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Quarterly Bulletin
 
 
Flash storage
 
 

Form factors, interfaces and technologies

 

The typical NAND Flash storage, in the average industrial computer for the past five years has been a CompactFlash or CF of a couple of hundred megabytes up to one gigabyte. The PATA interface has been predominant and the faster wear out of the Multi-level cell NAND, MLC has made the Single-level cell NAND, SLC type Flash the only option due to its robustness and longer life span.  

 
Lately the number options have increased, both in terms of interfaces and form factors. Advanced on-board flash controllers have made the less costly MLC Flash good enough for some embedded applications. Cost for Flash may be even lower from choosing consumer Flash storage. That is if it the application can somehow handle the fact that the specified number of write cycles for the typical MultiMediaCard, MMC or Secure Digital, SD aren’t impressive.
 
Let’s begin with an overview of form factor options. A handful of new form factors have been added to the two common ones mentioned above. The CFast form factor looks like a CF card. Size is similar and the connector is the same but the SATA communication interface has replaced PATA and the data speed is therefore increased. CFast is an encapsulated module easily attached in and removed from its slot.

The second one on the list of new form factors, mSATA, is a board attached to the slot and fixed with screws to the motherboard or carrier board. The exterior of an mSATA board is similar to that of the PCI Express Mini Card. The interface is SATA. An advantage with mSATA is the small size. The dimensions are 50.80 mm x 29.85 mm and there is even a half-size version.
 

Encapsulated module or board fastened with screws

The slim SATA form factor measures 39 mm x 54 mm which is half the size of a 1.8 inch disc drive. The communication interface and connector is SATA. Slim SATA is, like mSATA, a board fastened with screws to the motherboard or carrier board. It’s a common form factor for tablet PCs using SSD instead of a hard disc drive.


Even more robust than the Flash boards fastened with screws are the onboard SSDs in BGA packages. It’s the type of Flash used on the Hectronic Qseven modules like H6049, H6055 and H6059. Like any component they are soldered to the board. Advantages are the small footprint and excellent shock and vibration resistance. Interfaces are PATA or SATA. Pinouts are proprietary but some initiatives have been taken for future standardization.

 

Then there are the MMC and SD form factors common in products on the consumer market such as digital cameras and smartphones. Low-end and mid-range ARM processors typically have interfaces for MMC or SD. The communication interfaces of these two are similar, sometimes even to the extent that one works in the slot for the other. MMC is license free which SD isn’t.

 

Limiting writing to draw advantages from commercial Flash

Production volumes are large and prices are thereby kept low, so low that it’s sometimes even worthwhile to consider MMC or SD for industrial applications.  When discussing Flash in this end of the quality range, a distinction needs to be made.

Flash is sometimes subdivided into three categories; industrial, commercial and consumer grade quality. The last category, consumer Flash products quite simply aren’t recommendable for industrial applications. Data integrity is typically not guaranteed. Problems in this Flash category include that data is nonexistent after a couple of years due to NAND leakage.

 

Commercial grade Flash MMC/SD cards on the other hand offers possibilities also in some industrial applications. A condition is that the application somehow can keep the MMC/SD card lifetime long enough by controlling and limiting writing. Wear out properties of MMC and SD cards are not generally a strongpoint.

 

Thus far about form factors. Next area to cover is communication interfaces. Low-end and mid-range ARM processors typically have interfaces for MMC or SD. Should there be a need for MMC or SD in your X86-based application it’s uncomplicated to add a controller, for instance converting an USB port to MMC/SD. SATA is found more and more in high-end ARM processors and are common in X86 platforms from entry-level to high-end.

 

PATA makes way for SATA

Among X86 platforms SATA has taken over from PATA. Technical limitations in combination with the need for increased data speeds are a couple of important reasons behind the change. Signal integrity is difficult to maintain in PATA’s parallel signaling when data speeds increase. Serial ATA, SATA quite simply offers much faster communication speed.

Apart from form factor and interface there is also the decision to go for Flash storage using the robust SLC technology or the cost-effective MLC ditto. The decision has until now been quite uncomplicated and straightforward. Industrial computers have been reduced to using SLC and in consumer products most often the cost-effective, but less robust, MLC Flash has been used. SLC can store one bit of information per Flash cell whereas MLC pushes the limits a bit and holds two bits.
 
 

Threshold voltage in SLC Flash cell

 
 

Threshold voltage in MLC Flash cell

 
 
Writing to a Flash cell is performed by charging the floating gate of the cell. The cell is read by measuring the voltage level.

The voltage range of the SLC is divided into two states depending on the voltage. Reading results in the logical 1 or the logical 0 depending on if the voltage level is over or under the voltage reference point.

The characteristic properties of the MLC Flash cell comes mainly from the fact that the voltage range of the cell is divided into four states (SLC has two states). The voltage span representing each state is narrower in the MLC cell and the charging and reading of the cell becomes more sensitive to the wear out of the cell.

Characteristic differences between SLC and MLC are found in the table below.
 
Property
High density
Low cost per bit
Operating temperature range
Endurance
Low power consumption
Write/erase speeds
Write/erase endurance
SLC
 
 
X
X
X
X
X
MLC
X
X
(X)
 
 
 
 
 

More and more industrial embedded applications include a graphical user interfaces and media playing possibilities and at the same time the general trend is that operating systems require increased storage. These are two factors driving the evolution towards larger Flash densities.

