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.
Continue to page 2