A closer look at the Samsung 840 EVO mSATA hardware.
The topside of the PCB hosts two 19nm toggle 2 TLC NAND chip
packages manufactured by Samsung. We can also see various support components.
The underside of the PCB hosts the Samsung MEX SSD
controller, another two NAND chip packages, and the cache chip.
The SSD controller
The SSD controller is the Samsung MEX. This controller is
based on an ARM Cortex R4 and has three cores, with a clock frequency of
400MHz, which is clocked 100MHz faster than the MDX controller found on the
original 840 series of SSDs.
The firmware onboard the review sample is EXT0AB0Q.
The NAND is Samsung’s own SSD grade 19nm toggle 2 TLC NAND,
with 400Mbps bandwidth per die. More interesting is that only four NAND chips
are used for the 1000GB capacity of this SSD. Each NAND chip package on the 1TB
model actually has 16 NAND dies stacked together, with each NAND chip package
giving an incredible 256GB of storage. This also creates the possibility of this
type of NAND chip package being used in a 2.5 inch form factor, since it is
possible to house 16 of these packages within a standard 2.5 inch drive
housing. This opens the prospect of having an SSD, sometime in the future, with
a staggering 4TB capacity.
How does TLC NAND differ from MLC NAND?
MLC NAND as we know stores 2 bits of data per cell, meaning
that each cell has four possible states, which all require a different voltage
to access one of these states. The four possible states for MLC NAND are as
- 00 (high voltage)
- 01 (medium high voltage)
- 10 (medium low voltage)
- 11 (low voltage)
From this you may expect TLC NAND to have six possible
states, but this isn’t so. TLC NAND has eight possible states, and requires
eight different voltages to access, erase, and program the NAND, which makes
TLC much more difficult to program. The eight possible states for TLC NAND are
- 000 (highest voltage)
- 001 (high voltage)
- 010 (medium high voltage)
- 100 (high medium voltage)
- 011 (low medium voltage)
- 101 (medium low voltage)
- 110 (low voltage)
- 111 (lowest voltage)
With all these different states and voltages required, not
only is TLC more complex, it is also less durable, and it takes longer to erase
the NAND as well.
Turbo write is a feature found on the Samsung 840 EVO mSATA series
of SSDs. On the 840 EVO mSATA SSDs, a given amount of NAND acts as a high
performance write buffer. On the 120GB, and 250GB models, 9GB of TLC NAND is
set aside for this purpose. The high performance buffer is actually emulated
SLC NAND, therefore the actual size of the SLC buffer is 3GB on the 120GB and
250GB 840 EVO mSATAs.
The 500GB, and 1000GB versions of the 840 EVO mSATA have
6GB, and 12GB of emulated SLC NAND respectively. SLC NAND is much faster to
program compared to TLC NAND, giving much higher performance.
When the 840 EVO mSATA is idle, the Turbo write buffer is
transferred over to the normal NAND, thus freeing the emulated SLC NAND for
further writes. This only takes a minute or two to complete.
There is up to 1GB of LPDDR2 (Low power DDR2) onboard the
840 EVO mSATA SSD, which is clocked at 1066MHz, and once again manufactured by
- 840 EVO mSATA 120GB = 256MB of cache
- 840 EVO mSATA 250GB and 500GB = 512MB of cache
- 840 EVO mSATA 1000GB = 1GB of cache
Dynamic thermal guard
Dynamic thermal guard monitors temperature within the SSD,
and will throttle the SSD controller clock speed if the temperature becomes
dangerously high. This could be very useful when the 840 EVO is installed in a
laptop where ventilation is limited.
- Available capacities: 120GB, 250GB, 500GB, and 1000GB
- Sequential read: Up to 540 MB/s
- Sequential write: Up to 520 MB/s
- 4K random read: Up to 98,000 IOPS
- 4K random write: Up to 90,000 IOPS
- NAND: 19nm toggle 2 TLC
- SSD Controller: Samsung MEX
- Data encryption supported: 256-bit AES, which is
compliant with TCG Opal and IEEE 1667 standards
Let’s head to the next page where we take a look at our
testing methods and the review PC….