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. EPROMs are easily recognisable by the transparent window in the top of the package, through which the silicon chip can be seen, and which admits UV light during erasing.
As the glass 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 EPROMs are being increasingly replaced by EEPROM chips, which offer similar price of silicon and reprogrammability in a windowless package.
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 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 useful for development, as using one-time programmable devices would be horribly wasteful for debugging.
The EPROM was invented by engineer Dov Frohman.
EPROMs come in several sizes both in physical packaging as well and storage capacity
| EPROM Type | Size — bits | Size — bytes | Length (hex) | Last address (hex) |
|---|---|---|---|---|
| 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 |
NOTE: 27x series EPROMs containing a C in the name are CMOS based, without the C are NMOS
See also
- Programmable read-only memory
- EEPROM, like EPROM but electrically erasable
- Flash memorybg:EPROM
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