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Erasable Programmable Read Only Memory (EPROM)
An amended form of a PROM wherein a customer or user can repeatedly erase the stored data or repeatedly program it according to his / her needs is called EPROM. In other words, an erasable PROM is called EPROM(Erasable Programmable Read Only Memory). It is a versatile type of memory which entirely depends on its user and there is also no restriction or limitation on its programming (i.e. programming can be done on it repeatedly. Remember that prior to programming, the entire previous data existing on this memory is erased or wiped out and only then programming can be undertaken). In ordinary life, example of an EPROM(Erasable Programmable Read Only Memory) operation is just similar to writing on a piece of paper with a pencil and erasing it repeatedly through an eraser, until you intentionally write something according to your wishes and leave it un-erased. Remember that programming in EPROM can be done by means of the application of a PROM burner or PROM programmer in a local laboratory or workshop. In EPROM(Erasable Programmable Read Only Memory), only MOS devices (generally N – MOSFETs) are used (i.e. these are manufactured through the application of MOS devices instead of bipolar devices). Further, most of EPROMs are compatible with MOS & TTL.
Figure 9.17 – Simplified PROM programming setup
A prevalent and popular EPROM(Erasable Programmable Read Only Memory) family is available in the 27XXX series, which is being fabricated by a number of manufacturers. A brief summary of some of the models available in this particular series, has been illustrated in figure 9.18. Upon viewing this table, it becomes apparent that outputs of all these models have been arranged in a 8- bit series or as one byte, as a consequence of which it is compatible with majority of the microprocessor systems. It must be also be inculcated in mind that after erasing EPROM(Erasable Programmable Read Only Memory) programming or after going through a fresh programming, its window is protected via an opaque sticker, so that it could be safeguarded against ultra – violet rays and sunlight. As a result of direct collision of sunlight on EPROM(Erasable Programmable Read Only Memory), erasing process gets accomplished in about one week’s time whereas it is completed in about 3 years period via fluorescent light within a room.
Figure 9.18 – 27XXX Series EPROMs
|EPROM 27XXX||Organization||Number of Bits|
According to erasing, there are following two types of an EPROM.
- UV EPROM
An EPROM(Erasable Programmable Read Only Memory), in which an ultra – violet light is reflected via a transparent window constructed above EPROM, for the purpose of erasing the stored data, is called UV EPROM(Erasable Programmable Read Only Memory). In the center of a UV EPROM’s IC chip, a round-shaped transparent quartz is built, from where light can be transmitted pretty easily. After programming once, whenever urgency of a re-programming is felt, first of all data previously existing on the memory has to be erased or wiped out. In order to eradicate or erase the previously stored data, a high-density ultra-violet light is thrown above a transparent window constructed in the middle of this IC chip, for a certain period of time (approx. 1 hour). As a result, the previous setting on the IC tends to change (i.e. previous data is wiped out completely) and the logical 1 state develops on the whole IC. As such, this IC gets ready once again for new programming. UV EPROM(Erasable Programmable Read Only Memory) is generally used where there is a need for changes in programs from time to time. In figure 9.19, an ultra-violet erasable PROM package has been illustrated.
Figure 9.19 -Ultra-violet erasable PROM package
Electrically Erasable PROM – EEPROM
An EPROM(Erasable Programmable Read Only Memory), in which stored data is erased or wiped out electrically, is called an electrically erasable PROM (i.e. EEPROM) or electrically PROM (EPROM). Sometimes, it is also known as E2 PROM. In other words, an ENPROM, which has the capacity of erasing data electrically and re – programming, is called E2PROM or EEPROM. This memory is non-volatile type just like an EPROM(Erasable Programmable Read Only Memory) and there is no need for application of any ultraviolet light for erasing its programs. Rather, data existing on memory can either totally or partially, or some of its bytes or any number of its bytes can be erased through the application of electrical pulses. As erasing operation takes place in an EEPROM electrically, therefore there is no need to unplug it from the circuit board (i.e. erasing operation can be performed without unplugging memory from the circuit board). Moreover, writing speed in an EEPROM is far less as compared to the writing speed of a RAM, therefore it is not commonly used on high speed circuits.
It must be remembered here that parts of EEPROM board can be re – programmed on one – time one – byte basis.
Previous Topic: Programmable Read Only Memory or PROM in Digital Electronics
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