Title: Design and Implementation of Read-Only Memory (ROM) Using Discrete Components

Objective:

In this lab we will learn how a decoder can be used to construct the read-only-memory (ROM).

Lab Equipment and Components:

• 74LS138 3x8 decoder

• 1N4148 signal diodes

Introduction:

Read-Only Memory (ROM) is the primary memory unit of any computer system along with the Random Access Memory (RAM), but unlike RAM, in ROM, the binary information is stored permanently. This information to be stored is provided by the designer and is then stored inside the ROM . Once, it is stored, it remains within the unit, even when power is turned off.

The information is embedded in the ROM, in the form of bits, by a process known as programming the ROM. Programming is used to refer to the hardware procedure which specifies the bits that are going to be inserted in the hardware configuration of the device.

A typical ROM can have hundreds to millions of gates. In order to show the internal logic diagram for such technologies in a concise form, it is necessary to have special symbols. Figure shows the conventional and array logic symbols for a multiple input AND and a multiple input OR gate.

The block diagram for the ROM is as given below

          +--------------+

          |  k           |

     k ---+ 2   X  n ROM |----> n

 (Address |              | (Data   

  Inputs) +--------------+   Outputs)

Fig. ROM with 2^k x n bit storage                       

Block Structure

• It consists of k input lines and n output lines.

• The k input lines is used to take the input address from where we want to access the content of the ROM.

• Since each of the k input lines can be either 0 or 1, so there are 2^k total addresses which can be referred to by these input lines and each of these addresses contain n bit information, which is given out as the output of the ROM.

• Such a ROM is specified as 2^k x n ROM.

Internal Structure

• It consists of two basic components – Decoder and OR gates.

• A Decoder is a combinational circuit which is used to decode any encoded form (such as binary, BCD) to a more known form (such as decimal form ) .

• In ROM, the input to a decoder will be in binary form and the output will represent its decimal equivalent.

• The Decoder is represented as l x 2^l, that is, it has l inputs and has 2^l outputs, which implies that it will take l-bit binary number and decode it into one of the 2^l decimal number.

• All the OR gates present in the ROM will have outputs of the decoder as their input.

• The output of one OR gate represents the data saved in that Bit. At the addresses where we need to store a 1, that decoder output is connected to the input of the OR gate.

It is simple to construct the ROM using diode based OR gates as shown in figure below. Wherever we want to write a 1, the corresponding switch will be closed, hence connecting the diode OR gate input to that decoder output.