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EPROM. The small quartz window admits UV light for erasure.

An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in electronic circuits. Once programmed, an EPROM can be erased only by exposing it to strong ultraviolet light. That UV light has usually a wavelength of 235nm (for optimum erasure time) and belongs to the UVC range of UV light. EPROMs are easily recognizable by the transparent quartz window in the top of the package, through which the silicon chip can be seen, and which permits UV light during erasing.

A 32KB (256Kbit) EPROM.

As the quartz window is expensive to make, OTP (one-time programmable) chips were introduced; the only difference is that the EPROM chip is packed in an opaque package, so it can not be erased after programming. OTP versions are manufactured for both EPROMs themselves and EPROM-based microcontrollers. However, OTP EPROM (whether separate or part of a larger chip) is being increasingly replaced by EEPROM for small amounts where the cell cost isn't too important and flash for larger amounts.

EPROM. 60x Closeup .

A programmed EPROM retains its data for about ten to twenty years and can be read an unlimited number of times. The erasing window must be kept covered with a foil label to prevent accidental erasure by sunlight. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with a label containing the BIOS publisher's name, the BIOS revision, and a copyright notice.

Some microcontrollers, often those from before the era of EEPROMs and flash memory, use EPROM to store their program. This is not very useful for development, as using one-time programmable devices would be horribly wasteful for debugging and windowed versions are expensive. Leaving the die of such a chip exposed to light can also change behavior in ways that may be disastrous when moving from a windowed part used for development to a non-windowed part for production. For example a device may power up with data memory random with the die covered but with it in a predictable state with the die exposed to light.

The EPROM was invented by engineer Dov Frohman.

This 8749 Microcontroller stores its program in internal EPROM.

EPROMs come in several sizes both in physical packaging as well and storage capacity

NOTE: 1702 EPROMs were PMOS, 27x series EPROMs containing a C in the name are CMOS based, without the C are NMOS

• Programmable read-only memory
• EEPROM, like EPROM but electrically erasable
• Flash memory