Overview
Read only memory devices are a special case of memory where, in normal system oper-
ation, the memory is read but not changed. Read only memories are non-volatile, that is,
stored information is retained when the power is removed. The main read only memory
devices are listed below:
ROM (Mask Programmable ROM —also called “MROMs”)
EPROM (UV Erasable and Electrically Programmable ROM)
OTP (One Time Programmable EPROM)
EEPROM (Electrically Erasable and Programmable ROM)
Flash Memory - These devices are covered in Section 10.
How the Device Works
The read only memory cell usu-
ally consists of a single transistor
(ROM and EPROM cells consist
of one transistor, EEPROM cells
consists of two transistors). The
gate threshold voltage of the
transistor determines whether it
is a “1” or “0”. During the read
cycle, a voltage is placed on the
gate of the cell. Depending on
the programmed threshold volt-
age, the transistor will or will not
drive a current. The sense ampli-
fier will transform this current,
or lack of current, into a “1” or
“0”. Figure 9-1 shows how a
read only memory works.
INTEGRATED CIRCUIT ENGINEERING CORPORATION
9-1
9ROM, EPROM, & EEPROM TECHNOLOGY
19956
Source: ICE, "Memory 1996"
Column
Row
Cell
Selected
Sense Amplifier
Current Detector
To Output Buffer
Figure 9-1. Read Only Memory Schematic
