Flash memory

Flash memory is a non-volatile electronic computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named after the NOR and NAND logic gates. It allows the reading and writing of multiple memory locations in the same operation. Thanks to this, the flash technology, through electrical impulses, allows higher operating speeds compared to the original EEPROM technology, which only allowed acting on a single memory cell in each programming operation.

This is the technology used in USB flash drives, solid state drives and current BIOSes.

History

Common storage devices, used to transport computer data.

The origins of flash memory can be traced back to the development of the Floating Gate MOSFET (FGMOS), also known as a Floating Gate Transistor. The original MOSFET (Metal Oxide Semiconductor Field Effect Transistor), also known as a MOS transistor, was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959. Kahng went on to develop a variation, the floating gate MOSFET, with Chinese engineer Simon Min Sze at Bell Labs in 1967. The history of flash memory has always been closely linked to the advancement of the rest of the technologies to which it provides its services such as routers, modems, PC BIOS, wireless, etc.

Invention and commercialization

In 1984, it was Fujio Masuoka who invented this type of memory as an evolution of the existing EEPROMs at that time, while working for Toshiba, he proposed a new type of floating gate memory that allowed entire sections of memory to be erased in a way quickly and easily, by applying a voltage to a single wire connected to a group of cells. This led to the invention of flash memory by Masuoka at Toshiba in 1980. According to Toshiba, the name "flash" it was suggested by Masuoka's colleague, Shōji Ariizumi, because the process of erasing the memory contents reminded him of a camera flash. Masuoka and his colleagues presented the invention of NOR flash in 1984 and then NAND flash at the IEEE in 1987 by the conference called the International Electron Devices Meeting (IEDM) held in San Francisco. Intel tried to claim the creation of this without success, although it did market the first flash memory in common use. In the peripherals are: fax-Modem, network card. Between the years 1994 and 1998, the main types of memory known were developed, such as SmartMedia or CompactFlash. The technology soon raised applications in other fields. In 1998, the Rio company marketed the first digital audio player without moving parts, taking advantage of the flash memory operating mode. This product would inaugurate a new class of players that would cause a revolution in the music industry leading to the Napster scandal, the launch of the iPod, and the eventual replacement of CD and tape players.

In 1994, SanDisk began marketing memory cards (CompactFlash) based on these circuits, and since then evolution has reached small handheld consumer electronics devices such as portable MP3 players, memory cards for game consoles and mobile phones, storage capacity for PC Cards that allow connection to wireless networks and a long etcetera, even reaching space aeronautics.

General information

Smart card, microSD card and three different USB sticks.

Economically speaking, the price in the market complies with Moore's law by increasing its capacity and decreasing the price.

Some of its advantages are high shock resistance, faster access times, low power consumption, and quiet operation, since it contains no mechanical actuators or moving parts compared to a conventional hard drive. Its small size is also a determining factor when choosing a portable device, as well as its lightness and versatility for all the uses for which it is oriented. In view of this, SSD drives that use flash memory instead of platters are beginning to become popular.

However, all types of flash memory only allow a limited number of writes and erases, typically between 10,000 and a million, depending on the cell, the precision of the manufacturing process, and the voltage required to erase it. In addition, its cost-capacity ratio is less favorable compared to other media such as optical discs and hard drives.

This type of memory is made with NOR and NAND logic gates to store the corresponding zeros (0) or ones (1).

The file systems for these memories are in full development, although they are already working, such as JFFS originally for NOR, evolved to JFFS2 to also support NAND or YAFFS, already in its second version, for NAND. However, in practice a FAT file system is used for compatibility, especially on removable memory units.

Another feature has been the thermal resistance of some memory card packages geared towards high-end digital cameras. This allows it to function in extreme temperature conditions such as deserts or glaciers since the supported temperature range ranges from –25 °C to 85 °C.

The most common applications are:

• The USB sticks that, in addition to storage, can include other services such as digital fingerprint reader, FM radio, voice recording and, above all, as MP3 portable players and other audio formats.
• PC Cards (discontinued).
• Flash memory cards that are used to store photos and videos on digital cameras. They are also common on mobile phones and tablets to expand storage capacity.

There are various packaging standards promoted and manufactured by the majority of multinationals dedicated to the production of hardware. The most common today are Secure Digital, Compact Flash and Memory Stick.

Low-level access

Flash, as type of EEPROM that is, contains a matrix of cells with a transistor evolved with two gates at each intersection. Traditionally they only store one bit of information. The new flash memories, also called multilevel cell devices, can store more than one bit per cell by varying the number of electrons they store.

