Page 192 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

MemoryICs w 173

Due to their regular structures and the numbers of components produced,
memory devices used to lead semiconductor technology by approximately two
years. For example, DRAMs based on 0.35 micron processes became available
as early 1994, but other devices with these geometries didn’t hit the main-
stream until 1996. By 2002, however, high-end general-purpose logic processes
had become just as advanced as memory processes, and both memory and logic
foundries routinely offered 0.13 micron technologies.

PROMs
Because ROMs are constructed containing pre-defined data, they are an
expensive design option unless used for large production runs. As an alter-
native, programmable read-only memories
(PROMs) l7 were developed in the alf of the 199Os,
mid-1970s. PROMs are manufactured as AM-based computer memories
re asynchronous, which means
standard devices that may be electrically
hey weren‘t synchronized to the
programmed by the designer and are non-
ystem clock. Every new generation
volatile. PROMs are one-time programmable f computers used a new trick to
(OTP) and slightly slower than their ROM boost speed, and even the engi-
equivalents, but are significantly cheaper neers started to become confused
for small- to medium-sized production runs. by the plethora of names, such as
fast page mode (FPM), extended data
EPROMs ut (€DO), and burst €DO (BEDO). In
To satisfy the designers’ desire to eality, these were all based on
re DRAM concepts; the differ-
combine the advantages of DRAMs
ces are largely in how you wire
(programmable) and ROMs (nonvolatile),
he chips together on the circuit
erasable programmable read-only memories ard (OK, sometimes there is a bit
(EPROMs) l8 were developed in the late tweaking on the chips also).
1970s. EPROMs are electrically program- Over time, the industry
mable by the designer and are non-volatile, grated to synchronous DRAM
but can be erased and reprogrammed should (SDRAM), which is synchronized to
the designer so desire. An EPROM device the system clock and makes every-
one’s fives much easier. Once again,
is presented in a ceramic package with a however, SDRAM is based on core

DRAM concepts-it‘s all in the way
you tweak the chips and connect
17 In conversation, PROM is pronounced just like
the high school dance of the same name.