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Updated 05-19-06

EPROM Primer

EPROM Construction

EPROM History


Erasable Programmable Read Only Memory

EPROM's have been a critical component to computers for over 30 years. An EPROM is an erasable memory device that can store a small amount of data. There are two common uses for EPROM's. One is for storing a program, usually a simple program like the BIOS in a computer or the application that runs a microcontroller. The second use is a LUT, or a Look Up Table. A LUT is useful when a set of known inputs values has one output value. For example, multiplication use to be a very slow operation for microcontrollers, to speed it up, the MCU would output the two numbers to an EPROM (as an address) and the data stored at that address would be the result.

There are several different types of EPROM's:
They have several distinct features.

UV-EPROM UV EPROM's are programmed at high voltages (usually 28V) and are erased by shining Ultra-Violet (high energy) light at them through a window. They have no limit to read cycles and can store data for over 20 years.
OTP One Time Programmable ROMS are generally identical to UV-EPROM's with one exception, they do not have the window for UV light and thus are not readily erasable.
This was done to save money, as they could be packaged in a plastic package instead of ceramic/glass.
There has been some success in erasing these using X-Rays.
EEPROM These are Electronically erasable. Again it requires a higher voltage (12.5V-28V) which can be generated on chip by a charge pump. Write times are usually rather slow and power requirements are hard to manage as writes take MUCH more power then reads.
EEPROM's also have a limited number of write cycles due to electron tunnel oxide degradation, a result of quantum mechanics.
Flash Flash is very similar to EEPROM's except they are built to write/erase entire blocks (multi-byte) at a time (EEPROM writes a byte at a time). This makes writes faster, and saves transistors.


We're going to concentrate on UV-EPROM's in this article as they are more historical these days.
(however the concept and physics is mostly the same)

An EPROM is a large array of Floating Gate Transistors.
Inside a floating gate MOSFET, the main components are a control gate, floating gate, and the thin oxide layer. The EPROM has a grid of columns and rows and the cell at each intersection has two transistors. The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as the floating gate and the other as the control gate. The floating gate's only link to the row (wordline) is through the control gate. As long as this link is in place, the cell has a value of 1. To change the value to 0 requires a process called Fowler-Nordheim tunneling. Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 28 volts, is applied to the floating gate. The charge comes from the column (bitline), enters the floating gate and drains to a ground.

This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EPROM has all of the gates fully open, giving each cell a value of 1.

To rewrite an EPROM, you must erase it first. To erase it, you must supply a level of energy strong enough to break through the negative electrons blocking the floating gate. In a standard EPROM, this is best accomplished with UV light at a wavelength of 253.7 nanometers. Contrary to popular belief, sunlight does not erase your EPROM. Because this particular frequency will not penetrate most plastics or glasses, each EPROM chip has a quartz window on top of it. The EPROM must be very close to the eraser's light source, within an inch or two, to work properly.

Too much UV-Light can excite the electrons TOO much resulting in an 'over-erased' condition that cannot be readily be fixed.
OTP EPROM's just lack the window, so there is no way to get UV-Light in them. Several individuals have gotten around this by using X-Rays which is just a higher energy wave in the electromagnetic spectrum. It WILL penetrate the plastic case and erase it, however, since it is MUCH higher energy, it is easy to over-erase the chip.

EPROM's are packaged in a ceramic package because of the embedded quartz crystal. During normal heating and cooling cycles a Quartz/Plastic package would fail. Ceramic and Quartz expand and contract at the same rate making the ceramic package the only acceptable form. This decreases failures but greatly increases cost. NOTE: Apparently the Soviets solved this, as I have 3 Soviet EPROM's that are in a plastic package. It appears to be all resin, with a small hole that has a resin lens planted in it.

Soviet Plastic EPROM

The first EPROM was the Intel C1701 back in 1971 which stored 256 bytes of information. Today's EPROM's hold 8Mb or more.
Below is a listing of most all Parallel EPROM's that were used. These EPROM's have separate pins for Data and Address (non-multiplexed) and take the data out in parallel. Modern day EPROM's now either multiplex the Address/Data ports or take the data out via a serial connection (often I2C)

EPROM Size (bits) Structure Pins Address Lines
1701 2k 256 x 8 24 8  
1702 - 4702 - 8702 - 5203 - 9702 - U552C 2k 256 x 8 24 8
6601 - 6653 - 6603 4k 1k x 4 24 10  

2704 - 6654 - 6604 8704 - 5204

4k 512 x 8 24 9  

2708 - 8708 - 2758 - U555C - K573RF1 -2508

8k 1k x 8 24 10  
2716 - 2717 - 4716 - 2516 - K573RF2 - K573RF5 - 27291 - 27245 16k 2k x 8 24 11 K573RF5
8755 16k (w/ IO) 2k x 8 40 11  
2732 - 2532 32k 4k x 8 24 12  
2764 - 8764 -2763 - 2564 - 57C49 - 68766 K573RF4 - K573RF6A 64k 8k x 8 28 13
27128 128k 16k x 8 28 14  
27256 - 87256 - 87257 256k 32k x 8 28 15
Atmel AT27C256R-15LC
27512 - 512k 64k x 8 28 16
AMD 27C512
27513 512k 4 x 16k x 8 28 14  
27010, 27101, 27100, 27301,
271000 - 271001 - 2710000 - 571001
1024k 128k x 8 32 17  
27011 1024k 8 x 16k x 8 28 14  
271024 - 27102 - 27210 - 571024 1024k 64k x 16 40 16
Hitachi HN27C1024HCC-10
272001 - 27201 - 27020 2048k 256k x 8 32 18  
272048 -27202 - 27220 2048k 128k x 16 40 17  
27040 - 274001
4096k 512k x 8 32 19  
274096 - 274002 - 27402 - 27240 4096k 256k x 16 40 18  
274100 - 27400 - 274000 4096k 512k x 8
256k x 16
40 18-19  
27801 - 27080 - 278001 8192k 1024k x 8 32 20  
278000 (Micronix) 8192k 1024k x 8 32 20  
27800 - 278000 8192k 1024k x 8
512k x 16
42 19-20  
27160 16384k 2048k x 8
1024k x 16
42 20-21  
27332 32768k 2048k x 16 42 21  
27642 65536k 4096k x 16 42 22  

27Cxxx - CMOS part (as opposed to NMOS
87xxxx - Denoted latched address, for use with multiplexed MPUs (like the 8086)
27LV or 27V Low Voltage Parts (3V - 3.6V Supplies)
27Hxxx Hispeed access times
27Wxxx used by several companies to denote 2.7V compliant parts
47xx Used for the MCS-4 Computer set
97xx AMD Version of the MCS-80 EPROM's (for the Am9080 etc)
57xxx Common Toshiba prefix

UV-EPROM's are at the end of their life. Newer technologies are smaller, cheaper, and faster. These chips served us well into the 21st Century, impressive for a technology that is over 30 years old. They will continue to serve in many control applications until old equipment is replaced.

Intel 1702A 1602A Die

(image courtesy of MrLaptop)

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