These memories are based on the FAMOS transistor (Floating Gate Avalanche-Injection Metal Oxide Semiconductor) which is essentially an NMOS transistor with an additional (metal oxide based) conductor located or between the control gate (CG – Control Gate) and the source/drain terminals contained in another gate (FG – Floating Gate) or around the FG containing the electrons that store the information.

NOR flash memory

Wired and silicon structure of NOR flash memory.

In NOR flash memories, when the electrons are in the FG (Floating Gate), they modify (practically cancel) the electric field that CG (Gate control) would generate if it were active. In this way, depending on whether the cell is at 1 or 0, the electric field of the cell exists or not. So, when the cell is read by putting a certain voltage on CG, the electric current flows or not depending on the voltage stored in the cell. The presence/absence of current is detected and interpreted as a 1 or a 0, thus reproducing the stored data. In multilevel cell devices, the intensity of the current is detected to control the number of electrons stored in FG and interpret them properly.

To program a NOR type cell (assign a certain value) the current is allowed to pass from the source terminal to the drain terminal, then a high voltage is placed in CG to absorb the electrons and retain them in the electric field that generate. This process is called hot-electron injection. To erase (set to “1”, the natural state of the transistor) the contents of a cell, expel these electrons, the Fowler-Nordheim tunneling technique is used, a quantum-mechanical tunneling process. That is, applying a high enough inverse voltage to the employee to attract the electrons, converting the transistor into an electron gun that allows, by opening the drain terminal, the electrons to leave it. This process is the one that causes the deterioration of the cells, when applying such a high voltage on such a thin conductor.

It is necessary to emphasize that flash memories are subdivided into blocks (sometimes called sectors) and therefore, for erasure, entire blocks are cleaned to speed up the process, since it is the slowest part of the process. For this reason, flash memories are much faster than conventional EEPROMs, as they erase byte by byte. However, to rewrite a data it is necessary to clean the block first and then rewrite its content.

NAND flash memory

Wired and silicon structure of NAND flash memory.

Flash memories based on NAND logic gates work slightly differently: they use an injection tunnel for writing and a 'drop' tunnel for erasure. Memories based on NAND have, in addition to the obvious base in other types of gates, a much lower cost, about ten times more resistance to operations, but they only allow sequential access (more oriented to mass storage devices), compared to NOR-based flash memories that allow random access reading. However, it has been NAND that has allowed the expansion of this type of memory, since the erasure mechanism is simpler (although it is also erased by blocks) which has provided a more profitable base for the creation of memory devices. memory card type. The popular USB memories or also called Pendrives, use NAND type flash memories.

Comparison of NOR and NAND flash memories

To compare these types of memory, the different aspects of memories traditionally valued are considered.

• The storage density of chips is currently quite higher in NAND memories.

• The cost of NOR is much higher.

• NOR access is random for reading and block-oriented for modification.

• In the writing of NOR we can modify a single bit. This highlights with the limited reprogramming of NANDs that must modify blocks or full words.

• The reading speed is very higher in NOR (50-100 ns) compared to NAND (10 μs of page search + 50 ns per byte).

• The write rate for NOR is 5 μs per byte compared to 200 μs per page in NAND.

• The erase speed for NOR is 1 ms per block of 64 KB compared to 2 ms per block of 16 KB in NAND.

• The reliability of NOR-based devices is really very high, it is relatively immune to data corruption and does not have erroneous blocks compared to the low reliability of NAND systems that require data correction and there is the possibility that there will be marked blocks such as erroneous and useless.

In summary, NAND-based systems are cheaper but lack the reliability to make them efficient, which demonstrates the imperative need for a good file system. Depending on what you are looking for, it will be worth opting for one type or another.

Standardization

The Open NAND Flash Interface (ONFI) group has developed a standardized low-level interface for NAND memory chips. This enables interoperability between NAND devices from different manufacturers. ONFI version 1.0 was released on December 28, 2006. It states:

• A standard physical interface (pinout) for nand memories.
• A standard command set to read, write and delete nand chips.
• A self-identification mechanism (comparable to the SDRAM memory modules presence detection function).

The ONFI group is supported by most major nand flash memory manufacturers, including Hynix, Intel, Micron Technology, and Numonyx^en), as well as major manufacturers of devices that incorporate nand flash chips. nand flash memory

A group of vendors (including Intel, Dell, and Microsoft) formed the NVM Express (Non-Volatile Memory Host Controller Interface) working group. The goal of the group is to provide standard software and hardware programming interfaces for non-volatile memory subsystems, including the "flash cache" device, attached to the PCI Express bus.