In large densities the SLC Flash option is quite costly. It’s the price paid for robustness. The less robust, but also less costly MLC Flash, therefore becomes an increasingly interesting option in industrial applications. Furthermore new controller technology tends to, at least to some extent, compensate for the shortcomings of MLC. Nowadays there is also, unlike in the past, MLC Flash for extended operating temperature range.

 

The lowest quality, consumer grade, products typically use a third and less trustworthy Flash cell technology, Triple-level Cell, TLC. Flash using TLC is not recommended for industrial applications for reasons described when discussing MMC and SD cards.

 

Clever controller functionality increases Flash life time

Wear leveling is an example from the list of smartness of the controller. Wear leveling means that writing is evenly distributed over all the cells on the Flash. This is not to prematurely wear out any single cell or block which will make the component as a whole useless.

Another example of the cleverness found in Flash is based on the addition of extra cells. For example a 32 gigabyte Flash storage module may contain 40 gigabyte in actual fact. The Flash controller monitors the cells in use to identify the ones that are close to wearing out and malfunction. Healthy cells from the unused additional 8 gigabytes replace the ones that are in danger of wear out. This functionality prolongs the life time of the Flash component.

Geometries decrease in the production process of Flash (as is the case with semiconductors in general) and storage densities grow. The downside to the reduced dimensions of the micro architecture is that wear out is turned into a bigger problem. An MLC Flash cell may have had a specified life of 10 000 write cycles but nowadays that figure may be decreased to 5000 cycles and in the future even 3000 cycles.
 

Determining Flash life time

Flash suppliers have used to specify the maximum number of write cycles before wear out, or the total number of bytes that are possible to write (TBW). Today it’s often different. Datasheets may describe the technologies in-depth, on a Flash cell level and offer details on wear leveling and error correction methods. Before you receive any figures on wear out and life time however you often need to contribute with information about the application. Flash suppliers may need to know how often the application writes to Flash, the average size of writes  and other details unique to the application in order to determine with any certainty the life time of the Flash component in your particular application.

Decisions on Flash storage have become a bit more complicated now than it has been. On the other hand there are possibilities for large densities, more form factors to choose from and interfaces to match application requirements and budget restrictions. However the challenge is that analysis of the application and adaptations to software and operating system may be necessary not to make mistakes that trigger wear out problems in Flash.
 
Form factor
CompactFlash

Disc On Modue

CFast

mSATA

Slim SATA

Onboard SSD

MultiMediCard

RS-MMC

eMMC

Secure Digital

Mini SD

Micro SD
Dimensions
43 x 36

Varies

43 x 36

50.80x29.85

39 x 54

BGA package

32 x 24

24 x 18

BGA package

32 x 24

21.5 x 20

15 x 11

Technology
SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

SLC/MLC

Interface
PATA
IDE/PATA/SATA/USB
SATA

SATA

SATA

SATA/PATA

Serial interface
Serial interface
Serial interface
Serial interface
Serial interface
Serial interface
Connector
PATA
IDE/PATA/SATA/USB
SATA
PCI Express Mini Card
SATA

Proprietary

MMC slot

MMC slot

-

SD slot

SD slot

SD slot

The table above is an overview over common Flash form factors and their respective dimensions, Flash cell technology, communicaiton interface and connector type.
 
A full Windows operating system using Flash instead of a hard disc drive runs the risk of malfunctioning due to Flash wear out in less than a year. Problems arise from the lack of a write filter. The latest Windows 7 and 8 operating systems however are well adapted to using Flash since it’s growing more common in laptops and desktops. Embedded versions of Linux, Windows XP and CE have and have had the possibility of controlling and minimizing the number of writes to reduce wear out problems in systems based on Flash storage, often referred to as write filters. It’s especially important to avoid “unnecessary” writes such as the operating system caching on the drive.

Nevertheless a reasonable knowledge and control over the writing habits of the software is needed not to turn the use of Flash into a problem over time. Measures may be needed to minimize writing to avoid wear out and prolong the life time of Flash.
 

Utilities to monitor and diagnose Flash cells

Read the small print in the specification of the Flash to avoid unpleasant surprises in the application. Find out what type of error correction methods are used. Some industrial grade Flash modules include functionality that will send a message to the system when there’s a couple months left on the Flash lifetime. The benefit being that visit from service personnel may be planned instead of acute. Flash suppliers may offer utilities and software tools to monitor Flash health status, day by day.

Information on what happens in Flash on power failure is crucial. No one wants Flash that jams and requires the visit from a service technician or even turns the system into an RMA case. The challenge is that information about consequences of power failure is not always found in the datasheet of the Flash storage. Some Flash products use intelligent controller functionality to handle power cuts but test and verification is sometimes necessary to avoid unpleasant surprises.

Some say that Flash will replace hard disc drives completely. Data speed, robustness and growing densities makes Flash look like the perfect overall hard disc replacement. It’s highly unlikely. More and more functionality is relocated to the Internet “cloud”. Huge amounts of data need to be stored in server halls and Flash quite simply isn’t cheap enough and densities aren’t large enough.

 

More likely Flash will strengthen its position as a complement to hard disc drives. The large amounts of data may be stored cost-efficiently in hard discs. Information that needs to be accessed quickly is stored in Flash. There are already today hybrid drives combining Flash SSD with a mechanical drive.

 

The requirements for storage size in the embedded/industrial sector are definitely growing, if not dramatically as is the case in the Internet “cloud”. We’ll probably see more cases where hard disc drives make way for Flash storage. It’s a development driven for instance from reduced Flash prices and larger densities. One or two applications, in let’s say Digital Signage, may still be inclined to use a hard disc. Such an example is when the two requirements for large storage capacities and low cost is combined.

 

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