Read Only Memory ( ROM)

 

A read-only memory (ROM) is a device that includes both the decoder and the OR gates within a single IC package. The connections between the outputs of the decoder and the inputs of the OR gates can be specified for each particular configuration.

A ROM is essentially a memory (or storage) device in which permanent binary information is stored. The binary information must be specified by the designer and is then embedded in the unit to form the required interconnection pattern. ROMs come with special internal electronic fuses that can be "programmed" for a specific configuration. Once the pattern is established, it stays within the unit even when power is turned off and on again.

It consists of n input lines and m output lines. Each bit combination of the input variables is called an address. Each bit combination that comes out of the output lines is called a word. The number of bits per word is equal to the number of output lines, m. An address is essentially a binary number that denotes one of the min terms of n variables. The number of distinct addresses possible with n input variables is 2n. An output word can be selected by a unique address, and since there are 2n distinct addresses in a ROM, there are 2n distinct words that are said to be stored in the unit.

The word available on the output lines at any given time depends on the address value applied to the input lines. A ROM is characterized by the number of words 2n and the number of bits per word m.

Basic structure of ROM

Consider a 32 x 8 ROM. The unit consists of 32 words of 8 bits each. This means that there are eight output lines and that there are 32, distinct words stored in the unit, each of which may be applied to the output lines. The particular word selected that is presently available on the output lines is determined from the five input lines. There are only five inputs in a 32 x 8 ROM because 25 = 32, and with five variables, we can specify 32 addresses or min terms. For each address input, there is a unique selected word. Thus, if the input address is 00000, word number 0 is selected and it appears on the output lines. If the input address is 11111, word number 31 is selected and applied to the output lines. In between, there are 30 other addresses that can select the other 30 words.

Applications of ROM 

• Preprogrammed toy circuit,
• Preprogrammed robot circuit,
• Standard look up table,
• Arithmetic function table generator,
• User defined code generator,
• Character generator,
• Printable or displayable fonts table