Flash memory

Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It is primarily used in memory cards, USB flash drives, MP3 players and solid-state drives for general storage and transfer of data between computers and other digital products(SONY VGP-BPS13Q Battery). It is a specific type of EEPROM (electrically erasable programmable read-only memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once(SONY VGP-BPS13B/Q Battery). Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid state storage is needed(SONY VGN NR11S/S battery). Example applications include PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. It has also gained popularity in console video game hardware, where it is often used instead of EEPROMs or battery-powered static RAM (SRAM) for game save data(SONY VGN NR11M/S battery).

Flash memory is non-volatile, meaning no power is needed to maintain the information stored in the chip. In addition, flash memory offers fast read access times (although not as fast as volatile DRAM memory used for main memory in PCs) and better kinetic shock resistance than hard disks(SONY VGN NR11Z/S battery). These characteristics explain the popularity of flash memory in portable devices. Another feature of flash memory is that when packaged in a "memory card," it is extremely durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water(SONY VGN NR11Z/T battery).

Although technically a type of EEPROM, the term "EEPROM" is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes. Because erase cycles are slow(SONY VGP-BPS13A/Q Battery), the large block sizes used in flash memory erasing give it a significant speed advantage over old-style EEPROM when writing large amounts of data(SONY Vaio PCG-F305 battery).

History

Flash memory (both NOR and NAND types) was invented by Dr. Fujio Masuoka while working for Toshiba circa 1980. According to Toshiba, the name "flash" was suggested by Dr. Masuoka's colleague, Mr. Shoji Ariizumi(SONY VGP-BPS21/S Battery), because the erasure process of the memory contents reminded him of the flash of a camera. Dr. Masuoka presented the invention at the IEEE 1984 International Electron Devices Meeting (IEDM) held in San Francisco, California(SONY VGP-BPS21 Battery).

Intel Corporation saw the massive potential of the invention and introduced the first commercial NOR type flash chip in 1988. NOR-based flash has long erase and write times, but provides full address and data buses(SONY VGP-BPS21B Battery), allowing random access to any memory location. This makes it a suitable replacement for older read-only memory (ROM) chips, which are used to store program code that rarely needs to be updated, such as a computer's BIOS or the firmware of set-top boxes(SONY VGP-BPS21A/B Battery). Its endurance is 10,000 to 1,000,000 erase cycles. NOR-based flash was the basis of early flash-based removable media; CompactFlash was originally based on it, though later cards moved to less expensive NAND flash(SONY VAIO PCG-5K1L battery).

Toshiba announced NAND flash at the 1987 International Electron Devices Meeting. It has reduced erase and write times, and requires less chip area per cell, thus allowing greater storage density and lower cost per bit than NOR flash; it also has up to ten times the endurance of NOR flash(SONY VAIO PCG-6W2L battery). However, the I/O interface of NAND flash does not provide a random-access external address bus. Rather, data must be read on a block-wise basis, with typical block sizes of hundreds to thousands of bits. This made NAND flash unsuitable as a drop-in replacement for program ROM since most microprocessors and microcontrollers required byte-level random access(SONY VAIO PCG-7112L battery). In this regard NAND flash is similar to other secondary storage devices such as hard disks and optical media, and is thus very suitable for use in mass-storage devices such as memory cards(SONY VAIO PCG-8Z1L battery). The first NAND-based removable media format was SmartMedia in 1995, and many others have followed, including MultiMediaCard, Secure Digital, Memory Stick and xD-Picture Card. A new generation of memory card formats(SONY VAIO PCG-8Z2L battery), including RS-MMC, miniSD and microSD, and Intelligent Stick, feature extremely small form factors. For example, the microSD card has an area of just over 1.5 cm2, with a thickness of less than 1 mm. microSD capacities range from 64 MB to 32 GB, as of March 2010(SONY VAIO PCG-8Y2L battery).

Principles of operation

Flash memory stores information in an array of memory cells made from floating-gate transistors. In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell (MLC) devices, can store more than one bit per cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells(SONY VAIO PCG-8Y1L battery).

The floating gate may be conductive (typically polysilicon in most kinds of flash memory) or non-conductive (as in SONOS flash memory).

