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What is ovonic unified memory?
Ovonic unified memory(also known as PCM,
PRAM, PCRAM, Phase-change memory and
Chalcogenide RAM C-RAM) is a type of non
volatile memory which uses a unique thin-film
phase change material to store information .
PRAM uses the unique behavior of
Chalcogenide glass, which can be "switched"
between two states, crystalline and amorphous,
with the application of heat .
OUM is based on the information storage
technology that allows rewriting of CD's and
DVD's.
While CD and DVD drives read and write ovonic
material with lasers, OUM uses electric current
to change the phase of memory cells.
What is a non volatile
memory?
Non-volatile memory is the general name used to
describe any type of memory which does not lose its
information when power is turned off.
NVRAM(non volatile random access memory) is a
subgroup of the more general class of non-volatile
memory types, the difference being that NVRAM devices
offer random access, as opposed to sequential access
like hard disks.
The best-known form of NVRAM memory today is
flash memory used in camera’s,scan devices and flash
drives.
Why chalcogenide glasses ?
The crystalline and amorphous states of chalcogenide
glass have dramatically different electrical resistivity
values, and this forms the basis by which data are
stored.
The amorphic, high resistance state is used to
represent a binary 0, and the crystalline, low
resistance state represents a binary 1 .
Chalcogenide is the same material utilized in re-writable
optical media (such as CD-RW and DVD-RW). In those
instances, the material's optical properties are
manipulated, rather than its electrical resistivity, as
chalcogenide's refractive index also changes with the
state of the material.
How chalcogenide glass switches
states?
It is heated to a high temperature (over
600°C), at which point the chalcogenide
becomes a liquid. Once cooled, it is frozen
into an amorphic glass-like state and its
electrical resistance is high.
By heating the chalcogenide to a temperature
above its crystallization point, but below the
melting point, it will transform into a
crystalline state with a much lower
resistance. This phase transition process can
be completed in as quickly as five nanoseconds
A schematic drawing of two
PRAM cells at different
states.
Baby step image of PRAM
A typical PRAM
Structure of FLASH memory
Flash memory works by modulating charge (electron)
stored within the gate of a MOS transistor .
The gate is constructed with a special "stack" designed
to trap charges .
Changing the bit's state requires removing the
accumulated charge, which demands a relatively large
voltage to "suck" the electrons off the floating gate. This
burst of voltage is provided by a charge pump which
takes some time to build up power.
write times for common Flash devices are on the order of
one ms (for a block of data), about 100 000 times the
typical 10 ns read time (for a byte).
PRAM vs FLASH memory
PRAM can offer much higher
performance in applications
where writing quickly is
important, both because the
memory element can be
switched more quickly, and
also because single bits may
be changed to either 1 or 0
without needing to first erase
an entire block of cells which
makes it effectively 30-times
faster than flash memory
A PRAM device may endure
around 100 million write
cycles.
Flash memory erases the
entire data block.
With Flash, each burst of
voltage across the cell causes
degradation, so most flash
devices are only rated for
something on the order of 10
000 to 100 000 writes per
sector,
PRAM exhibits higher
resistance to radiation.
Flash devices trap
electrons to store
information, they are
susceptible to data
corruption from radiation,
making them unsuitable
for many space and
military applications .
PRAM requires 20
percent fewer process
steps than those for the
manufacturing of NOR
flash, making it cheaper
to produce
More processing steps to
fabricate a flash memory
Where can PRAM’s be useful?
PRAM is a promising technology in the military
and aerospace industries where radiation effects
make the use of standard non-volatile memories
such as Flash impractical .
A write cycle endurance of 10^8, which will allow
it to be a contender for replacing PROM’sand
EEPROMs in space systems.
Other potential PRAM applications include much
faster and capacious USB thumb drives and
solid-state disk drives .
Hybrid disk drives and the motherboard with
flash cache, could also have larger and
significantly faster cache memory leading to
better performance.
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PRAM is expected to be especially popular in
the future designs of multi-function handsets
and for other mobile applications .
Fundamental tradeoff:
unintentional vs. intentional
phase-change
PRAM lifetime is limited by mechanisms such as
degradation due to GST thermal expansion
during programming, metal (and other material)
migration, and other mechanisms still unknown.
The contents of a PRAM are lost because of the
high temperatures needed to solder the device
to a board. The manufacturer using PRAM parts
must provide a mechanism to program the
PRAM "in-system" after it has been soldered in
place.
Reference
www.BAEsystems.com
www.samsung.com
www.techworld.com