Programmable Logic Devices
- A programmable logic device is an IC that is user configurable and is capable of implementing logic functions.
- It is an LSI chip that contains a ‘regular’ structure and allows the designer to customize it for any specific application, i.e. it is programmed by the user to perform a function required for this application.
- A PLD contains a large number of gates, flip-flops, and registers that interconnected on the chip. Many of the connections, however, are fusible links that can be broken.
- The IC is said to be programmable because of the specific function of the IC for a given application determined by the selective breaking of some of the interconnections while leaving others intact.
- The ‘fuse blowing’ process can do either by the manufacturer in accordance with the customer’s instructions, or by the customer himself which called ‘programming’ because it produces the desired circuit pattern interconnecting the gates, flip-flops, registers, and so on.
- PLDs can reprogrammed in a few seconds and hence give more flexibility to experiment with designs.
The advantages of PLDs over fixed function ICs are as follows:
Low development cost
Less space requirement
Less power requirement
Easy circuit testing
Easy design modification
High design security
Less design time
High switching speed
Read Only Memory (ROM)
- A read-only memory (ROM) essentially a memory device in which permanent binary information stored.
- Once the pattern established, it stays within the unit when the power turned off and on again.
- ROMs used to store information which is of a fixed type, such as tables for various functions, fixed data, and instructions.
The advantages of using a ROM as a PLD are the following:
Ease of design since no simplification or minimization of the logic function required.
Designs can change, modified rapidly.
It is usually faster than discrete MSI/SSI circuit.
There are a few disadvantages also of ROM based circuits, such as:
- Non-utilization of complete circuit
- Increased power requirement
- Enormous increase in size with increase in the number of input variables making it impractical