Floating-gate transistor

In flash memory, each memory cell resembles a standard MOSFET, except the transistor has two gates instead of one(SONY VAIO PCG-7Z2L battery). On top is the control gate (CG), as in other MOS transistors, but below this there is a floating gate (FG) insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer(SONY VAIO PCG-7Z1L battery), any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years. When the FG holds a charge, it screens (partially cancels) the electric field from the CG, which modifies the threshold voltage (VT) of the cell. During read-out(SONY VAIO PCG-7133L battery), a voltage intermediate between the possible threshold voltages is applied to the CG, and the MOSFET channel will become conducting or remain insulating, depending on the VT of the cell, which is in turn controlled by charge on the FG(SONY VAIO PCG-7113L battery). The current flow through the MOSFET channel is sensed and forms a binary code, reproducing the stored data. In a multi-level cell device, which stores more than one bit per cell, the amount of current flow is sensed (rather than simply its presence or absence), in order to determine more precisely the level of charge on the FG(SONY VAIO PCG-6W3L battery).

NOR flash

In NOR gate flash, each cell has one end connected directly to ground, and the other end connected directly to a bit line.

This arrangement is called "NOR flash" because it acts like a NOR gate: when one of the word lines is brought high, the corresponding storage transistor acts to pull the output bit line low(SONY VAIO PCG-7111L battery).

Programming

A single-level NOR flash cell in its default state is logically equivalent to a binary "1" value, because current will flow through the channel under application of an appropriate voltage to the control gate. A NOR flash cell can be programmed, or set to a binary "0" value, by the following procedure(SONY VAIO PCG-6W1L battery):

an elevated on-voltage (typically >5 V) is applied to the CG

the channel is now turned on, so electrons can flow from the source to the drain (assuming an NMOS transistor)

the source-drain current is sufficiently high to cause some high energy electrons to jump through the insulating layer onto the FG, via a process called hot-electron injection(SONY VAIO PCG-6V1L battery)

Erasing

To erase a NOR flash cell (resetting it to the "1" state), a large voltage of the opposite polarity is applied between the CG and source, pulling the electrons off the FG through quantum tunneling. Modern NOR flash memory chips are divided into erase segments (often called blocks or sectors) (SONY VAIO PCG-6S3L battery). The erase operation can only be performed on a block-wise basis; all the cells in an erase segment must be erased together. Programming of NOR cells, however, can generally be performed one byte or word at a time(SONY VAIO PCG-6S2L battery).

Internal charge pumps

Despite the need for high programming and erasing voltages, virtually all flash chips today require only a single supply voltage, and produce the high voltages via on-chip charge pumps(SONY VAIO PCG-5J2L battery).

NAND flash

NAND flash also uses floating-gate transistors, but they are connected in a way that resembles a NAND gate: several transistors are connected in series, and only if all word lines are pulled high (above the transistors' VT) is the bit line pulled low. These groups are then connected via some additional transistors to a NOR-style bit line array(SONY VAIO PCG-5L1L battery).

To read, most of the word lines are pulled up above the VT of a programmed bit, while one of them is pulled up to just over the VT of an erased bit. The series group will conduct (and pull the bit line low) if the selected bit has not been programmed(SONY VAIO PCG-5K2L battery).

Despite the additional transistors, the reduction in ground wires and bit lines allows a denser layout and greater storage capacity per chip. In addition, NAND flash is typically permitted to contain a certain number of faults (NOR flash, as is used for a BIOS ROM, is expected to be fault-free) (SONY VAIO PCG-5J1L battery). Manufacturers try to maximize the amount of usable storage by shrinking the size of the transistor below the size where they can be made reliably, to the size where further reductions would increase the number of faults faster than it would increase the total storage available(SONY VAIO PCG-5G3L battery).

NAND flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB storage devices known as USB flash drives, as well as most memory card formats and solid-state drives available today(SONY VAIO PCG-5G2L battery).

Block erasure

One limitation of flash memory is that although it can be read or programmed a byte or a word at a time in a random access fashion, it can only be erased a "block" at a time. This generally sets all bits in the block to 1. Starting with a freshly erased block, any location within that block can be programmed(SONY VGP-BPS13B/S Battery). However, once a bit has been set to 0, only by erasing the entire block can it be changed back to 1. In other words, flash memory (specifically NOR flash) offers random-access read and programming operations, but cannot offer arbitrary random-access rewrite or erase operations(SONY VGP-BPS13S Battery). A location can, however, be rewritten as long as the new value's 0 bits are a superset of the over-written value's. For example, a nibble value may be erased to 1111, then written as 1110. Successive writes to that nibble can change it to 1010, then 0010, and finally 0000(SONY VGP-BPS13AS Battery). Essentially, erasure sets (all) bits, and programming can only clear bits. Filesystems designed for flash devices can make use of this capability to represent sector metadata(SONY VGP-BPS13A/S Battery).

