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Electric motors are the backbone of modern automation. Motors convert electrical energy into mechanical energy, and the resulting motion and torque drives a load. The most effective motor control solution is the variable frequency drive, or adjustable speed drive. Drives vary motor input frequency and voltage to control motor speed and torque.
The NAND type is found mainly in memory cards, USB flash drives, solid-state drives (those produced since 2009), feature phones, smartphones, and similar products, for general storage and transfer of data. NAND or NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROM or battery-powered static RAM. A key disadvantage of flash memory is that it can endure only a relatively small number of write cycles in a specific block.[2]
Flash memory[3] is used in computers, PDAs, digital audio players, digital cameras, mobile phones, synthesizers, video games, scientific instrumentation, industrial robotics, and medical electronics. Flash memory has fast read access time, but it is not as fast as static RAM or ROM. In portable devices, it is preferred to use flash memory because of its mechanical shock resistance since mechanical drives are more prone to mechanical damage.[4]
NAND flash 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. 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 makes NAND flash unsuitable as a drop-in replacement for program ROM, since most microprocessors and microcontrollers require byte-level random access. In this regard, NAND flash is similar to other secondary data storage devices, such as hard disks and optical media, and is thus highly suitable for use in mass-storage devices, such as memory cards and solid-state drives (SSD). Flash memory cards and SSDs store data using multiple NAND flash memory chips.
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.
Common flash devices such as USB flash drives 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. This prevents incremental writing within a block; however, it does help the device from being prematurely worn out by intensive write patterns.
In practice, flash file systems are used only for memory technology devices (MTDs), which are embedded flash memories that do not have a controller. Removable flash memory cards, SSDs, eMMC/eUFS chips and USB flash drives have built-in controllers to perform wear leveling and error correction so use of a specific flash file system may not add benefit.
Consumer flash storage devices typically are advertised with usable sizes expressed as a small integer power of two (2, 4, 8, etc.) and a designation of megabytes (MB) or gigabytes (GB); e.g., 512 MB, 8 GB. This includes SSDs marketed as hard drive replacements, in accordance with traditional hard drives, which use decimal prefixes.[143] Thus, an SSD marked as \"64 GB\" is at least 64 10003 bytes (64 GB). Most users will have slightly less capacity than this available for their files, due to the space taken by file system metadata.
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.[145]In January 2008, SanDisk announced availability of their 16 GB MicroSDHC and 32 GB SDHC Plus cards.[146][147]
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 chip, and then copied into SDRAM or SRAM when the device is powered-up.[155] Using an external serial flash device rather than on-chip flash removes the need for significant process compromise (a manufacturing 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. Since 2005, many devices use serial NOR flash to deprecate parallel NOR flash for firmware storage. Typical applications for serial flash include storing firmware for hard drives, Ethernet network interface adapters, DSL modems, etc.
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 (SSD) is attractive when considering speed, noise, power consumption, and reliability. Flash drives are gaining traction as mobile device secondary storage devices; they are also used as substitutes for hard drives in high-performance desktop computers and some servers with RAID and SAN architectures.
There remain some aspects of flash-based SSDs that make them unattractive. The cost per gigabyte of flash memory remains significantly higher than that of hard disks.[156] Also flash memory has a finite number of P/E (program/erase) cycles, but this seems to be currently under control since warranties on flash-based SSDs are approaching those of current hard drives.[157] In addition, deleted files on SSDs can remain for an indefinite period of time before being overwritten by fresh data; erasure or shred techniques or software that work well on magnetic hard disk drives have no effect on SSDs, compromising security and forensic examination. However, due to the so-called TRIM command employed by most solid state drives, which marks the logical block addresses occupied by the deleted file as unused to enable garbage collection, data recovery software is not able to restore files deleted from such. 153554b96e
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