What is Look-Up Tables (LUTs) in FPGA (Look-Up Tables, LUT là gì?)

In this article we explore two questions:

• Location of LUT in an FPGA

• The role of LUT in an FPGA

1./ What is Look-Up Table (LUT)? Look-Up Table là gì?

+ What is Look-Up Tables (LUT): LUT or Look-Up Table, is a table that determines output values ​​for combinations of input values, it can be thought of similarly to the truth table of digital circuits. K-input LUT (k-LUT, LUT_k) is used to describe a LUT with k-input.

+ LUT classification: LUTs are classified based on the number of inputs to the LUT.

Example: two-input LUT (LUT2), three-input LUT (LUT3), four-input LUT (LUT4), five-input LUT (LUT5), six-input LUT (LUT6).

We see the truth table of basic gate logic, and how a LUT2 describes those basic gate logic. And the truth table of logic gates is the same as the truth table of that logic gate when described by LUT2.

2./ Location of LUT in an FPGA

An FPGA circuit consists of many configurable logic blocks called CLBs (Configuarable Logic Block).

=> Each CLB contains a number of logic elements called Logic Cells (LC).

==> Each LC includes binary inputs, and programmable outputs.

===> Inside each LC contains a logic function generator. The logic function generator can be implemented using a LUT (Look-Up Table memory) or a combination of mutiplexers and gates.

Example with Cyclone II Altera FPGA:

Altera FPGA with Logic Array Blocks (LABs),

=> Each LAB will include Logic Elements (LEs).

==> Each LE will include LUT4s (four-input look-up tables)

Example with Virtex-6, Spartan-6 and 7 series XILINX FPGA:

XILINX FPGA with CLBs (Configuarable Logic Blocks)

=> Each CLB include a pair of Slices

==> Each Slice will include two Logic Cells (LCs)

===> Each LC include one LUT6 (6-input look-up table)

3./ The role of LUT in an FPGA

+ Do you also have the same question as me: when I learn about Digital System, I learn about AND, OR, NOT Gate,…, Boolean algebra logic, but when I learn about structure of FPGA, I don’t see anything, I never saw the appearance of these gates in the structure of the FPGA. I was also confused by this.

Does the FPGA have anything to do with what I have learned about Digital System?

+ LUT is an FPGA solution to implement logic functions. You can also use LUT to create AND, OR, NOT, … gates.
Instead of connecting logic gates, you describe a digital circuit with the input and output logic values of that digital circuit in look-up tables (LUT).
This is similar to creating a truth table for logic gates, or logic functions, or digital circuits.

Creating look-up tables will be easier to understand, easier to implement, and more convenient in storing information than connecting circuit lines for logic gates and logic blocks.

+ LUT is an important component in FPGA.

+ Advantages of LUT: Flexibility allowing the implementation of many complex logic functions and many types of digital circuits.

+ Disadvantages of LUT: there is a limitation in the number of inputs and outputs, it creates propagation delay due to the need for memory access time to look up the output value of the LUTs.