR are that it’s popular, it’s well known, the specs are defined and it’s available. The low-power optimization makes LPDDR4 only a little more expensive than DDR, and it still uses the I/O pins that DDR uses. That makes for ease of migration, because LPDDR4 runs in the same frequency range that DDR runs in.





There are other memory options besides LPDDR vying for the opportunity to be the next memory of choice. Serial memory is emerging as a viable alternative and is a completely different way of looking at the memory space. As far as an FPGA goes, memory is the last frontier, the last section to go serial. The reason for that is latency. The time it takes to turn the data from a parallel stream into serial, sending it down the serial link to then turn it back from serial to parallel, always took too long. Instead of using I/O pins, serial memory leverages SerDes technology. In FPGAs, it is possible to use serial interfacing (transceivers) to run at high rates. More recently, based on the need to reduce latency, vendors have addressed those performance concerns as well.


The takeaway is that DDR3 is here and is very strong, while DDR4 is still in its growth-and-adoption phase. DDR4 will also experience its own staying power that will probably stretch longer than the popular DDR3 simply because it is the last offering in a line of extremely successful memories. LPDDR4 is the most likely candidate to fill the gap, but won’t replace DDR4 in all areas unless there are very rapid read/write iterations. Otherwise, serial memory is the newcomer to watch.


This blog post was adapted from the recent Xcell Journal article titled “Goodbye DDR, Hello Serial Memory.” For much more in-depth technical information about this topic, see this article in the latest issue of Xcell Journal. Click here to read it online or download the PDF.