Every processor needs storage. A few systems may get by with a single type, but more often a hierarchy of technologies is employed, such as a server with a redundant array of disks (RAID) storage system (Fig. 1). Each component brings something to the system, be it high capacity, fast access, or nonvolatility.
The mix is changing, too, as new technologies come online and existing technologies change due to improvements. What was once unheard of is now commonplace. For example, small netbooks are available with only solid-state memory, leading to better battery life due to lower power requirements.
The drive toward higher capacities requires tradeoffs and different approaches to implementation. For instance, multilevel-cell (MLC) flash memory is delivering higher capacities than singlelevel- cell (SLC) flash memory, but at a price in terms of performance and hardware lifetime.
Likewise, hard drives are shrinking. The 2.5-in. drive is a far cry from the full-height, 5.25-in. hard drive of old, but the smaller drivers have a higher capacity and faster response time. There also is wider use of RAID to provide more reliable storage as well as smaller and more scalable solutions.
FORGET ME, NOT Random access memory (RAM) is the centerpoint of commercial computing devices. These days, it’s typically volatile storage, including implementations such as static RAM (SRAM) and dynamic RAM (DRAM) replacing magnetic core memory that was nonvolatile. Also, new technologies like ferroelectric RAM (FRAM) and magnetic RAM (MRAM) look to bring this feature back into the fold.
Standalone SRAM chips are still used, but most SRAM is typically found on-chip as part of microcontrollers providing functions ranging from register files to multilevel cache. Its primary features include high performance. The downsides tend to be chip real estate and higher power requirements.
DRAM is where things get more interesting and varied. On-chip DRAM is becoming more common, although the differing semiconductor technologies for DRAM and logic have tended to keep the two on separate chips. Furthermore, DRAM’s higher capacity tends to move it outside the processor chip. As a result, designers can choose how much capacity to provide, or end users can even add their own memory.
Embedded designers have a number of other challenges when choosing DRAM. That’s because the microprocessors in use have a wide range of performance characteristics, as does DRAM. Embedded designers also need to consider product lifetime, whereas PC users tend to chase the latest and greatest and lowest cost per bit when it comes to memory. The move to virtualization is pushing for ever-higher densities, fulfilling the adage that there is no such thing as enough memory.
At the low end resides the venerable but still highly utilized synchronous DRAM (SDRAM), at least in embedded applications. SDRAM has been available and inexpensive, and a major benefit these days is its easy interface requirements. Its slower speed compared to DDR2 and DDR3, used on the bulk of PC-based systems, is an advantage to designers, especially when trying to mate it with slower processors, relatively speaking. The downside is capacity and efficiency compared to DDR2 and DDR3.
Another problem microprocessor designers are running into is speed. Pushing the upper speed bound usually means dragging the lower bound up the scale. This isn’t a problem when dealing with the latest x86 gigahertz multicore processors from the likes of AMD, Intel, and VIA, but becomes so when trying to support 200-MHz processors.
Of course, the processor clock could be sped up with a corresponding increase in cost and power requirements. These two factors are definitely not on the list of preferable features. Almost any microcontroller could handle SDRAM. Some can handle DDR2, and few can handle DDR3’s higher speeds.
DDR2 is the commodity king. It handles the bulk of server, PC, and laptop systems, but those are quickly moving toward DDR3. Still, DDR2 will be the darling of embedded systems for some time to come even as its availability begins to fall and prices begin to climb. This won’t happen overnight, but it’s trending in that direction. The challenge in the embedded market is meeting DDR2’s performance requirements for lower-end micros.
Samsung’s new 16-Gbyte DDR3 memory targets server motherboards designed to handle only DDR3 memory (Fig. 2). When using these new modules, a server motherboard can host up to 192 Gbytes of DDR3 memory at transfer rates up to 1333 Mbits/s with a 60% power consumption improvement over DDR2. Most higher- end motherboards have chip sets that handle DDR2 or DDR3. DDR3-only chip sets are typically smaller and more efficient.
On the horizon, Innovative Silicon’s Z-RAM single-transistor memory technology is supposed to be more scalable and areaefficient than existing DRAM technologies. Hynix and AMD have licensed Z-RAM technology, but for different purposes. Hynix may incorporate it into mainline memory, while AMD is looking at large on-chip L3 caches. Z-RAM likely won’t show up for another year or so, but it could significantly impact the market when it arrives.
An interface on the horizon is serial port memory. Designed to bring high-speed serial interfaces to memory, it’s sponsored by the Serial Port Memory Technology Working Group. It will, in theory, cut the number of pins needed for memory by 40% and deliver 3.2- to 12.6-Gbyte/s throughput. It initially addresses multimedia mobile devices where real estate is a premium and power must be minimized.
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