Programmable Logic Array

In Digital design, we often use a device to perform multiple applications. The device configuration is changed (reconfigured) by programming it. Such devices are known as programmable devices. It is used to build reconfigurable digital circuits. The following are the popular programmable device

  • PLA - Programmable Logic Array
  • PAL - Programmable Array Logic
  • CPLD - Complex Programmable Logic Device (Click here for more details)
  • FPGA - Field-Programmable Gate Array (Click here for more details)

PLA: Programmable Logic Array is a programmable device used to implement combinational logic circuits. The PLA has a set of programmable AND planes, which link to a set of programmable OR planes, which can then be conditionally complemented to produce an output. This layout allows for a large number of logic functions to be synthesized in the sum of products canonical forms.

Suppose we need to implement the functions: X = A'BC + ABC + A'B'C' and Y = ABC + AB'C. The following figures shows how PLA is configured. The big dots in the diagram are connections. For the first AND gate (left most), A complement, B, and C are connected, which is first minterm of function X. For second AND gate (from left), A, B, and C are connected, which forms ABC. Similarly for A'B'C', and AB'C. Once the minterms are implemented. Now we have to combine them using OR gates to the functions X, and Y.

One application of a PLA is to implement the control over a data path. It defines various states in an instruction set, and produces the next state (by conditional branching).

Note that the use of the word "Programmable" does not indicate that all PLAs are field-programmable; in fact many are mask-programmed during manufacture in the same manner as a ROM. This is particularly true of PLAs that are embedded in more complex and numerous integrated circuits such as microprocessors. PLAs that can be programmed after manufacture are called FPLA (Field-programmable logic array).


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