Page 211 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes EPROM
This is an Erasable Programmable Read-Only Memory. This form of semiconductor memory
can be programmed and then erased at a later time. This is normally achieved by exposing the
silicon to ultraviolet light. To enable this to happen there is a circular window in the package
of the EPROM to enable the light to reach the silicon of the chip. When the PROM is in use, this
window is normally covered by a label, especially when the data may need to be preserved
for an extended period. The PROM stores its data as a charge on a capacitor. There is a charge
storage capacitor for each cell and this can be read repeatedly as required. However it is found
that after many years the charge may leak away and the data may be lost. Nevertheless, this type
of semiconductor memory used to be widely used in applications where a form of ROM was
required, but where the data needed to be changed periodically, as in a development environment,
or where quantities were low.
EEPROM
This is an Electrically Erasable Programmable Read-Only Memory. Data can be written to it and
it can be erased using an electrical voltage. This is typically applied to an erase pin on the chip.
Like other types of PROM, EEPROM retains the contents of the memory even when the power is
turned off. Also like other types of ROM, EEPROM is not as fast as RAM.
Flash memory

Flash memory may be considered as a development of EEPROM technology. Data can be written
to it and it can be erased, although only in blocks, but data can be read on an individual cell basis.
To erase and re-program areas of the chip, programming voltages at levels that are available within
electronic equipment are used. It is also non-volatile, and this makes it particularly useful. As
a result Flash memory is widely used in many applications including memory cards for digital
cameras, mobile phones, computer memory sticks and many other applications.
DRAM
Dynamic RAM is a form of random access memory. The DRAM uses a capacitor to store each
bit of data, and the level of charge on each capacitor determines whether that bit is a logical
1 or 0. However, these capacitors do not hold their charge indefinitely, and therefore the
data needs to be refreshed periodically. As a result of this dynamic refreshing it gains its
name of being a dynamic RAM. The DRAM is the form of semiconductor memory that is
often used in equipment including personal computers and workstations where it forms
the main RAM for the computer.
SRAM
Static Random Access Memory. This form of semiconductor memory gains its name from
the fact that, unlike DRAM, the data does not need to be refreshed dynamically. It is able to
support faster read and write times than DRAM (typically 10 ns against 60 ns for DRAM),
and in addition its cycle time is much shorter because it does not need to pause between
accesses. However it consumes more power, is less dense and more expensive than DRAM. As
a result of this it is normally used for caches, while DRAM is used as the main semiconductor
memory technology.
SDRAM
Synchronous DRAM. This form of semiconductor memory can run at faster speeds than
conventional DRAM. It is synchronized to the clock of the processor and is capable of keeping two
sets of memory addresses open simultaneously. By transferring data alternately from one set of
addresses, and then the other, SDRAM cuts down on the delays associated with non-synchronous
RAM, which must close one address bank before opening the next.

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