Flash Filesystems

Designing an efficient file system for flash memory has become a dizzying and complex race, because although both are types of flash memory (NOR and NAND), they have very different characteristics when it comes to accessing those files. data. This is because a file system that works with NOR memory incorporates several unnecessary mechanisms for NAND and, in turn, NAND requires additional, unnecessary mechanisms to manage NOR memory. usb keychain

An example might be a "garbage collector". This tool is conditioned by the performance of the delete functions which, in the case of NOR, is very slow and, furthermore, a NOR garbage collector requires a fairly high relative complexity and limits the design options of the file system. Compared to NAND systems, which erase much faster, these limitations don't make sense.

Another major difference between these systems is the use of bad blocks that can exist in NAND but make no sense in NOR systems that guarantee integrity. The size that both systems must handle also differs significantly and therefore is another factor to take into account. These systems should be designed based on the orientation that you want to give to the system.

The two file systems that compete for leadership for the internal organization of flash memories are JFFS (Journaling Flash File System) and YAFFS (Yet Another Flash File System), ExFAT (Extended File Allocation Table) is Microsoft's option.

Background to flash memory

Memories have come a long way since the early days of computing. It is convenient to remember the types of semiconductor memories used as main memory and a few light brushstrokes about each one of them to frame flash memories within their context.

Organizing these types of memory, it is convenient to highlight three categories if we classify them based on the operations that we can perform on them, that is, read-only memories, mostly read memories and read/write memories.

• Reading-only memories.
  • ROM (Read Only Memory): are mainly used in microprogramming systems. Manufacturers often employ them when they produce components in a massive way.
  • PROM (Programmable Read Only Memory): the writing process is electronic. It can then be recorded to the manufacture of the chip, unlike the previous ones recorded during the manufacture. It allows a single recording and is more expensive than the ROM.
• Memories of mostly reading.
  • EPROM (Erasable Programmable Read Only Memory): You can write several times electrically, however, the deletion of the contents is complete and through the exposure to ultraviolet rays (of this they usually have a small ‘ventanite’ on the chip).
  • EEPROM (Electrically Erasable Programmable Read Only Memory): You can selectively delete byte to byte with electric current. It's more expensive than EPROM.
  • Flash memory: is based on EEPROM memories but allows the erased block to block and is cheaper and dense.
• Reading / Writing Memories (RAM).
  • DRAM (Dynamic Random Access Memory): data is stored as in the load of a capacitor. It tends to be downloaded and, therefore, a process of periodic refreshment is necessary. They're simpler and cheaper than SRAMs.
  • SRAM (Static Random Access Memory): the data is stored forming bistables, so it does not require refreshment. Just like DRAM is volatile. They are faster than the DRAM and more faces.

Flash card holder

A flash card reader or memory card reader is a peripheral that reads or writes flash memory. Currently, those installed in computers (included in a motherboard or via USB port), digital frames, DVD readers and other devices, usually read various types of cards.

Future

The future of the world of flash memory is quite bright, as computers and smart embedded appliances tend to become ubiquitous, and therefore the demand for small, cheap and flexible memories will continue to rise until they appear new systems that surpass it in both features and cost. Apparently, this did not seem very feasible, even in the medium term, since the miniaturization and density of flash memories was still far from reaching worrying levels from a physical point of view. But with the appearance of the memristor, the future of flash memories begins to fade.

The development of flash memories is, compared to other types of memory, surprisingly fast in terms of capacity, speed and performance. However, the communication standards of these memories, especially in communication with PCs, are notably lower, which can delay the progress achieved.

The commitment of consumer computing giants, such as AMD and Fujitsu, to form new companies dedicated exclusively to this type of memory, such as Spansion^(en) in July 2003, augur strong investments in research, development and innovation in a market that in 2005 continued to grow and that in 2004 recorded astonishing growth to 15,000 million dollars (after having overcome the technological bubble of the so-called "boom dot com") according to industry analyst Gartner, who endorses all these ideas.

It is curious that this new company, specifically, is turning the tables on speeds with a technique that is as simple in form as it is complex in essence to combine the two types of technologies that reign in the world of memories flash in such a short time. Undoubtedly, many efforts of all kinds are being invested on this point.

However, flash memory will continue to be strongly specialized, taking advantage of the characteristics of each type of memory for specific functions. Let's assume a Harvard Architecture for a small device like a PDA; the instruction memory would be made up of ORNAND-type memory (using second-generation MirrorBit technology) dedicated to system programs, this would offer sustained speeds of up to 150 MB/s in burst mode according to the company with a negligible energy cost and that implements really advanced hardware security; for data memory we could use systems based on high capacity NAND gates at a really affordable price. All that remains is to reduce the consumption of the current powerful PC processors and we would have a very small system with features that today would be the envy of most desktop computers. And there isn't much time left until these systems take, with redoubled effort, the streets.

