Ovonic Unified Memory

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Ovonic Unified Memory
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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.

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• While CD and DVD drives read and write ovonic
  material with lasers, OUM uses electric current
  to change the phase of memory cells.

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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 cameras,scan devices and
  flash drives.

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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.

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How chalcogenide glass switches states?

• It is heated to a high temperature (over 600C),
  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

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A schematic drawing of two PRAM cells at
different states.
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Baby step image of PRAM
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A typical PRAM
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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).

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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,

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• PRAM exhibits higher resistance to radiation.
• PRAM requires 20 percent fewer process steps
  than those for the manufacturing of NOR flash,
  making it cheaper to produce

• Flash devices trap electrons to store
  information, they are susceptible to data
  corruption from radiation, making them unsuitable
  for many space and military applications .
• More processing steps to fabricate a flash memory

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Where can PRAMs 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 108, which will allow
  it to be a contender for replacing PROMsand
  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 .

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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.

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Reference

• www.BAEsystems.com
• www.samsung.com
• www.techworld.com