Read-only memory (ROM) is similar in design to
static or dynamic RAM circuits, except that the "latching" mechanism is made
for one-time (or limited) operation. The simplest type of ROM is that which
uses tiny "fuses" which can be selectively blown or left alone to represent
the two binary states. Obviously, once one of the little fuses is blown, it
cannot be made whole again, so the writing of such ROM circuits is one-time
only. Because it can be written (programmed) once, these circuits are
sometimes referred to as PROMs (Programmable Read-Only Memory).
However, not all writing methods are as permanent as blown fuses. If a
transistor latch can be made which is resettable only with significant
effort, a memory device that's something of a cross between a RAM and a ROM
can be built. Such a device is given a rather oxymoronic name: the EPROM
(Erasable Programmable Read-Only Memory). EPROMs come in two basic
varieties: Electrically-erasable (EEPROM) and Ultraviolet-erasable
(UV/EPROM). Both types of EPROMs use capacitive charge MOSFET devices to
latch on or off. UV/EPROMs are "cleared" by long-term exposure to
ultraviolet light. They are easy to identify: they have a transparent glass
window which exposes the silicon chip material to light. Once programmed,
you must cover that glass window with tape to prevent ambient light from
degrading the data over time. EPROMs are often programmed using higher
signal voltages than what is used during "read-only" mode. |