Although data structures in flash memory cannot be updated in completely general ways, this allows members to be "removed" by marking them as invalid. This technique may need to be modified for Multi-level Cell devices, where one memory cell holds more than one bit(Sony VAIO VGN-FZ15G Battery).

Common flash devices such as USB keys and memory cards provide only a block-level interface, or flash translation layer (FTL), which writes to a different cell each time to wear-level the device(Sony VAIO VGN-FZ15T Battery). This prevents incremental writing within a block, however it does help the device from being prematurely worn out by abusive and/or poorly designed hardware/software. For example, nearly all consumer devices ship formatted with MS-FAT file system, which pre-dates flash memory, having been designed for DOS, and disk media(SONY VGP-BPS13/B Battery).

Memory wear

Another limitation is that flash memory has a finite number of program-erase cycles (typically written as P/E cycles). Most commercially available flash products are guaranteed to withstand around 100,000 P/E cycles(Dell N3010 Battery), before the wear begins to deteriorate the integrity of the storage. Micron Technology and Sun Microsystems announced an SLC flash memory chip rated for 1,000,000 P/E cycles on December 17, 2008(Dell INSPIRON 1764 battery).

The guaranteed cycle count may apply only to block zero (as is the case with TSOP NAND parts), or to all blocks (as in NOR). This effect is partially offset in some chip firmware or file system drivers by counting the writes and dynamically remapping blocks in order to spread write operations between sectors; this technique is called wear leveling(Dell INSPIRON 1564 battery). Another approach is to perform write verification and remapping to spare sectors in case of write failure, a technique called Bad Block Management (BBM). For portable consumer devices, these wearout management techniques typically extend the life of the flash memory beyond the life of the device itself, and some data loss may be acceptable in these applications(Dell INSPIRON 1464 battery). For high reliability data storage, however, it is not advisable to use flash memory that would have to go through a large number of programming cycles. This limitation is meaningless for 'read-only' applications such as thin clients and routers, which are only programmed once or at most a few times during their lifetimes(Dell INSPIRON 1520 battery).

he low-level interface to flash memory chips differs from those of other memory types such as DRAM, ROM, and EEPROM, which support bit-alterability (both zero to one and one to zero) and random-access via externally accessible address buses(Dell INSPIRON 1420 battery).

While NOR memory provides an external address bus for read and program operations (and thus supports random-access); unlocking and erasing NOR memory must proceed on a block-by-block basis. With NAND flash memory, read and programming operations must be performed page-at-a-time while unlocking and erasing must happen in block-wise fashion(Dell INSPIRON E1505 battery).

NOR memories

Reading from NOR flash is similar to reading from random-access memory, provided the address and data bus are mapped correctly. Because of this, most microprocessors can use NOR flash memory as execute in place (XIP) memory(Dell INSPIRON 1525 battery), meaning that programs stored in NOR flash can be executed directly from the NOR flash without needing to be copied into RAM first. NOR flash may be programmed in a random-access manner similar to reading(SONY VGP-BPS13B/B Battery). Programming changes bits from a logical one to a zero. Bits that are already zero are left unchanged. Erasure must happen a block at a time, and resets all the bits in the erased block back to one. Typical block sizes are 64, 128, or 256 KB(SONY VGP-BPS13 Battery).

Bad block management is a relatively new feature in NOR chips. In older NOR devices not supporting bad block management, the software or device driver controlling the memory chip must correct for blocks that wear out, or the device will cease to work reliably(HP PAVILION DV6000 battery).

The specific commands used to lock, unlock, program, or erase NOR memories differ for each manufacturer. To avoid needing unique driver software for every device made, a special set of Common Flash Memory Interface (CFI) commands allow the device to identify itself and its critical operating parameters(SONY VAIO VGN-FZ31B Battery).

Apart from being used as random-access ROM, NOR memories can also be used as storage devices by taking advantage of random-access programming(SONY VAIO VGN-FZ31M Battery). Some devices offer read-while-write functionality so that code continues to execute even while a program or erase operation is occurring in the background. For sequential data writes, NOR flash chips typically have slow write speeds compared with NAND flash(SONY VAIO VGN-FZ31J Battery).

