Hardware Reference
In-Depth Information
Althoughwesaythesechipsareblankwhennew,theyaretechnicallypreloadedwithbin-
ary 1s. In other words, a 1Mb ROM chip used in a PC would come with 1 million (actu-
ally 1,048,576) bit locations, each containing a binary 1. A blank PROM can then be pro-
grammed, which is the act of writing to it. This usually requires a special machine called
a device programmer, ROM programmer, or ROM burner (see
Figure 5.3
).
Figure 5.3
Typical gang (multisocket) device programmer (PROM burner).
Programming the ROM is sometimes referred to as
burning
it because that is technically
an apt description of the process. You can think of each binary 1 bit as a fuse, which is
intact. Most chips run on 5 volts, but when a PROM is programmed, a higher voltage
(normally 12 volts) is placed at the various addresses within the chip. This higher voltage
actually blows or burns the fuses at the desired locations, thus turning any given 1 into a
0. Although you can turn a 1 into a 0, you should note that the process is irreversible; that
is, you can't turn a 0 back into a 1.
The device programmer examines the program you want to write into the chip and then
selectively changes only the 1s to 0s where necessary in the chip.
PROM chips are often referred to as
one-time programmable (OTP)
chips for this reason.
They can be programmed once and never erased. Most PROMs are inexpensive (about $3
for a typical PC motherboard PROM), so if you want to change the program in a PROM,
you discard it and program a fresh one with the new data.
The act of programming a PROM takes anywhere from a few seconds to a few minutes,
depending on the size of the chip and the algorithm the programming device uses.
Figure
5.3
shows an illustration of a typical PROM programmer that has multiple sockets. This
is called a
gang programmer,
and it can program several chips at once, saving time if you
