|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
There are several different types of EPROM's:
They have several distinct features.
We're going to concentrate on UV-EPROM's in this article as they
are more historical these days.
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.
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.
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.
(image courtesy of MrLaptop)