NAND memories

NAND flash architecture was introduced by Toshiba in 1989. These memories are accessed much like block devices such as hard disks or memory cards. Each block consists of a number of pages. The pages are typically 512 or 2,048 or 4,096 bytes in size. Associated with each page are a few bytes (typically 1/32 of the data size) that can be used for storage of an error correcting code (ECC) checksum(SONY VAIO VGN-FZ31E Battery).

Typical block sizes include:

32 pages of 512+16 bytes each for a block size of 16 KB

64 pages of 2,048+64 bytes each for a block size of 128 KB

64 pages of 4,096+128 bytes each for a block size of 256 KB

128 pages of 4,096+128 bytes each for a block size of 512 KB(Sony Vaio VGN-FZ31S battery).

While reading and programming is performed on a page basis, erasure can only be performed on a block basis.Another limitation of NAND flash is data in a block can only be written sequentially. Number of Operations (NOPs) is the number of times the sectors can be programmed. So far this number for MLC flash is always one whereas for SLC flash it is four(SONY VAIO VGN-FZ31z Battery).

NAND devices also require bad block management by the device driver software, or by a separate controller chip. SD cards, for example, include controller circuitry to perform bad block management and wear leveling(SONY VAIO VGN-FZ38M Battery). When a logical block is accessed by high-level software, it is mapped to a physical block by the device driver or controller. A number of blocks on the flash chip may be set aside for storing mapping tables to deal with bad blocks, or the system may simply check each block at power-up to create a bad block map in RAM(SONY VAIO VGN-FZ210CE Battery). The overall memory capacity gradually shrinks as more blocks are marked as bad.

NAND relies on ECC to compensate for bits that may spontaneously fail during normal device operation. A typical ECC will correct a one bit error in each 2048 bits (256 bytes) using 22 bits of ECC code, or a one bit error in each 4096 bits (512 bytes) using 24 bits of ECC code. If ECC cannot correct the error during read, it may still detect the error(SONY VAIO VGN-FZ160 Battery). When doing erase or program operations, the device can detect blocks that fail to program or erase and mark them bad. The data is then written to a different, good block, and the bad block map is updated(SONY VAIO VGN-FZ21 Battery).

Most NAND devices are shipped from the factory with some bad blocks which are typically identified and marked according to a specified bad block marking strategy. By allowing some bad blocks(SONY VAIO VGN-FZ410 Battery), the manufacturers achieve far higher yields than would be possible if all blocks had to be verified good. This significantly reduces NAND flash costs and only slightly decreases the storage capacity of the parts(SONY VAIO VGN-FZ21m Battery).

When executing software from NAND memories, virtual memory strategies are often used: memory contents must first be paged or copied into memory-mapped RAM and executed there (leading to the common combination of NAND + RAM). A memory management unit (MMU) in the system is helpful(SONY VAIO VGN-FZ18m Battery), but this can also be accomplished with overlays. For this reason, some systems will use a combination of NOR and NAND memories, where a smaller NOR memory is used as software ROM and a larger NAND memory is partitioned with a file system for use as a non-volatile data storage area(SONY VAIO VGN-FZ11m Battery).

NAND is best suited to systems requiring high capacity data storage. This type of flash architecture offers higher densities and larger capacities at lower cost with faster erase, sequential write, and sequential read speeds, sacrificing the random-access and execute in place advantage of the NOR architecture(SONY VAIO VGN-FZ11z Battery).

Standardization

A group called the Open NAND Flash Interface Working Group (ONFI) has developed a standardized low-level interface for NAND flash chips. This allows interoperability between conforming NAND devices from different vendors. The ONFI specification version 1.0 was released on December 28, 2006. It specifies(SONY VAIO VGN-FZ11l Battery):

a standard physical interface (pinout) for NAND flash in TSOP-48, WSOP-48, LGA-52, and BGA-63 packages

a standard command set for reading, writing, and erasing NAND flash chips

a mechanism for self-identification (comparable to the Serial Presence Detection feature of SDRAM memory modules) (SONY VAIO VGN-FZ140E Battery)

The ONFI group is supported by major NAND flash manufacturers, including Hynix, Intel, Micron Technology, and Numonyx, as well as by major manufacturers of devices incorporating NAND flash chips(SONY VAIO VGN-FZ260E Battery).

