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What are FPGA LUTs?

Published in FPGA Logic 3 mins read

FPGA LUTs (Field Programmable Gate Array Look-Up Tables) are fundamental building blocks within FPGAs used to implement digital logic functions. They are essentially configurable memory units functioning as truth tables.

Understanding LUTs

  • Basic Function: A LUT is a small memory array that stores the output values for every possible combination of input values for a particular logic function.

  • Truth Table Implementation: The LUT implements a logic function by storing its truth table. For example, a 2-input LUT can implement any logic function of two variables (AND, OR, XOR, etc.) because it has 22 = 4 memory locations, one for each possible input combination (00, 01, 10, 11).

  • Configuration: During FPGA configuration, the memory cells of the LUT are programmed with the desired output values for each input combination, effectively defining the logic function it performs.

  • Size and Complexity: LUTs come in various sizes, defined by the number of inputs they can handle (e.g., 4-input LUT, 6-input LUT). A larger LUT can implement more complex logic functions directly. Modern FPGAs commonly use 6-input LUTs (6-LUTs).

How LUTs Work

  1. Inputs: The input signals to the LUT serve as the address lines to its internal memory.

  2. Memory Lookup: Based on the input combination, the LUT selects a specific memory location.

  3. Output: The value stored in that memory location is output as the result of the logic function.

Example: 2-Input AND Gate using a LUT

To implement a 2-input AND gate using a 2-input LUT:

Input A Input B Output (A AND B)
0 0 0
0 1 0
1 0 0
1 1 1

The LUT's memory would be programmed with the values 0, 0, 0, and 1, corresponding to the output column in the truth table. When the inputs A and B are both 1, the LUT would address the location containing '1' and output a '1'.

Benefits of LUTs in FPGAs

  • Flexibility: LUTs provide enormous flexibility, allowing designers to implement a wide range of digital circuits using the same hardware.

  • Reconfigurability: The logic function of a LUT can be changed simply by reprogramming its memory cells, enabling FPGAs to be reconfigured for different applications.

  • Custom Logic: LUTs are essential for implementing custom logic tailored to the specific needs of an application.

In conclusion, FPGA LUTs serve as the fundamental building blocks that enable the flexible and reconfigurable nature of FPGAs by acting as programmable truth tables for implementing diverse digital logic functions.

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