In digital electronics, a Programmable Logic Array (PLA) is a type of programmable logic device used for implementing combinational logic circuits.
Understanding PLA
A PLA is essentially a logic device that can be configured by the user to perform specific logical functions. Its structure allows for flexibility in creating complex logic circuits without the need for traditional fixed-function logic gates like AND, OR, NAND, or NOR gates wired together manually.
How PLA Works
The core structure of a PLA, as highlighted in the reference, consists of two main programmable parts:
- Programmable AND gate planes: This plane takes the input variables and their complements. Each AND gate in this plane can be programmed to form a specific product term (an ANDing of selected input variables or their complements).
- Programmable OR gate planes: This plane receives the outputs from the AND gates. Each OR gate in this plane can be programmed to sum selected product terms from the AND plane.
The outputs from the OR gates can then be conditionally complemented, meaning they can be output directly or inverted.
This two-level AND-OR structure allows the PLA to implement sum-of-products (SOP) logic expressions, which can represent any combinational logic function.
Key Features and Use Cases
PLAs are valuable in digital design for several reasons:
- Flexibility: They can implement a wide range of combinational logic functions.
- Reduced Complexity: They can often replace multiple discrete logic gates, simplifying circuit design and reducing board space.
- Prototyping and Development: Their programmability makes them useful for quick prototyping and design iteration.
- Custom Logic: They allow engineers to create custom integrated circuits (ICs) for specific applications.
Common Applications:
- Implementing truth tables
- Decoding addresses in memory systems
- Creating custom control logic
- Realizing finite state machine logic (specifically the combinational logic part)
PLA vs. Other Programmable Logic Devices
While PLAs are a type of programmable logic device, other types exist, such as Programmable Array Logic (PALs) and Complex Programmable Logic Devices (CPLDs). A key distinction often made is that in a PAL, only the AND plane is programmable, while the OR plane is fixed. The PLA, with both planes programmable, offers greater flexibility but can sometimes be more complex or slower than a PAL for certain functions.
Understanding PLAs is fundamental to the study of digital logic design and the evolution of integrated circuits.