A group of vendors, including Intel, Dell, and Microsoft formed a Non-Volatile Memory Host Controller Interface (NVMHCI) Working Group. The goal of the group is to provide standard software and hardware programming interfaces for nonvolatile memory subsystems, including the "flash cache" device connected to the PCI Express bus(SONY VAIO VGN-FZ150E Battery).

Distinction between NOR and NAND flash

NOR and NAND flash differ in two important ways:

the connections of the individual memory cells are different

the interface provided for reading and writing the memory is different (NOR allows random-access for reading, NAND allows only page access) (SONY VAIO VGN-FZ190 Battery)

These two are linked by the design choices made in the development of NAND flash. A goal of NAND flash development was to reduce the chip area required to implement a given capacity of flash memory, and thereby to reduce cost per bit and increase maximum chip capacity so that flash memory could compete with magnetic storage devices like hard disks(SONY VAIO VGN-FZ460E Battery).

NOR and NAND flash get their names from the structure of the interconnections between memory cells.In NOR flash, cells are connected in parallel to the bit lines, allowing cells to be read and programmed individually(SONY VAIO VGN-FZ280E Battery). The parallel connection of cells resembles the parallel connection of transistors in a CMOS NOR gate. In NAND flash, cells are connected in series, resembling a NAND gate. The series connections consume less space than parallel ones, reducing the cost of NAND flash(SONY VAIO VGN-FZ440N Battery). It does not, by itself, prevent NAND cells from being read and programmed individually.

When NOR flash was developed, it was envisioned as a more economical and conveniently rewritable ROM than contemporary EPROM and EEPROM memories. Thus random-access reading circuitry was necessary(SONY VAIO VGN-FZ32 Battery). However, it was expected that NOR flash ROM would be read much more often than written, so the write circuitry included was fairly slow and could only erase in a block-wise fashion(sony vgp-bpl9 battery). On the other hand, applications that use flash as a replacement for disk drives do not require word-level write address, which would only add to the complexity and cost unnecessarily(SONY vgp-bps9 battery).

Because of the series connection and removal of wordline contacts, a large grid of NAND flash memory cells will occupy perhaps only 60% of the area of equivalent NOR cells(assuming the same CMOS process resolution, e.g(SONY VAIO VGN-FZ11S Battery). 130nm, 90 nm, 65 nm). NAND flash's designers realized that the area of a NAND chip, and thus the cost, could be further reduced by removing the external address and data bus circuitry(SONY VAIO VGN-FZ Battery). 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(SONY VGP-BPS8 battery).

Write endurance

The write endurance of SLC floating-gate NOR flash is typically equal or greater than that of NAND flash, while MLC NOR and NAND flash have similar endurance capabilities. Example Endurance cycle ratings listed in datasheets for NAND and NOR flash are provided(Sony VAIO PCG-5K1L battery).

SLC NAND flash is typically rated at about 100k cycles (Samsung OneNAND KFW4G16Q2M)

MLC NAND flash is typically rated at about 5–10k cycles (Samsung K9G8G08U0M)

SLC floating-gate NOR flash has typical endurance rating of 100k to 1M cycles (Numonyx M58BW 100k; Spansion S29CD016J 1,000k) (Sony VAIO PCG-6W2L battery)

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(Sony VAIO PCG-7112L battery).

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(Sony VAIO PCG-8Z1L battery). 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(Sony VAIO PCG-8Z2L battery).

In practice, flash file systems are only used for memory technology devices (MTDs), 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(Sony VAIO PCG-8Y2L battery).

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(Sony VAIO PCG-8Y1L battery).

Consumer flash drives typically have sizes measured in powers of two (e.g., 512 MB, 8 GB). This includes SSDs as hard drive replacements, even though traditional hard drives tend to use decimal units(Sony VAIO PCG-7Z2L battery). Thus, an SSD marked as "64 GB" is actually 64 × 1,0243 bytes (64 GB). In reality, most users will have slightly less capacity than this available, due to the space taken by file system metadata(Sony VAIO PCG-7Z1L battery).

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(Sony VAIO PCG-7133L battery).

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.[21] In January 2008, Sandisk announced availability of their 16 GB MicroSDHC and 32 GB SDHC Plus cards(Sony VAIO PCG-7113L battery).

