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reduced by removing the external address and data bus circuitry. Instead, external devices could communicate with NAND flash via sequential-accessed command and data registers, which would internally retrieve and output the necessary data. This design choice made random-access of NAND flash memory impossible, but the goal of NAND flash was to replace hard disks, not to replace ROMs.

Write Endurance

The write endurance of SLC Floating Gate NOR flash is typically equal or greater than that of NAND flash, while MLC NOR & NAND Flash have similar Endurance capabilities. Example Endurance cycle ratings listed in datasheets for NAND and NOR Flash are provided.

  • SLC NAND Flash is typically rated at about 100K cycles (Samsung OneNAND KFW4G16Q2M)
  • MLC NAND Flash is typically rated at about 5K-10K cycles (Samsung K9G8G08U0M)
  • SLC Floating Gate NOR Flash has typical Endurance rating of 100K to 1,000K cycles (Numonyx M58BW 100K; Spansion S29CD016J 1000K)
  • MLC Floating Gate NOR Flash has typical Endurance rating of 100K cycles (Numonyx J3 Flash)

However, by applying certain algorithms and design paradigms such as wear leveling and memory over-provisioning, the endurance of a storage system can be tuned to serve specific requirements.[1]

Flash file systems

Because of the particular characteristics of flash memory, it is best used with either a controller to perform wear leveling and error correction or specifically designed flash file systems, which spread writes over the media and deal with the long erase times of NOR flash blocks[citation needed]. The basic concept behind flash file systems is: When the flash store is to be updated, the file system will write a new copy of the changed data to a fresh block, remap the file pointers, then erase the old block later when it has time.

In practice, flash file systems are only used for "Memory Technology Devices" ("MTD"), which are embedded flash memories that do not have a controller. Removable flash memory cards and USB flash drives have built-in controllers to perform wear leveling and error correction so use of a specific flash file system does not add any benefit[citation needed].

Capacity

Multiple chips are often arrayed to achieve higher capacities for use in consumer electronic devices such as multimedia players or GPS. The capacity of flash chips generally follows Moore's Law because they are manufactured with many of the same integrated circuits techniques and equipment.

Consumer flash drives typically have sizes measured in powers of two (e.g. 512 MB, 8 GB). This includes SSDs as hard drive replacements[citation needed], even though traditional hard drives tend to use decimal units. Thus, a 64 GB SSD is actually 64×1024³ bytes. In reality, most users will have slightly less capacity than this available, due to the space taken by filesystem metadata.

In 2005, Toshiba and SanDisk developed a NAND flash chip capable of storing 1 GB of data using Multi-level Cell (MLC) technology, capable of storing two bits of data per cell. In September 2005, Samsung Electronics announced that it had developed the world’s first 2 GB chip.[2]

In March 2006, Samsung announced flash hard drives with a capacity of 4 GB, essentially the same order of magnitude as smaller laptop hard drives, and in September 2006, Samsung announced an 8 GB chip produced using a 40-nm manufacturing process.[3]

In January 2008, Sandisk announced availability of their 16 GB MicroSDHC and 32 GB SDHC Plus cards.[4][5]

In 2009, Kingston announced a 256 GB Flash Drive available only in the UK and other parts of Europe. As of 2010, however, it is available in the USA.

There are still flash-chips manufactured with capacities under or around 1 MB, e.g., for BIOS-ROMs and embedded applications.

Transfer rates

NAND flash memory cards are much faster at reading than writing so it is the maximum read speed that is commonly advertised. As a chip wears out, its erase/program operations slow down considerably[citation needed], requiring more retries and bad block remapping. Transferring multiple small files, each smaller than the chip-specific block size, could lead to much a lower rate. Access latency also influences performance, but less so than with their hard drive counterpart.

The speed is sometimes quoted in MB/s (megabytes per second), or as a multiple of that of a legacy single speed CD-ROM, such as 60x, 100x or 150x. Here 1x is equivalent to 150 KB per second. For example, a 100x memory card gives 150 KB x 100 = 15,000KB/s = 14.65 MB/s.

Performance also depends on the quality of memory controllers. Even when the only change to manufacturing is die-shrink, the absence of an appropriate controller can result in degraded speeds.[6]

Applications

Serial flash

Serial flash is a small, low-power flash memory that uses a serial interface, typically SPI, for sequential data access. When incorporated into an embedded system, serial flash requires fewer wires on the PCB than parallel flash memories, since it transmits and receives data one bit at a time. This may permit a reduction in board space, power consumption, and total system cost.

