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Thursday, 28 November 2019

ROM Working, Types and Application

Working Principle of ROM

Though the current ROMs do not make use of diode matrix but it will help in understanding the working of ROM. The principle on which ROM is based can be explained with the help of Figure which shows storage in a diode ROM. A storage cell is capable of storing a 1 or a 0. A ROM can be formed from a matrix of diodes and such memories are called diode matrix ROM.
Storage in a Diode ROM
A storage cell in the diode matrix is located at each intersection of a row line and column line. For storing a 0 the diode is not connected and for storing a 1 the diode is connected at the intersection of row line and column line as shown in Figure. These coupling binary cells are normally fixed at the time of manufacture and cannot be altered later. These ROMs are either made as per users requirement or are available in standard form for special purposes such as binary-to-gray code conversion for determining the squares of numbers etc.

For explaining the working of diode matrix ROM let us consider a memory which can store eight words each of 4 bits and any one of these words can be read whenever required. The connections for such a ROM are shown in Figure. The words to be stored from location 0 to 7 are given in Table - 1.

For addressing or accessing eight locations we need eight lines or we may use a three line to eight line decoder and we will have only three address lines. This decoder is made inside the ROM and with the three input address line we can access any of the eight locations by assigning proper input address signal. The address signal required for accessing any location from 0 - 7 is shown in Table - 2 and is called look up table for ROM.

Table – 1, Location or Address and Words to be Stored

Location or Address
Word to be stored

Table – 2, Binary Address or Input Signal and Stored Word

Address A2 A1 A0
Stored word D2 D1 D0

In Figure, we have shown the use of three to eight line decoder. We may extend the capacity of such memories. In general, a ROM with capacity to store in words for n bits is known as m x n ROM and is represented as shown in Figure.

Working : Let us see how the circuit of ROM (Figure) works. The address of location which we wish to read is given as input signal to the decoder say we wish to read the contents of 5th location so we give input 101 to the decoder and decoder will activate the line corresponding to 5th row. As no diode is connected between 5th row line and 1, 2, 3 and 4th column line so all the outputs will be 0s, i.e., the output word will be 0000 and this is what we expect. Now let us examine what happens when 011 is the address signal, this signal will make the decoder to activate line corresponding to 3rd row line. As no diode is connected between this row line and first column line the output D3 will be a 0, at the junction of row line 3 and column line 2, a diode is connected, so D2 will be a 1. Similarly D1 will be a 0 and D0 will be a 1. Hence the word at the output D3 D2 D1 D0 will be 0101 as we should have.

Types of ROM in Digital Electronics

Depending upon the methodology of programming, crusing and reprogramming information into ROMs, these are classified as

1. Mask-Programmed ROM (MROM)
2. Programmable ROM (PROM)
3. Erasable Programmable ROM (EPROM)
4. Electrically Erasable Programmable ROM (EEPROM)
5. Flash ROM.

• MROMs are permanently programmed by the manufacturer during the fabrication process by using a custom designed mask as per the system design specification. These are non-programmable ROMs.

• In case of PROMs, the programming is done by the customer with the help of a special gadget called PROM programmer. These are one time programmable ROMs.

• An EPROM can be erased and reprogrammed as many times as desired. Once programmed, it is non-volatile, i.e., it holds the stored data indefinitely.

• The stored information can however be erased by exposing the chip to ultra-violet (UV) radiation through a transparent window on the top of the chip meant for the purpose. These EPROMs are referred to as UVEPROMs.

• EEPROMs, also called EAPROMs (Electrically Alterable PROMs) can be erased and programmed by the application of controlled electric pulses the IC. EEPROM is a rugged, low power low density semiconductor device and it occupies less space. The cost is higher as compared to EPROMs.

• The flash memory combines the low cost and high density features of EPROM and in-circuit electrical erasability of EEPROM without compromising on the high speed access of both.

Differences between EPROM and UVEPROM

S. No.
Erasure and programming is done with electrical pulses.
Erasure and programming is done with ultra violet light.
Erasure and reprogramming is possible when the EEPROM is still in the circuit.
Erasure and reprogramming should be done by taking out the UVEPROM chip from the circuit.
Easy to construct
Difficult to construct.
The voltage on the floating gate permits the storage.
The photocurrent from the insulated gate structure permits the storage.
Speed of operation is more.
Speed of operation is less.
Shorter time of erasure.
Longer time of erasure.
Ability to erase and write individual bytes
Ability to erase and write memory array.
Low density
High density.
Suitable for field and remote control applications.
Suitable for experimental projects, product development and college labs.

Applications of ROM in Digital Electronics

Some of the applications of ROM in Digital Electronics are

1. ROMs are used for a variety of tasks within a digital system. They can be used as a direct substitute for any random logic of AND, OR and NOR gates.

2. ROMs are used to store bootstrap program that loads operating system program available in secondary memory and language interpreters in personal and business computers and to store, monitor or control programs in microcomputer and microprocessor based systems like electronic games, electronic cash registers, electronic scales and microcomputer controlled automobile fuel injection.

3. A very significant application of MOS ROM is for character generation. This includes display control for moving billboards and LED arrays.

4. They can also be used for code conversion, e.g., ASCII to EBCDIC conversion.

5. A ROM and a DAC can be used to generate sine waves, saw-tooth waves, triangular waves and square waves.

6. A ROM can be used to implement any or a set of logic expressions and is therefore used in the design of combinational circuits.

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