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(Sony VAIO PCG-6W3L battery).

Transfer rates

NAND flash memory cards are much faster at reading than writing so it is the maximum read speed that is commonly advertised(Sony VAIO PCG-7111L battery).

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(Sony VAIO PCG-6W1L battery).

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 60×, 100× or 150×. Here 1× is equivalent to 150 kB/s. For example, a 100× memory card gives 150 kB/s × 100 = 15,000 kB/s = 14.65 MB/s(Sony VAIO PCG-6V1L battery).

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(Sony VAIO PCG-6S3L battery).

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(Sony VAIO PCG-6S2L battery).

There are several reasons why a serial device, with fewer external pins than a parallel device, can significantly reduce overall cost(Sony VAIO PCG-5L1L battery):

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(Sony VAIO PCG-5K2L battery). 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(Sony VAIO PCG-5J2L battery).

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(Sony VAIO PCG-5J1L battery).

Smaller and lower pin-count packages occupy less PCB area.

Lower pin-count devices simplify PCB routing.

There are two major SPI flash types. The Atmel AT45 DataFlas was the first type and is characterized by small pages and one or more internal SRAM page buffers allowing a complete page to be read to the buffer(Sony VAIO PCG-5G3L battery), partially modified, and then written back. The second type is called SPI flash and typically has larger sectors. The smallest sectors typically found in an SPI flash are 4 kB, but they can be as large as 64 kB. Since the SPI flash lacks an internal SRAM buffer, the complete page must be read out and modified before written back, making it slow to manage(Sony VAIO PCG-5G2L battery). SPI flash is cheaper than DataFlash and is therefore a good choice when the application is code shadowing.

The two types are not easily exchangeable, since they do not have the same pinout. and the command sets are also incompatible(Sony VGP-BPS21A/B battery).

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(Sony VGP-BPS21/S battery); 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(Sony VGP-BPS21B battery). 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) (Sony VGP-BPS21 battery). 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(Sony VGP-BPS13A/S battery).

Flash memory as a replacement for hard drives

One more recent application for flash memory is as a replacement for hard disks. Flash memory does not have the mechanical limitations and latencies of hard drives, so a solid-state drive, or SSD, is attractive when considering speed(Sony VGP-BPS13B/S battery), noise, power consumption, and reliability. Flash drives are gaining traction as mobile device secondary storage devices; they are also used as hard drives in high-performance desktop computers and some servers with RAID and SAN architectures(Sony VGP-BPS13S battery).

There remain some aspects of flash-based SSDs that make them unattractive. Most important, the cost per gigabyte of flash memory remains significantly higher than that of platter-based hard drives(Sony VGP-BPS13AS battery). 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(Sony PCGA-BP2EA battery).

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(Sony VGP-BPS13/B battery).

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(Sony VGP-BPS13/S battery). 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. 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. Sandisk announced an OEM 32 GB 1.8" SSD drive at CES 2007(Sony VGP-BPS13A/B battery). 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(Sony VGP-BPS13B/B battery).

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(Sony VGN-FZ11E battery), but rarely modified, such as application and operating system executable files. Also, Addonics has a PCI adapter for four CF cards, creating a RAID-able array of solid-state storage that is much cheaper than the hardwired-chips PCI card kind(Sony VGN-FZ430E battery).

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(Sony VGN-FZ32 battery).

Sharkoon has developed a device that uses six SDHC cards in RAID-0 configuration 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(Sony VGN-FZ440N battery).

In Mid 2010, Apple Inc, introduced Mac Book Air notebooks which use flash as the primary device of storage instead of hard drives. However, Apple has already seen success with this hardware model in its most successful iPhones and iPads(Sony VGN-FZ52B2 battery).

Flash scalability

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(Sony VGN-FZ52B battery). 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(Sony VGN-FZ51B battery).

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 improvements to the manufacturing process(Sony VGN-FZ50B battery). The decrease in endurance and increase in uncorrectable bit error rates that accompany feature size shrinking can be compensated by improved error correction mechanisms. Even with these advances, it may be impossible to economically scale flash to smaller and smaller dimensions(Sony VGN-FZ340E/B battery). Many promising new technologies (such as FeRAM, MRAM, PMC, PCM, and others) are under investigation and development as possible more scalable replacements for flash(Sony VGN-FZ38 battery).