Hardware Reference
In-Depth Information
Erasable programmable read-only memory
(EPROM)
was invented by the Israeli engineer
Dov Frohman in 1971. It is an array of floating-gate transistors individually programmed by an
electronic device that supplies higher voltages than those normally used in electronic circuits.
Programming is achieved via
hot carrier injection
onto the floating gate. Once programmed, an
EPROM can be erased only by exposing it to strong ultraviolet (UV) light. That UV light usually
has a wavelength of 235 nm for optimum erasure time. EPROMs are easily recognizable by the
transparent fused quartz window in the top of the package, through which the silicon chip can
be seen and which permits UV light to go through during erasing.
As the quartz window is expensive to make, one-time programmable (OTP) chips were
introduced; the only difference is that the EPROM chip is packed in an opaque package, so it
cannot be erased after programming. OTP versions are manufactured for both EPROMs them-
selves and EPROM-based microcontrollers. However, OTP EPROM (whether separate or part of
a larger chip) is being increasingly replaced by EEPROM for small amounts where the cell cost
isn't too important and flash memory is used for larger amounts.
A programmed EPROM retains its data for about 10 to 20 years and can be read an unlimited
number of times. The erasing window must be kept covered with a foil label to prevent acci-
dental erasure by sunlight. Old PC
basic input/output system
(BIOS) chips were often EPROMs,
and the erasing window was often covered with a label containing the BIOS publisher's name,
the BIOS revision, and a copyright notice.
Electrically erasable programmable read-only memory
(EEPROM) was developed in 1983
by George Perlegos at Intel. It was built on earlier EPROM technology, but used a thin gate ox-
ide layer so that the chip could erase its own bits without requiring a UV source. EEPROM is
programmed and erased using the process called field emission (more commonly known in the
industry as
Fowler-Nordheim tunneling
). EEPROM allows the user to selectively erase a single
location, a row, or the whole chip. This feature requires a complicated programming circuitry.
Because of this, the EEPROM cannot achieve the density of the EPROM technology.
Flash memory
was invented by Fujio Masuoka while working for Toshiba in 1984. Flash
memory incorporates the advantages and avoids the drawbacks of EPROM and EEPROM tech-
nologies. The flash memory can be erased and reprogrammed in the system without using a
dedicated programmer. It achieves the density of EPROM, but it does not require a window for
erasure. Like EEPROM, flash memory can be programmed and erased electrically. However, it
does not allow individual locations to be erased; the user can only erase a block or the whole
chip. Today, the BIOS programs of many high-performance PCs are stored in flash memory.
Most microcontrollers introduced today use on-chip flash memory as their program memory.
Flash memory chips have also been used in flash disk memory, personal digital assistants,
digital cameras, cell phones, and so on.
A simplified memory system block diagram is shown in Figure 1.2. A memory system may
consist of one or multiple memory chips. Both memory chips and memory systems are orga-
nized as an array of memory locations. A memory location may hold any number of bits (most
common numbers are 4 bits, 8 bits, 16 bits, 32 bits, and 64 bits). The memory organization of a
memory chip or a memory system is often indicated by
m
3
n;
where
m
specifies the number of
memory locations in the memory chip or memory system and
n
specifies the number of bits in
each location. Every memory location has two components:
contents
and
address
.
A memory location can be used to store data, instruction, and the status of peripheral
devices. The size of memory is measured in bytes; a byte consists of 8 bits. A 4-bit quantity is
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