For today’s media-rich mobile consumer
electronics, NAND Flash has earned the reputation as the
non-volatile memory-of-choice for multimedia and Internet
capability. There are two types of NAND Flash memory architectures:
Single-Level Cell (SLC) and Multi-Level Cell (MLC). SLC NAND Flash
is the higher performing technology that is used to enable streaming
video and Internet capability in handheld electronics such as 3G
cell phones, PDAs, digital still / video cameras and MP3 players,
etc. MLC NAND Flash is appropriate for short-term, price-sensitive
applications, such as voice recorders, where streaming video is not
required.
SLC NAND Flash: Straightforward
Architecture
SLC NAND Flash’s architecture is
straightforward, providing a significant performance advantage over
MLC NAND Flash whose architecture is convoluted. SLC NAND Flash ICs
have one bit of data stored per memory cell and two states: erased
(1) or programmed (0). MLC NAND Flash ICs have two bits of data
stored per memory cell and four states: erased (11), two thirds
(10), one third (01) or programmed (00). The complex architecture of
MLC NAND Flash is a performance disadvantage when compared to SLC
NAND Flash.
SLC NAND Flash Uses Less Energy
With only two states and one bit of data
stored, SLC NAND Flash’s Control Logic is able to conserve energy
when managing the electrical charge during operations. This is why
during the program operation SLC NAND Flash is faster than MLC NAND
Flash.
Since MLC NAND Flash has four states, and two bits of
data stored, it must expend more energy to manage the electrical
charge during operations. Applying higher voltage to program MLC
NAND Flash degrades its memory cell characteristics. And with
smaller margins, the chances of an incorrect reading are higher.
Plus, due to leakage and other effects, the logic level of the cell
may change. MLC NAND Flash must compensate with enhanced EDC (Error
Detection and Correction).
Analogy : A Glass of Water
This glass of water analogy demonstrates
how SLC NAND Flash outperforms MLC NAND Flash. SLC NAND Flash has
only two states: erased (empty) or programmed (full). MLC NAND Flash
has four states: erased (empty), 1/3, 2/3, and programmed (full).
It’s easier to read the correct fill status when a glass is either
empty or full, as in SLC NAND Flash. When a glass is partially full,
as in MLC NAND Flash, the fill status is more difficult to read,
taking more time and energy.
The SLC NAND Flash Performance Advantage
The Write Data Rate of SLC is three to six
times faster that of MLC, as shown
SLC NAND Flash: Durability
The program and erase operations of SLC
NAND Flash last 100,000 cycles - ten times longer than MLC NAND
Flash, whose program and erase operations only last 10,000 cycles.
After 10,000 cycles, the reliability of MLC NAND Flash’s program and
erase operations is not guaranteed.
Analogy : Different Types of Tires
The endurance of SLC NAND Flash versus MLC
NAND Flash is like the difference between a lower grade(thin) tire
and a higher grade (thicker) tire. This means that SLC NAND Flash
has a significant longer life span than MLC NAND Flash.
SLC NAND Flash: Partial Programming
SLC NAND Flash allows for partial page
programming, while MLC NAND Flash does not. For instance, page size
is usually 2KBytes. With SLC NAND Flash, 512Byte sectors can be
written all at once, saving space. When programming partial data
with MLC NAND Flash, the whole 2KByte page must be used.
SLC NAND Flash optimizes information
transfer and data flow, since all the pipes which are
program-to-program communications, are the same. However, with MLC
NAND Flash, the pipes are different and the information transfer and
data flow can be uneven and slow.
SLC NAND Flash: A Generation Ahead
SLC NAND Flash is a technology generation
ahead of MLC NAND Flash, enabling the same density using smaller
geometry. Currently, there is a six to nine month time gap between
the two technologies. And, as SLC NAND Flash technology moves to
smaller geometry, this time gap becomes larger.
