Read-Only Memory (ROM): Definition, Working and Types

Read-Only Memory (ROM): Definition, Working and Types

Read-Only Memory (ROM): Definition, Working and Types

These memories in true sense are the read only memories, because they are programmed at the time of manufacturing.

The programming is done as per the requirements of the user and it is the last process in manufacturing of ROMs.

Such ROMs are called as custom programmed or mask programmed. We cannot change the programmed data after fabrication.

These ROMs are costly so are suitable only for bulk use (number of chips in millions).

They are used in TV, games, home computers, toy, etc.

ROM organisation array

A ROM is an array of selectively unidirectional contacts. The contacts can be open closed selectively to program the required information into a ROM.

A 16-bit ROM array is shown in Fig. It uses two 2-4 address decoders, 16 locations and 1-data output. So this is a 16 x 1 ROM.

The method of addressing is called as two dimensional, X-Y or coincident selection, addressing.

A unidirectional switch is included at the junction of every row and column.

The lower two address bits (A1  A0) are decoded by the decoder DL. The outputs of this decoder are used to select one out of the four rows.

The high two address bits (A3  A2) are decoded by the decoder DH The four outputs of this decoder are used to select one out of the four column sense amplifiers.

Diode Matrix

Diode matrix is the heart of the ROM array shown in Fig. It is formed by connecting a diode and a switch between each row and column. For example refer Fig.  which shows diode D03 connected between row 0 and column 3.

The chip select (CS) input is used to enable the output.

Programmable Read Only Memories (PROM)

A user can program these ROMs using a PROM programmer.

A PROM can be programmed only once after its fabrication. After programming the contents will become permanently fixed in the ROM.

The PROMs are manufactured without any data stored onto them (they are blank).

This memory is programmed by the user, It has nichrome or polysilicon wires that are arranged in a matrix form.

The functionality of this memory can be viewed as diodes or fuses. Fig. shows a single fused PROM cell.

Single Fused PROM cell

When the fuse is intact, the memory cell is configured as logic while if the fuse is the memory cell is configured as logic 0.

This memory can be programmed by the user with the help of a special PROM programmer that selectively burns the fuses according to the bit pattern to be stored. This method is called as "burning the PROM". The PROMs are one time programmable. The information that is stored is permanent.