EPROM

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 usually has a wavelength of 235nm (for optimum erasure time) and belongs to the UVC range of UV light. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip can be seen, and which permits UV light during erasing.
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.

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 Israeli engineer Dov Frohman in 1971.

EPROMs come in several sizes both in physical packaging as well and storage capacity. While parts of the same type number from different manufacturers are compatible as long as they're only being read, there are subtle differences in the programming process.
Most EPROMS could be identified by the programmer through "signature mode" by forcing 12V on pin A9 and reading out two bytes of data. However, as this was not universal, programmer software also would allow manual setting of the manufacturer and device type of the chip to ensure proper programming.

EPROM Type	Size — bits	Size — bytes	Length (hex)	Last address (hex)
1702, 1702A	2 Kibit	256	100	000FF
2704	4 Kibit	512	200	001FF
2708	8 Kibit	1 KiB	400	003FF
2716, 27C16	16 Kibit	2 KiB	800	007FF
2732, 27C32	32 Kibit	4 KiB	1000	00FFF
2764, 27C64	64 Kibit	8 KiB	2000	01FFF
27128, 27C128	128 Kibit	16 KiB	4000	03FFF
27256, 27C256	256 Kibit	32 KiB	8000	07FFF
27512, 27C512	512 Kibit	64 KiB	10000	0FFFF
27C010, 27C100	1 Mibit	128 KiB	20000	1FFFF
27C020	2 Mibit	256 KiB	40000	3FFFF
27C040	4 Mibit	512 KiB	80000	7FFFF
27C080	8 Mibit	1 MiB	100000	FFFFF

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

Contents

See also

External links

See also

★ Programmable read-only memory

★ EEPROM, like EPROM but electrically erasable

★ Flash memory

External links

★ Detailed information about EPROMs types and EPROM programming

★ EPROM vs. Other Memory Types