There are several reasons why a serial device, with fewer external pins than a parallel device, can significantly reduce overall cost:

  • Many ASICs are pad-limited, meaning that the size of the die is constrained by the number of wire bond pads, rather than the complexity and number of gates used for the device logic. Eliminating bond pads thus permits a more compact integrated circuit, on a smaller die; this increases the number of dies that may be fabricated on a wafer, and thus reduces the cost per die.
  • Reducing the number of external pins also reduces assembly and packaging costs. A serial device may be packaged in a smaller and simpler package than a parallel device.
  • Smaller and lower pin-count packages occupy less PCB area.
  • Lower pin-count devices simplify PCB routing.

Firmware storage

With the increasing speed of modern CPUs, parallel flash devices are often much slower than the memory bus of the computer they are connected to. Conversely, modern SRAM offers access times below 10 ns, while DDR2 SDRAM offers access times below 20 ns. Because of this, it is often desirable to shadow code stored in flash into RAM; that is, the code is copied from flash into RAM before execution, so that the CPU may access it at full speed. Device firmware may be stored in a serial flash device, and then copied into SDRAM or SRAM when the device is powered-up.[7] Using an external serial flash device rather than on-chip flash removes the need for significant process compromise (a process that is good for high speed logic is generally not good for flash and vice-versa). Once it is decided to read the firmware in as one big block it is common to add compression to allow a smaller flash chip to be used. Typical applications for serial flash include storing firmware for hard drives, Ethernet controllers, DSL modems, wireless network devices, etc.

Flash memory as a replacement for hard drives


An obvious extension of flash memory would be as a replacement for hard disks. Flash memory does not have the mechanical limitations and latencies of hard drives, so the idea of a solid-state drive, or SSD, is attractive when considering speed, noise, power consumption, and reliability. Flash drives are considered serious candidates for mobile device secondary storage; they are not yet competitors for hard drives in desktop computers or servers with RAID and SAN architectures.

There remain some aspects of flash-based SSDs that make the idea unattractive. Most important, the cost per gigabyte of flash memory remains significantly higher than that of platter-based hard drives. Although this ratio is decreasing rapidly for flash memory, it is not yet clear that flash memory will catch up to the capacities and affordability offered by platter-based storage. Still, research and development is sufficiently vigorous that it is not clear that it will not happen, either.

There is also some concern that the finite number of P/E cycles of flash memory would render flash memory unable to support an operating system. This seems to be a decreasing issue as warranties on flash-based SSDs are approaching those of current hard drives.[8][9]

In June 2006, Samsung Electronics released the first flash-memory based PCs, the Q1-SSD and Q30-SSD, both of which used 32 GB SSDs, and were at least initially available only in South Korea.[10] Dell Computer introduced a 32GB SSD option on its Latitude D420 and D620 ATG laptops in April 2007—at $549 more than a hard-drive equipped version.[11]

At the Las Vegas CES 2007 Summit Taiwanese memory company A-DATA showcased SSD hard disk drives based on Flash technology in capacities of 32 GB, 64 GB and 128 GB.[12] Sandisk announced an OEM 32 GB 1.8" SSD drive at CES 2007.[13] The XO-1, developed by the One Laptop Per Child (OLPC) association, uses flash memory rather than a hard drive. As of March 2009, a Salt Lake City company called Fusion-io claims the fastest SSD with sequential read/write speeds of 1500 MB/1400 MB's per second.[14]

Rather than entirely replacing the hard drive, hybrid techniques such as hybrid drive and ReadyBoost attempt to combine the advantages of both technologies, using flash as a high-speed cache for files on the disk that are often referenced, but rarely modified, such as application and operating system executable files. Also, Addonics has a PCI adapter for four CF cards,[15] creating a RAID-able array of solid-state storage that is much cheaper than the hardwired-chips PCI card kind.

Early versions of the ASUS Eee PC used a flash-based SSD of 2 GB to 20 GB, depending on model, although later versions of the machine use conventional hard disks. The Apple Inc. Macbook Air has the option to upgrade the standard hard drive to a 128 GB Solid State hard drive. The Lenovo ThinkPad X300 also features a built-in 64 GB Solid State Drive. The Apple iPad has flash-based SSD's of 16, 32, and 64 GB.

Sharkoon has developed a device that uses six SDHC cards in RAID-0 as an SSD alternative; users may use more affordable High-Speed 8 GB SDHC cards to get similar or better results than can be obtained from traditional SSDs at a lower cost.

