Read Only Memory
There are five basic ROM types:
All of them share the same characteristics: they are non-volatile and requires a special process to reprogram it.
PROM
PROM
have a grid of columns and rows just as ordinary ROMs do. The difference is
that every intersection of a column and row in a PROM chip has a fuse connecting
them. A charge sent through a column will pass through the fuse in a cell
to a grounded row indicating a value of "1." Since all the cells
have a fuse, the initial, or blank, state of a PROM chip is all "1"s.
To change the value of a cell to "0," you use a special tool called
a programmer to send a specific amount of current to the cell. The higher
voltage breaks the connection between the column and row by burning out the
fuse. This process is known as burning the PROM.
PROMs can only be programmed once. They are more fragile than ROMs. A jolt of static electricity can easily cause fuses in the PROM to burn out, changing essential bits from "1" to "0". But blank PROMs are inexpensive and are great for prototyping the data for a ROM before committing to the costly ROM fabrication process.
Erasable Programmable Read-Only Memory
EPROM addresses the issue of reprogramming a ROM; these chips can be rewritten many times. Erasing an EPROM requires a special tool that emits a certain frequency of ultraviolet (UV) light. EPROMs are configured using an EPROM programmer that provides voltage at specified levels, depending on the type of EPROM used.
To rewrite an EPROM, you must erase it first. To erase it, you must supply a level of energy strong enough to break through the negative electrons blocking. In a standard EPROM, this is best accomplished with UV light at a frequency of 253.7. Because this particular frequency will not penetrate most plastics or glasses, each EPROM chip has a quartz window on top of the chip. The EPROM must be very close to the eraser's light source, within an inch or two, to work properly. An EPROM eraser is not selective; it will erase the entire EPROM. The EPROM must be removed from the device it is in and placed under the UV light of the EPROM eraser for several minutes.
EEPROMs and
Flash Memory
Though EPROMs are a big step up from PROMs in terms of reusability, they still require dedicated equipment and a labor-intensive process to remove and reinstall them each time a change is necessary. Also, changes cannot be made incrementally to an EPROM. The whole chip must be erased. Electrically Erasable Programmable Read-Only Memory (EEPROM) chips remove the biggest drawbacks of EPROMs.
The EEPROM chips can be reprogrammed electrically. The chip doesn't have to be removed from its place, and programming can be selective. The only drawback is the speed of programming. For applications that require fast writing, EEPROM cannot perform well.
Manufacturers responded to this limitation with flash memory, a type of EEPROM that uses in-circuit wiring to erase by applying an electrical field to the entire chip or predetermined sections of it called blocks. Flash memory works much faster than traditional EEPROMs because it writes data in chunks, usually 512 bytes in size, instead of a byte at a time.
NEXT STOP: Memory: memory address decoding
