EPROM

EPROM

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An EPROM or erasable programmable read only memory, is a type of memory chip
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

 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 by exposing it to strong ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light from a mercury-vapor light source. EPROMs are easily recognizable by the transparent fused quartz
Fused quartz
Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...

 window in the top of the package, through which the silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 chip is visible, and which permits exposure to UV light during erasing.

Operation


Development of the EPROM memory cell started with investigation of faulty integrated circuits where the gate connections of transistors had broken. Stored charge on these isolated gates changed their properties. The EPROM was invented by Dov Frohman
Dov Frohman
Dov Frohman is an Israeli electrical engineer and business executive. A former vice president of Intel Corporation, he is the inventor of the erasable programmable read only memory and the founder and first general manager of Intel Israel, Intel’s extensive operations in Israel...

 of Intel in 1971, who was awarded US patent 3660819 in 1972.

Each storage location of an EPROM consists of a single field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...

. Each field-effect transistor consists of a channel in the semiconductor body of the device. Source and drain contacts are made to regions at the end of the channel. An insulating layer of oxide is grown over the channel, then a conductive (silicon or aluminum) gate electrode is deposited, and a further thick layer of oxide is deposited over the gate electrode. The floating gate electrode has no connections to other parts of the integrated circuit and is completely insulated by the surrounding layers of oxide. A control gate electrode is deposited and further oxide covers it.

To retrieve data from the EPROM, the address represented by the values at the address pins of the EPROM is decoded and used to connect one word (usually an 8-bit byte) of storage to the output buffer amplifiers. Each bit of the word is a 1 or 0, depending on the storage transistor being switched on or off, conducting or non-conducting.

The switching state of the field-effect transistor is controlled by the voltage on the control gate of the transistor. Presence of a voltage on this gate creates a conductive channel in the transistor, switching it on. In effect, the stored charge on the floating gate allows the threshold voltage of the transistor to be programmed.

Storing data in the memory requires selecting a given address and applying a higher voltage to the transistors. This creates an avalanche discharge of electrons, which have enough energy to pass through the insulating oxide layer and accumulate on the gate electrode. When the high voltage is removed, the electrons are trapped on the electrode. Because of the high insulation value of the silicon oxide surrounding the gate, the stored charge cannot readily leak away and the data can be retained for decades.

Unlike EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

s, the programming process is not electrically reversible. To erase the data stored in the array of transistors, ultraviolet light is directed onto the die
Die (integrated circuit)
A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon or other semiconductor through processes such as...

. Photons of the UV light create ionization within the silicon oxide, which allow the stored charge on the floating gate to dissipate. Since the whole memory array is exposed, all the memory is erased at the same time. The process takes several minutes for UV lamps of convenient sizes; sunlight would erase a chip in weeks, and indoor fluorescent lighting
Fluorescent lamp
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...

 over several years. Generally the EPROMs must be removed from equipment to be erased, since it's not usually practical to build in a UV lamp to erase parts in-circuit.

Details


As the quartz window is expensive to make, OTP
Programmable read-only memory
A programmable read-only memory or field programmable read-only memory or one-time programmable non-volatile memory is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. Such PROMs are used to store programs permanently...

 (one-time programmable) chips were introduced; here, the die is mounted in an opaque package so it cannot be erased after programming - this also eliminates the need to test the erase function, further reducing cost. OTP versions of both EPROMs and EPROM-based microcontrollers are manufactured. However, OTP EPROM (whether separate or part of a larger chip) is being increasingly replaced by EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 for small amounts where the cell cost isn't too important and flash
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

 for larger amounts.

A programmed EPROM retains its data for a minimum of ten to twenty years , with many still retaining data after 35 or more years, and can be read an unlimited number of times. The erasing window must be kept covered with an opaque label to prevent accidental erasure by the UV found in sunlight or camera flashes. Old PC BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 chips were often EPROMs, and the erasing window was often covered with an adhesive label containing the BIOS publisher's name, the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 revision, and a copyright notice. Often this label was foil-backed to ensure its opacity to UV.

Erasure of the EPROM begins to occur with wavelengths shorter than 400 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

. 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 253.7 nm of at least 15 W-sec/cm2 for 20 to 30 minutes, with the lamp at a distance of about 1 inch.

