Types of ROM:
Memories
in the ROM family are distinguished by the methods used to write new data to
them (usually called programming or burning) and the number of times they can
be rewritten. This classification reflects the evolution of ROM devices from
hardwired to one-time programmable to erasable-and-programmable. A common
feature across all these devices is their ability to retain data and programs
forever, even when power is removed.
The very
first ROMs were hardwired devices that contained a preprogrammed set of data or
instructions. The contents of the ROM had to be specified before chip
production, so the actual data could be used to arrange the transistors inside
the chip! Hardwired memories are still used, though they are now called masked
ROMs to distinguish them from other types of ROM. The main advantage of a
masked ROM is a low production cost. Unfortunately, the cost is low only when
hundreds of thousands of copies of the same ROM are required, and no changes
are ever needed.
Another
type of ROM is the programmable ROM (PROM), which is purchased in an
unprogrammed state. If you were to look at the contents of an unprogrammed
PROM, you would see that all the bits are 1s. The process of writing your data
to the PROM involves a special piece of equipment called a device programmer,
which writes data to the device by applying a higher-than-normal voltage to
special input pins of the chip. Once a PROM has been programmed in this way,
its contents can never be changed. If the code or data stored in the PROM must
be changed, the chip must be discarded and replaced with a new one. As a
result, PROMs are also known as one-time programmable (OTP) devices. Many small
embedded microcontrollers are also considered one-time programmable, because
they contain built-in PROM.
An
erasable-and-programmable ROM (EPROM) is programmed in exactly the same manner
as a PROM. However, EPROMs can be erased and reprogrammed repeatedly. To erase
an EPROM, simply expose the device to a strong source of ultraviolet light.
(There is a "window" in the top of the device to let the ultraviolet
light reach the silicon. You can buy an EPROM eraser containing this light.) By
doing this, you essentially reset the entire chip to its
initialunprogrammedstate. The erasure time of an EPROM can be anything from 10
to 45 minutes, which can make software debugging a slow process.
Though
more expensive than PROMs, their ability to be reprogrammed made EPROMs a
common feature of the embedded software development and testing process for
many years. It is now relatively rare to see EPROMs used in embedded systems,
as they have been supplanted by newer technologies.
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