what is flash memory
Flash memory chips are electrically erasable, programmable, read-only memory (EEPROM) chips that can be erased and reprogrammed in blocks instead of one byte at a time. Because they are non-volatile, Flash memory chips do not need a constant power supply to retain their data. Flash memory chips offer extremely fast access times, low power consumption, and relative immunity to severe shock or vibration. They have a lifespan of approximately 100,000 write cycles – a fact that makes Flash unsuitable for use as computer main memory. Typically, Flash memory chips are used in portable or compact devices such as digital cameras, cell phones, pagers, and scanners. Flash memory chips are also used as solid-state disks in laptops and as memory cards for video game consoles.
Flash memory chips vary in terms of density, boot block size, number of words, bits per word, gate technology, and special features. Density is the capacity of the chip in bits. Boot block size is a secured block used to store boot codes. The number of words equals the number of rows, each of which stores a memory word and connects to a word line for addressing purposes. The bits per word are the number of columns, each of which connects to a sense / write circuit. Some Flash memory chips support NAND or serial access gate technology. Other devices support NOR or random access gate technology. In terms of special features, Flash memory chips can be read either in bursts of bits or page-by-page. Flash memory chips that provide read-while-write (RWW) operation can be read and written to at the same time.
Selecting Flash memory chips requires an analysis of performance specifications such as access time, data retention, endurance, supply voltage, and operating temperature. Measured in nanoseconds (ns), access time indicates the speed of memory and represents a cycle that begins when the CPU sends a request to memory and ends when the CPU receives the data requested. Data retention is the number of years that chips can retain data without reloading. Endurance is the maximum number of read/write cycles that chips can support. Supply voltages range from – 5 V to 5 V and include intermediate voltages such as -4.5 V, -3.3 V, -3 V, 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3 V, 3.3 V, and 3.6 V. Some Flash memory chips support a specific temperature range and feature mechanical and electrical specifications that are suitable for commercial or industrial applications. Other Flash memory chips meet screening levels for military specifications (MIL-SPEC).
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