|Reference||STATE OF THE ART||Stan Augarten|
|A Piece of Electronic Magic|
The First Erasable Programmable Read-Only Memory, the 1702
Intel's third major contribution to the development of the IC was the erasable programmable read-only memory, or EPROM. It is a ROM with a special difference; its contents can be erased by ultraviolet light. An EPROM can thus be reprogrammed again and again, giving a single chip more lives than a cat, whereas the contents of an ordinary ROM are set for all time at the semiconductor factory and cannot be altered.
The Key to the EPROM, which was invented by Dov Frohman, is a charge-storing capacitor embedded in the chip's memory cells; the capacitor is surrounded by an insulator, silicon dioxide, that turns into a modest electrical conductor when exposed to ultraviolet light for five to ten minutes, allowing the stored charges to drain away. An erased EPROM thus becomes a tabula rasa, easily reprogrammed by relatively simple electronic equipment. EPROMs are easy to recognize, because they come in packages with small quartz windows above the chips.
Erasable Programmable Read-Only Memory. In a typical EPROM memory cell, charges are stored in "floating" gates made out of polycrystalline silicon and embedded in silicon dioxide, an insulator.
EPROMs are ideal tools for product development or for the manufacture of equipment with a total production run of a thousand or less. Since they can be erased and reset many times over, these chips are, in the long run, less expensive alternatives to ROMs, which must be junked and replaced if their instructions prove faulty, or if the developing product's specifications change. The invention of the EPROM helped Intel sell the microprocessor, which was invented at about the same time, by enabling the firm's customers to program their own memory chips instead of paying Intel to do it for them.
Since the development of the EPROM, other forms of programmable memory chips have proliferated like rabbits. The most notable types are programmable ROMs, or PROMs, which contain miniscule fuses that may be blown to burn in data; electrically erasable PROMs, or EEPROMs, which can be erased and reset electrically; and electrically alterable ROMs, or EAROMs, often termed read-mostly memories. With only limited applications, EAROMs can be read in as little as a millionth of a second but require much more time for data to be written in.
|The 1702 stores up to 2K bits of data in two 1K grids in the center of the chip. A bit can be read out of this chip in one millionth of a second. The irregular circuits around the grids are column and row decoders, used to retrieve information from the memory cells. Actual size: 0.147 x 0.161 inches.|
|STATE OF THE ART
©Copyright Stan Augarten
|This book is provided for general reference. The National Museum of American History and the Smithsonian Institution make no claims as to the accuracy or completeness of this work.|
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