EPROM memory

EPROM. The small quartz window receives UV light during deletion.

EPROM window.

A 32KB (256Kbit) EPROM.

This microcontroller 8749 stores your program on an internal EPROM.

A 4KB EPROM (32Kbit) 1984.

Detailed view of the interior of a memory.

EPROM stands for Erasable Programmable Read-Only Memory (erasable programmable ROM). It is a type of non-volatile ROM memory chip invented by Intel engineer Dov Frohman. It is made up of Floating Gate Avalanche-Injection Metal-Oxide Semiconductor (FAMOS) cells, or "floating gate transistors", each of which comes from the factory without a load, so they are read as 1 (for this reason, a blank EPROM is read as FF in all its cells).

Features

EPROM memories are programmed by an electronic device, such as the Cromemco Bytesaver, which provides higher voltages than those normally used in electronic circuits. Cells receiving charge are then read as 0.

Once programmed, an EPROM can be erased only by exposure to strong ultraviolet light. This is because photons of light excite the electrons in the cells causing them to discharge. EPROMs are easily recognized by a transparent window at the top of the package, through which the silicon chip can be seen and which admits ultraviolet light during erasing.

As window quartz is expensive to manufacture, OTP (One-Time Programmable) chips were introduced. The only difference with the EPROM is the absence of the quartz window, so it cannot be erased. OTP versions are manufactured to replace both regular EPROMs and the EPROMs included in some microcontrollers. The latter were gradually being replaced by EEPROMs (for manufacturing small quantities where cost is not important) and by flash memory (in those with the highest use).

A programmed EPROM retains its data for ten to twenty years, and can be read an unlimited number of times. To prevent accidental erasure by sunlight, the erasure window must remain covered. Older personal computer BIOSes were often EPROMs, and the erase window was usually covered by a label containing the name of the BIOS maker, its revision, and a copyright notice.

EPROMs can come in different sizes and capacities. Thus, for the 2700 family you can find:

NOTE: 1702 EPROMs are PMOS, 27x series EPROMs that contain a C in the name are CMOS based, the rest are NMOS

Pins of the EPROM 2764

 +------------------
VPP UD1 +--+ 28ATA VCC
A12 UD2 27VA / PGM
A7 UD3 26IND NC
A6 UD4 25 A8
A5 UDI5 24IVA A9
A4 UDI6 23 A11
A3 UDI7 2764 22UDI /OE
A2 UD8 21VA A10
A1 UDI9 20UDI / ECE
A0 UD10 19IND D7
D0 UD11 18VA D6
D1 LIC12 17IND D5
D2 UD13 16IND D4
GND UDI14 15 short D3
+------------------

Erasing an EPROM

An EPROM memory can be erased with a UV light lamp, of the UV-C type, which emits radiation around 2537 Å (Angstrom) or 254nm, at a distance of about 2.5 cm from the memory. The radiation reaches the memory cells through a transparent quartz window located in the upper part of it.

To erase an EPROM it is necessary that the amount of radiation received by it is around 15 W/cm^2 for one second. The actual erasing time is usually around 20 minutes because the lamps used usually have powers around 12 mW/cm² (12 mW x 20 x 60 s = 14.4 W of supplied power). This time also depends on the manufacturer of the memory to be erased. During this time all its bits are set to 1.

It is important to avoid overexposure of radiation time to EPROMs; that is to say, the luminous power supplied to the memory, since it produces a premature aging of the same.

Because solar radiation and even artificial light from fluorescent tubes erase memory slowly (from a week to several months), it is necessary to cover said window with an opaque label to avoid it, once they are recorded.

It should be clarified that an EPROM cannot be partially or selectively erased; hence, no matter how small the possible modification to be made to its content, it must inevitably be deleted and reprogrammed in its entirety.

EPROM Eraser

An EPROM eraser is an optically opaque box, with a type C UV light source, which is also used to sterilize surgical instruments and/or as a germicide.

EPROM eraser lamp.

To erase EPROMs, you cannot use "UV Black" light, (which is commonly used to verify banknotes, tickets, etc.), which emit in the UV-A region, (365 nm). The only light that works is UV-C, (254 nm), which emits "dangerous light" or "germicidal", (kills germs). It is "dangerous light" because prolonged exposure can cause long-term cataracts and skin damage; however, a brief exposure, about 5 continuous seconds on the skin, should not cause more than a slight dryness, so it is necessary to take all precautions to avoid these problems. Since this type of UV-C light is found in sunlight, leaving an EPROM directly under it will erase it in a few days or weeks; so it is required to protect the EPROMs once they have been programmed.

You can use a normal 4 W tube lamp of the type F4T5 (4 watt, 5 inches) that gives white light (see photo). Also a G4T5 "Germicidal UVC" tube light, which has clear glass, to erase EPROMs. The "G" is for germicides, same as "F" It is for phosphorescent (even if they do not have phosphor). Another commonly used type of lamp is the PHILIPS TUV 4W-G4T5-240805D-4WTUV.

Simple circuit for EPROM eraser, (you can add timer).

UV lamps that have purple or lilac glass are for the UV-Visible or "UV Black" spectrum, they are usually marked U4T5 or similar and will not work to erase EPROMs.

A white light fluorescent tube has a phosphor coating on the inside of the glass. UV light from mercury excites the phosphor, which re-radiates energy into the visible range. UV lamps for EPROM erasers or germicides use light from mercury vapor directly. The glass should be made of quartz, rather than ordinary glass, to prevent the glass from absorbing most of the UV rays. Quartz is more transparent at UV lengths than mercury.

They could also be erased if they are exposed to light from electric welding (electrode), with the risk that a spark burns the chip, because the EPROM must be brought close to about 10 or 15 cm so that they receive enough radiation to erase them. (in which case it could be protected with a transparent glass barrier, glass, window, etc.); In theory they can also be erased with X-rays, "by taking X-rays of the EPROM", the erasure time will depend on the calibration/emission of the X-ray equipment used.

Difference between Eprom C and Non-C

The only difference between the 27256 and the 27C256 is that the 27256 uses NMOS while the 27C uses CMOS technology. CMOS only consumes appreciable power when a signal is changing. NMOS uses N-channel FET's with resistor elements, while CMOS avoids power-wasting resistors by using both N-channel and P-channel FETs. In addition, CMOS avoids heat production, allowing for more compact transistor arrangements than NMOS are capable of. The high density of CMOS elements reduces interconnection distances which increases speed. Also CMOS glows when there is a limited amount of power such as when using a battery powered system.

There are some problems with CMOS EPROMs using older programmers, due to differences in programming voltages (CMOS is 12.5 Vpp). CMOS EPROMs also require a voltage source, (Vcc), of exactly 6 Volts. CMOS are easy to erase but tend to be damaged if overexposed to UV light.

Decoding EPROM Numbers

• 27(C)XXX are EPROM or OTPROMS.
• 57(C)XXX are EPROM or OTPROMS that allow 8 low direction lines to be multipleized with the data line (Some MCU's multiplexes together the lower addresses and the data line). These are still programmed as EPROM 27(C) XXX in pocket programmers, because the software algorithm takes it into account.
• 28(C)XXX are EEPROM with C indicating for CMOS.
• 28FXXX are Flash EEPROM with F pointing for Flash. Don't confuse with EPROM.

Cross list

The following parts may be the same - (according to manufacturers' reference guides), there may be some differences including the algorithm used to program them.