Erasure can also be accomplished with X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s:
"Erasure, however, has to be accomplished by non-electrical methods, since the gate electrode is not accessible electrically. Shining ultraviolet light on any part of an unpackaged device causes a photocurrent to flow from the floating gate back to the silicon substrate, thereby discharging the gate to its initial, uncharged condition. This method of erasure allows complete testing and correction of a complex memory array before the package is finally sealed. Once the package is sealed, information can still be erased by exposing it to X radiation in excess of 5*104 rad
Rad (unit)
The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

s, a dose which is easily attained with commercial X-ray generators." (5*104 rad
Rad (unit)
The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

 = 500 J
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

/kg)

"In other words, to erase your EPROM, you would first have to X-ray it and then put it in an oven at about 600 degrees Celsius (to anneal semiconductor alterations caused by the x-rays). The effects of this process on the reliability of the part would have required extensive testing so they decided on the window instead." (any temperature between 450 - 1410 °C should work).

EPROMs had a limited but large number of erase cycles; the silicon dioxide around the gates would accumulate damage from each cycle, making the chip unreliable after several thousand cycles. EPROM programming is slow compared to other forms of memory. Because higher-density parts have little exposed oxide between the layers of interconnects and gate, ultraviolet erasing becomes less practical for very large memories. Even dust inside the package can prevent some cells from being erased.

Application


For large volumes of parts (thousands of pieces or more), mask-programmed ROMs are the lowest cost devices to produce. However, these require many weeks lead time to make, since the artwork for an IC mask layer must be altered to store data on the ROMs. Initially, it was thought that the EPROM would be too expensive for mass production use and that it would be confined to development only. It was soon found that small-volume production was economical with EPROM parts, particularly when the advantage of rapid upgrades of firmware was considered.

Some microcontrollers, from before the era of EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

s and flash memory
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

, use an on-chip EPROM to store their program. Such microcontrollers include some versions of the Intel 8048
Intel 8048
The MCS-48 microcontroller series, Intel's first microcontroller, was originally released in 1976. Its first members were 8048, 8035 and 8748....

, the Freescale 68HC11
Freescale 68HC11
The 68HC11 is an 8-bit microcontroller family introduced by Motorola in 1985. Now produced by Freescale Semiconductor, it descended from the Motorola 6800 microprocessor. It is a CISC microcontroller...

, and the "C" versions of the PIC microcontroller
PIC microcontroller
PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division...

. Like EPROM chips, such microcontrollers came in windowed (expensive) versions that were useful for debugging and program development. 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.

EPROM sizes and types


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 Year Size — bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s
Size — byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

s
Length (hex
Hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

)
Last address (hex
Hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

)
1702, 1702A 1971 2 Kbit  256 100 FF
2704 1975 4 Kbit 512 200 1FF
2708 1975 8 Kbit 1 KB
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...

 
400 3FF
2716, 27C16, TMS2716, 2516 1977 16 Kbit 2 KB 800 7FF
2732, 27C32, 2532 1979 32 Kbit 4 KB 1000 FFF
2764, 27C64, 2564 64 Kbit 8 KB 2000 1FFF
27128, 27C128 128 Kbit 16 KB 4000 3FFF
27256, 27C256 256 Kbit 32 KB 8000 7FFF
27512, 27C512 512 Kbit 64 KB 10000 FFFF
27C010, 27C100 1 Mbit
Megabit
The megabit is a multiple of the unit bit for digital information or computer storage. The prefix mega is defined in the International System of Units as a multiplier of 106 , and therefore...

 
128 KB 20000 1FFFF
27C020 2 Mbit 256 KB 40000 3FFFF
27C040, 27C400 4 Mbit 512 KB 80000 7FFFF
27C080 8 Mbit 1 MB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...

 
100000 FFFFF
27C160 16 Mbit 2 MB 200000 1FFFFF
27C320 32 Mbit 4 MB 400000 3FFFFF

See also

  • Programmable read-only memory
    Programmable read-only memory
    A programmable read-only memory or field programmable read-only memory or one-time programmable non-volatile memory is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. Such PROMs are used to store programs permanently...

  • EEPROM
    EEPROM
    EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

  • Flash memory
    Flash memory
    Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

  • Intel HEX
    Intel HEX
    Intel HEX is a file format for conveying binary information for applications like programming microcontrollers, EPROMs, and other kinds of chips. It is one of the oldest file formats available for this purpose, having been in use since the 1970s...

     - File format
  • SREC
    SREC (file format)
    The Motorola S-record format is an ASCII hexadecimal text encoding for binary data. It is also known as the SREC or S19 format. Each record contains a checksum to detect data that has been corrupted during transmission. The first record may include arbitrary comments such as a program name or...

    - File format

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