Industry

One source states that, in 2008, the flash memory industry includes about US$9.1 billion in production and sales. Apple Inc. is the third largest purchaser of flash memory, consuming about 13% of production by itself.[16] Other sources put the flash memory market at a size of more than US$20 billion in 2006, accounting for more than eight percent of the overall semiconductor market and more than 34 percent of the total semiconductor memory market.[17]

Flash scalability

File:NAND scaling timeline.png

Due to its relatively simple structure and high demand for higher capacity, NAND flash memory is the most aggressively scaled technology among electronic devices. The heavy competition among the top few manufacturers only adds to the aggressiveness. Current projections show the technology to reach approximately 20 nm by around late 2011. While the expected shrink timeline is a factor of two every three years per original version of Moore's law, this has recently been accelerated in the case of NAND flash to a factor of two every two years.

As the feature size of flash memory cells reach the minimum limit (currently estimated ~20 nm), further Flash density increases will be driven by greater levels of MLC, possibly 3-D stacking of transistors, and process improvements. Even with these advances, it may be impossible to economically scale Flash to smaller and smaller dimensions. Many promising new technologies (such as FeRAM, MRAM, PMC, PCM, and others) are under investigation and development as possible more scalable replacements for Flash.[18]

See also

References

  1. [1] Western Digital White Paper describing calculation and effects of SSD endurance
  2. Shilov, Anton (September 12, 2005). "Samsung Unveils 2GB Flash Memory Chip". X-bit labs. http://www.xbitlabs.com/news/memory/display/20050912212649.html. Retrieved 2008-11-30. 
  3. Gruener, Wolfgang (September 11, 2006). "Samsung announces 40 nm Flash, predicts 20 nm devices". TG Daily. http://www.tgdaily.com/content/view/28504/135/. Retrieved 2008-11-30. 
  4. 12 GB MicroSDHC
  5. 32 GB SDHC Plus
  6. Samsung Confirms 32nm Flash Problems, Working on New SSD Controller
  7. Many serial flash devices implement a bulk read mode and incorporate an internal address counter, so that it is trivial to configure them to transfer their entire contents to RAM on power-up. When clocked at 50 MHz, for example, a serial flash could transfer a 64 Mbit firmware image in less than two seconds.
  8. "Flash Memory vs. HDD - Who Will Win?". STORAGEsearch. http://www.storagesearch.com/semico-art1.html. Retrieved 2008-11-30. 
  9. "Flash Solid State Disks - Inferior Technology or Closet Superstar?". STORAGEsearch. http://www.storagesearch.com/bitmicro-art1.html. Retrieved 2008-11-30. 
  10. "Samsung Electronics Launches the World’s First PCs with NAND Flash-based Solid State Disk". Press Release. Samsung. May 24, 2006. http://www.samsung.com/he/presscenter/pressrelease/pressrelease_20060524_0000257996.asp. Retrieved 2008-11-30. 
  11. "Dell joins the fray, offers SSD in Latitude D420, D620". Engadget. April 24, 2007. http://www.engadget.com/2007/04/24/dell-joins-the-fray-offers-ssd-in-latitude-d420-d620/. Retrieved 2009-10-27. 
  12. "Future of Flash revealed". http://www.theinquirer.net/default.aspx?article=36841. 
  13. "SanDisk SSD Solid State Drives". http://www.sandisk.com/Oem/Default.aspx?CatID=1477. 
  14. Fusion-io
  15. "Addonics PCI adapter for 4 CF cards". http://www.addonics.com/products/flash_memory_reader/ad4cfprj.asp. 
  16. Deffree, Suzanne (April 2008). "Apple sneezes, flash industry gets sick". EDN 2008 (7): 74. http://www.edn.com/index.asp?layout=article&articleid=CA6544754. Retrieved 2008-04-19. 
  17. Yinug, Christopher Falan (July 2007). "The Rise of the Flash Memory Market: Its Impact on Firm Behavior and Global Semiconductor Trade Patterns" (PDF). Journal of International Commerce and Economics. http://www.usitc.gov/journal/Final_falan_article1.pdf. Retrieved 2008-04-19. 
  18. Kim, Kinam; Koh, Gwan-Hyeob (2004-05-16). Future Memory Technology including Emerging New Memories. Serbia and Montenegro: Proceedings of the 24th International Conference on Microelectronics (published 2004-05). pp. 377–384. http://ieeexplore.ieee.org/iel5/9193/29143/01314646.pdf?tp=&isnumber=&arnumber=1314646. Retrieved 2008-08-15. 

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