Read-only memory (ROM), also known as
firmware, is an integrated circuit programmed with
specific data when it is manufactured. ROM chips are used not
only in computers, but
in most other electronic items as well. In this edition of HowStuffWorks,
you will learn about the different types of ROM and how each
works. This article is one in a series of articles dealing
with computer memory, including:
Let's start by identifying the different types of ROM.
ROM Types
There are five basic ROM types:
- ROM
- PROM
- EPROM
- EEPROM
- Flash memory
Each type has unique
characteristics, which you'll learn about in this article, but
they are all types of memory
with two things in common:
- Data stored in these chips is nonvolatile -- it
is not lost when power is removed.
- Data stored in these chips is either unchangeable
or requires a special operation to change (unlike RAM, which
can be changed as easily as it is read).
This means
that removing the power source from the chip will not cause it
to lose any data.
ROM at Work
Similar to RAM, ROM chips
(Figure 1) contain a grid of columns and rows. But where the
columns and rows intersect, ROM chips are fundamentally
different from RAM chips. While RAM uses transistors
to turn on or off access to a capacitor
at each intersection, ROM uses a diode to connect the
lines if the value is 1. If the value is 0, then the lines are
not connected at all.
Figure 1. BIOS uses Flash memory, a type of
ROM. |
A diode
normally allows current to flow in only one direction and has
a certain threshold, known as the forward breakover,
that determines how much current is required before the diode
will pass it on. In silicon-based items such as processors
and memory chips, the forward breakover voltage is
approximately 0.6 volts. By taking advantage of the unique
properties of a diode, a ROM chip can send a charge that is
above the forward breakover down the appropriate column with
the selected row grounded to connect at a specific cell. If a
diode is present at that cell, the charge will be conducted
through to the ground, and, under the binary
system, the cell will be read as being "on" (a value of
1). The neat part of ROM is that if the cell's value is 0,
there is no diode at that intersection to connect the column
and row. So the charge on the column does not get transferred
to the row.
As you can see, the way a ROM chip works necessitates the
programming of perfect and complete data when the chip is
created. You cannot reprogram or rewrite a standard ROM chip.
If it is incorrect, or the data needs to be updated, you have
to throw it away and start over. Creating the original
template for a ROM chip is often a laborious process full of
trial and error. But the benefits of ROM chips outweigh the
drawbacks. Once the template is completed, the actual chips
can cost as little as a few cents each. They use very little
power, are extremely reliable and, in the case of most small
electronic devices, contain all the necessary programming to
control the device. A great example is the small chip in the
singing
fish toy. This chip, about the size of your fingernail,
contains the 30-second song clips in ROM and the control codes
to synchronize the motors to
the music.
PROM
Creating ROM chips totally from scratch
is time-consuming and very expensive in small quantities. For
this reason, mainly, developers created a type of ROM known as
programmable read-only memory (PROM). Blank PROM chips
can be bought inexpensively and coded by anyone with a special
tool called a programmer.
PROM chips (Figure 2) have a grid of columns and rows just
as ordinary ROMs do. The difference is that every intersection
of a column and row in a PROM chip has a fuse
connecting them. A charge sent through a column will pass
through the fuse in a cell to a grounded row indicating a
value of 1. Since all the cells have a fuse, the initial
(blank) state of a PROM chip is all 1s. To change the
value of a cell to 0, you use a programmer to send a specific
amount of current to the cell. The higher voltage breaks the
connection between the column and row by burning out
the fuse. This process is known as burning the PROM.
Figure
2 |
PROMs can only be programmed once. They are more fragile
than ROMs. A jolt of static electricity can easily cause fuses
in the PROM to burn out, changing essential bits from 1
to 0. But blank PROMs are inexpensive and are great for
prototyping the data for a ROM before committing to the costly
ROM fabrication process.
EPROM
Working with ROMs and PROMs can be a
wasteful business. Even though they are inexpensive per chip,
the cost can add up over time. Erasable programmable
read-only memory (EPROM) addresses this issue. EPROM chips
can be rewritten many times. Erasing an EPROM requires a
special tool that emits a certain frequency of ultraviolet (UV)
light. EPROMs are configured using an EPROM programmer
that provides voltage at specified levels depending on the
type of EPROM used.
Once again we have a grid of columns and rows. In an EPROM,
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
curious process called Fowler-Nordheim tunneling.
Tunneling is used to alter the placement of electrons
in the floating gate.
An electrical charge, usually 10 to 13 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 frequency of 253.7. 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.
An EPROM eraser is not selective, it will erase the entire
EPROM. The EPROM must be removed from the device it is in and
placed under the UV light of the EPROM eraser for several
minutes. An EPROM that is left under too long can become
over-erased. In such a case, the EPROM's floating gates
are charged to the point that they are unable to hold the
electrons at all.
For more information on ROM and other types of computer
memory, check out the links on the next page!
