Types of ROM

Masked ROM:

In this type of ROM bits are stored permanently by marking and metallization process. This is done by manufacturers. This type of ROM can be programmed only one-by the manufacture.

PROM:

Data are written into a ROM at the time of manufacture. However, a programmable ROM (PROM) allows the data to be loaded by the user, by connecting a fuse between the emitter and the bit-line. PROMs provide flexible and economical storage for fixed programs and data, where high production volumes are involved. Initially, if the PROM contains all 1s, then at the ‘required location the user can insert 0’s by burning out the fuses using high current pulses of course, this process is irreversible. However, the storing in ROMs will be very expensive when only a small number is required. Thus, PROMs provide a faster and less expensive approach for storing.

EPROM:

The Erasable PROM chip allows the stored data to erased and new data can be reprogrammed. It provides more flexibility during the development phase of digital system. With resemblance to the dynamic memory cell, information is stored in a capacitor is very well insulated and its rate of discharge is low. Hence it retains the stored information for more than a year. Due to high insulation, the process of writing new information into a cell involves the application of a higher voltage. The high voltage is used to cause a temporary breakdown in insulation, and allow charge to be stored in the capacitor. The contents of EPROM cells can be erased by increasing the discharge rate of the storage capacitors. This can be accomplished by exposing the chip to ultraviolet light. All cells in the chip are erased at the same time.

EEPROM:

In an electricity erasable PROM, the contents of cells can be erased by the application of a high voltage. Advantages with EEPROMs are : it need not be physically removed for reprogramming and the process can be made selective since electrical erasure is used.

The advantages of ROM are:

• They are non-volatile
• They are cheaper than RAM
• They are static and do not refreshing
• They are more reliable than RAM as their circuit is simple.
• They are available in longer sizes than RAM.
• They are easier to interface than RAM.

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