An EPROM, or Erasable Programmable Read-Only Memory, is a type of 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 digital 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.
Erasure of the EPROM begins to occur with wavelengths shorter than 400nm. Exposure time for sunlight of 1 week or 3 years for room fluorescent lighting may cause erasure. The recommended erasure procedure is exposure to UV light at 253.7nm of at least 15 W-sec/cm2 for 20 to 30 minutes, with the lamp at a distance of about 1 inch.
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.
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|
|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|
CMOS EPROM offers 4 Mbits with 100-nsec data access. (Complementary Metal Oxide Semiconductor; Erasable Programmable Read Only Memory)
Sep 20, 1990; CMOS EPROM offers 4 Mbits with 100-nsec data access Any serious manufacturer of semiconductor memories must recognize that...
US Patent Issued to the United States of America as Represented by the Secretary of the Navy on April 16 for "EPROM Erasing Apparatus and Method for Erasing an EPROM(s)" (California Inventors)
Apr 16, 2013; ALEXANDRIA, Va., April 16 -- United States Patent no. 8,422,311, issued on April 16, was assigned to The United States of America...
4-bit microcontroller supports 16 kbytes of EPROM and 1k nibbles of static RAM. (NEC Electronics Inc.'s UPD75P316A, erasable programmable read-only memory)(Electro '92 Show Guide & Product Spotlight)(EDN-Product Update) (Product Announcement)
May 07, 1992; Engineers shouldn't treat 4-bit (1-nibble) microcontrollers ([nu]Cs) as outdated technology. Four-bit [nu]Cs are alive and...