(Electrically Erasable Programmable Read Only Memory) A rewritable storage chip that holds its content without power. EEPROMs are byte addressable but must be erased before being rewritten. In flash memory, which evolved from EEPROMs and is almost identical in architecture, an entire block of bytes must first be erased. In addition, EEPROMs are typically used on circuit boards to store small amounts of instructions and data, whereas flash memory modules hold gigabytes of data for camera and computer storage (see flash memory). EEPROMs store location data in LTO tape cartridges, and they are actually used in flash memory-based SSDs; not as data storage but as boot code. See LTO.A Floating Gate Holds the ChargeThe bit cells in EEPROM and flash memory are CMOS-based transistors that hold a charge on a "floating gate." With no charge on the floating gate, the transistor acts normally, and a pulse on the control gate causes current to flow. When charged, it blocks the control gate action, and current does not flow. Charging is accomplished by grounding the source and drain terminals and placing sufficient voltage on the control gate tunnel through the oxide to the floating gate. A reverse voltage channeled from another transistor clears the charge by causing it to dissipate into the substrate.EEPROMs have a lifespan of between 10K and 100K write cycles, which is considerably greater than the EPROMs (single "E") that preceded them. See EPROM, SEEPROM, memory types and flash memory.
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