EPROM
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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.
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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[citation needed] 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. Such microcontrollers include some versions of the Intel 8048, the Freescale 68HC11, and the "C" versions of the PIC microcontroller. Like EPROM chips, such microcontrollers came in windowed (expensive) versions that were useful for debugging and program development, and also the same chip came in (somewhat cheaper) opaque OTP packages for production. Leaving the die of such a chip exposed to light can also change behavior in unexpected ways when moving from a windowed part used for development to a non-windowed part for production.
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 Kbit | 256 | 100 | 000FF |
| 2704 | 4 Kbit | 512 | 200 | 001FF |
| 2708 | 8 Kbit | 1 KB | 400 | 003FF |
| 2716, 27C16 | 16 Kbit | 2 KB | 800 | 007FF |
| 2732, 27C32 | 32 Kbit | 4 KB | 1000 | 00FFF |
| 2764, 27C64 | 64 Kbit | 8 KB | 2000 | 01FFF |
| 27128, 27C128 | 128 Kbit | 16 KB | 4000 | 03FFF |
| 27256, 27C256 | 256 Kbit | 32 KB | 8000 | 07FFF |
| 27512, 27C512 | 512 Kbit | 64 KB | 10000 | 0FFFF |
| 27C010, 27C100 | 1 Mbit | 128 KB | 20000 | 1FFFF |
| 27C020 | 2 Mbit | 256 KB | 40000 | 3FFFF |
| 27C040 | 4 Mbit | 512 KB | 80000 | 7FFFF |
| 27C080 | 8 Mbit | 1 MB | 100000 | FFFFF |
[edit] Gallery
 Close up of a EPROM die |
 A 32KB (256Kbit) EPROM |
 EPROM. 60x closeup |
 This 8749 Microcontroller stores its program in internal EPROM |
