How PAL, PLA, and FPGA work?

Here's a clear explanation of how PAL, PLA, and FPGA work, focusing on their internal logic and how they implement digital functions:

 1. PLA (Programmable Logic Array)

How it works:

A PLA consists of:

• A programmable AND array

• A programmable OR array

Concept:

• Implements sum-of-products (SOP) logic:
  F = (A AND B) OR (A' AND C) etc.

• You program which input combinations go to the AND gates.

• Then, you program which product terms (AND outputs) go to the OR gates.

Structure:

ngin

Inputs → Inverters → Programmable AND → Programmable OR → Outputs

Features:

• Fully programmable logic.

• Flexible and general-purpose.

• Good for implementing multiple logic functions sharing terms.

 2. PAL (Programmable Array Logic)

How it works:

A PAL is a simplified version of a PLA:

• A programmable AND array

• A fixed OR array

Concept:

• You still implement SOP logic, but with less flexibility:

• You can only OR together a fixed number of product terms.

• So, complex functions may not fit directly.

Structure:

scss

Inputs → Inverters → Programmable AND → Fixed OR → Outputs

Features:

• Faster than PLA due to simpler structure.

• Less flexible, but more reliable and cheaper.

• Often used for simple control logic or glue logic.

 3. FPGA (Field-Programmable Gate Array)

How it works:

An FPGA is a large grid of:

• Configurable Logic Blocks (CLBs) with Look-Up Tables (LUTs)

• Programmable interconnects

• I/O blocks

Concept:

• Each LUT stores the truth table of a logic function (like a tiny ROM).

• You program which logic goes into each LUT.

• Then you configure how the logic blocks are wired together using programmable switches.

Structure:

objectivec

Inputs → Routing → CLBs (LUT + FF) → Routing → Outputs

Features:

• Very flexible: can implement complex digital systems (even CPUs).

• Supports sequential logic (flip-flops), RAM, DSP, etc.

• Programmable via HDL (VHDL/Verilog) and tools like Vivado or Quartus.

Comparison Summary