An FPGA lookup table, or LUT, stores the truth table of a small Boolean function. Input bits select the stored result. Configurable logic blocks combine LUTs with registers and other resources, while programmable routing connects them into a larger design.
Worked example
A two-input XOR LUT stores outputs 0, 1, 1, 0 for addresses 00, 01, 10, 11. If input I0 is the least-significant address bit, the corresponding four-bit INIT value written from bit 3 to bit 0 is 0110. A nearby flip-flop can register the LUT output.
What to watch for
An FPGA is more than a collection of LUTs. Routing, clock resources, memory and arithmetic resources affect achievable performance and implementation choices.
Try it yourself
How many truth-table entries does a six-input LUT need?
Answer: 2^6 = 64 entries for a single-output Boolean function. Each input combination selects one stored result bit.
How to study this topic
Separate the ideal logical model from the physical device interface. Write units beside capacities, voltages and timing values. The architecture explains the idea; the chosen part's documentation supplies the operating limits.
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Digital Electronics