EPROM : Definition, Types And Comparison

Erasable and Programmable ROM (EPROM)

EPROM : Definition, Types And Comparison

As the name suggests, these ROMs can be erased and programmed again and again. The commonly used techniques for erasing are as follows:

• Erasing using ultraviolet radiation.
• Erasing using electricity.

The erasable programmable ROM using ultraviolet erasing is called as EPROM.

And the ROM that uses the electrical voltage for erasing is known as electrically alterable ROM (EAROM).

Ultraviolet Erasable Programmable ROM

The EPROM which can be erased using the ultra violet rays is shown in Fig.                

General Structure Of EPROM

This EPROM there is a quartz lid or window on

the package.

We can erase this PROM by exposing it to the ultraviolet (UV) light through the quartz window.

The exposure period should be 10 to 15 minutes.

Note that the UV rays will erase all the cells at time and all the locations in the erased EPROM will store a 1.

This memory chip can be reused many times. Hence, it is suitable for product development, experimental projects and college laboratories.

EEPROM (E²PROM : Electrically Erasable Programmable Read Only Memory)

The long form of EEPROM is Electrically Erasable PROM.

These are very similar to EPROMs and use the MOS circuitry.

The data is stored in the form of charge. Presence of charge represents 1 and its absence represents a 0.

A voltage as low as 20 to 25 volts can be used for erasing or programming.

It is possible to erase and program a particular memory location. This is called selective erasing. It is not necessary to erase all the memory locations.

It is however possible to erase all the location within about 10 ms by operating the EEPROM in a special mode.

This period is very short as compared to that required for the ultraviolet radiation erasing used for EPROM.

Another advantage of EEPROM is that it can be erased and reprogrammed when the memory chip is connected in the circuit.

However the disadvantage of EEPROM is very expensive and has very low density.

Comparison of EPROM and EEPROM

Comparison Between EPROM and EEPROM

Flash Memory

It is a special type of RAM.

It is a non-volatile memory which is powered continuously.

The erasing and programming of this memory takes place block by block.

Due to this process, the flash memories are faster than EEPROM which erase and write new data at the byte level.

Flash memory cannot be used as a random  access memory because RAM is to be addressed at a byte level.

The sections of flash memory chip are organized in such a way that they can be erased instantaneously or in a flash. Hence its name is "Flash Memory".

A special technique called Fowler-Nordheim tunnelling is used for erasing the contents.

Important features of a flash memory are high speed, low operating voltages, low power consumption and durability.

Typical application areas are digital camera, embedded controllers, cellular phones, etc.