Read Only Memory(ROM)

READ-ONLY MEMORY

A ROM is essentially a memory device in which permanent binary information is stored.

The inputs provide the address for memory and the outputs give the data bits of the stored word that is selected by the address. The number of words in a ROM is determined from the fact that k address input lines are needed to specify 2^k words.

Fig: Internal logic of a 32× 8 ROM _×

The five inputs are decoded into 32 distinct outputs by means of a 5 32 decoder. Each output of the decoder represents a memory address. The 32 outputs of the decoder are connected to each of the eight OR gates. Each OR gate must be considered as having 32 inputs. Each output of the decoder is connected to one of the inputs of each OR gate. Since each OR gate has 32 input connections and there are 8 OR gates, the ROM contains 32 x 8 = 256 internal connections.

A programmable connection between two lines is logically equivalent to a switch that can be altered to be either closed (meaning that the two lines are connected) or open (meaning that the two lines are disconnected). The programmable intersection between two lines is sometimes called a cross point.

For example, programming the ROM according to the truth table given by table. Every 0 listed in the truth table specifies the absence of a connection and every 1 listed specifies a path that is obtained by a connection.

Types of ROMs

The required paths in a ROM may be programmed in four different ways.

ü    The first is called mask programming and is done by the semiconductor company during the last fabrication process of the unit. This procedure is costly because the vendor charges the customer a special fee for custom masking the particular ROM.

ü    For small quantities, it is more economical to use a second type of ROM called programmable read-only memory- PROM. The fuses in the PROM are blown by the application of a high-voltage pulse to the device through a special pin. A blown fuse defines a binary 0 state and an intact fuse gives a binary 1 state. The hardware procedure for programming ROMs or PROMs is irreversible and once programmed, the fixed pattern is permanent and cannot be altered.

ü    A third type of R OM is the erasable PROM or EPROM, which can be restructured to the initial state even though it has been programmed previously. When the EPROM is placed under a special ultraviolet light for a given length of time. the shortwave radiation discharges the intern al floating gates that serve as the programmed connections. After erasure, the EPROM returns to its initial state and can be reprogrammed to a new set of values.

The fourth type of ROM is the electrically erasable PROM (EEPROM or E²PROM). This device is like the EPROM except that the previously programmed connections can be erased with an electrical signal instead of ultraviolet light. The advantage is that the device can be erased without removing it from its socket.

ü Flash memory devices are similar to EEPROMs, but have additional built-in circuitry to selectively program and erase the device in-circuit, without the need for a special programmer. They have widespread application in modern technology in cell phones, digital cameras, set-top boxes, digital TV, telecommunications, non volatile data storage and microcontrollers. Their low consumption of power makes them an attractive storage medium for laptop and notebook computers.

Combinational PLDs

The PROM is a combinational programmable logic device (PLD)-an integrated circuit with programmable gates divided into an AND array and an OR array to provide an AND-OR sum of-product implementation.

ü    There are three major types of combinational PLDs, differing in the placement of the programmable connections in the AND-OR array.

The PROM has a fixed AND array constructed as a decoder and a programmable OR array. The programmable OR gates implement the Boolean functions in sum-of-mintenns form.

The PAL has a programmable AND array and a fixed OR array. The AND gates are programmed to provide the product terms for the Boolean functions, which are logically summed in each OR gate.

The most flexible PLD is the PLA, in which both the AND and OR arrays can be programmed. The product terms in the AND array may be shared by any OR gate to provide the required sum-of-products implementation.