Any device with critical data will use the technologies based on NOR or ORNAND if we take into account that a failure can render a mobile phone terminal or a medical system useless in an extreme case. However, personal consumer electronics will continue to bet on NAND-based memories due to their immensely low cost and large capacity, such as portable MP3 players or even portable DVD players. The reduction in the voltage used (currently 1.8 V, the lowest), in addition to lower consumption, will allow the useful life of these devices to be significantly extended. However, the new challenges will be the problems that processors suffer today due to their miniaturization and high clock frequencies of the microprocessors.

File systems for flash memories, with projects available via CVS (Concurrent Version System) and open source allow really fast development, as is the case with YAFFS2, which has even achieved several sponsors and there are companies really interested in a project of this magnitude.

Integration with wireless systems will allow favorable conditions for greater integration and ubiquity of digital devices, turning the world around us into the dream of many since the 1980s. But not only that, the Brazilian Space Agency, To cite a space agency, it has already been officially interested in this type of memory to integrate it into its designs; NASA already did it and demonstrated its operation on Mars in Spirit (NASA's rover, Opportunity's twin), where orders were stored incorrectly as you can remember. This is just the beginning. And closer than we think. Intel ensures that 90% of PCs, close to 90% of mobiles, 50% of modems, etc., in 1997 already had this type of memory.

Flash memory as a substitute for hard drives

TDK is currently manufacturing 320Gb or larger NAND flash hard drives with a size similar to a 2½-inch hard drive, similar to laptop hard drives with a speed of 33.3Mb/ s. The problem with this hard drive is that, unlike conventional hard drives, it has a limited number of accesses. Samsung has also developed NAND memories up to 32Gb.

Apple introduced a new version of the MacBook Air laptop computer at an event called 'Back to the Mac' (Back to the Mac) on October 20, 2010, at its headquarters in Cupertino in California (United States). One of the most outstanding features of this new equipment is that it does not have a hard drive, but flash memory, which makes it a faster and lighter machine.

According to David Cuen, a specialist consulted by BBC Mundo, “flash memory is an interesting but risky bet. The question is: is the market ready to ditch hard drives? Apple seems to think so."

The expansion of flash memory is virtually infinite. On January 7, 2013, Kingston released a flash drive (DataTraveler HyperX Predator 3.0) with a maximum capacity of 1TB.

Flash memory as RAM

As of 2012, there are attempts to use flash memory as the main memory of the computer. It is currently slower than conventional DRAM, but it uses up to ten times less power, and is also significantly cheaper. The reference shows a photo of the device that looks like a PCI-Express card, supported by the corresponding driver.

Similarly, the Windows operating system since Windows Vista offers the ReadyBoost feature to use USB flash drives and memory cards as cache to help the computer be faster.

Care

• Change or recharge your device batteries when discharged or lowered: the discharge of batteries is one of the frequent causes of damage and data loss on memory cards. If the battery is finished while the device is saving something on the card, not only could the file be damaged but the entire card.
• Remove the device card correctly: it is important that the device has finished using the card before extracting it. If extracted while in use, the card could be damaged, causing the loss of information.
• Store the cards correctly in your plastic cases: despite being reliable, memory cards can suffer damage by falling on hard surfaces. Keep cards on your cases when you don't use them.
• Static electricity: most of today's cards are manufactured to tolerate static electricity, but strong discharges can cause damage.
• Format: format the cards in a format that the device can handle (usually: FAT for cards up to 2 GB; FAT 32 for cards from 4 to 32 GB; exFAT for larger capabilities). In case of doubts, check the manufacturer's manual. Never remove the card during the formatting process.
• Avoiding water: many cards are reasonably waterproof. For safety, save the left-over cards (and also the device) in a plastic bag by being near the water. And if any card gets wet, make sure it's completely dry before using it. The interiors of the devices are not waterproof.
• X-rays: so far no one has been able to confirm that the X-rays of airports damage memory cards, but just in case, do not carry the cards with you when traveling.
• Postal service: do not send the cards for the postal service, since the revisions with radiation could damage them. Instead, use a private messaging or packing service.
• When entering the card, do not force it: the cards are made to fit only in one way. If introduced in the wrong way and force, the cards or slots of the devices could be damaged. To know how to insert them, check the device manual.
• Heat: Unlike CD, DVD and BD discs, cards are heat resistant. Try not to leave them in a car parked under the sun, but they're possibly doing well. If needs require, consider enhanced cards for extreme conditions.
• Make backups: cards are not perfect and may fail because of the problems shown above. Make backups in different media and even save information on external hard drive or record it on DVD or BD disks to save it in the long term. Do not keep important information only on memory cards.