LUTs (Lookup Tables)

Lookup Tables (LUTs)

Much of the logic in a CLB is implemented using very small amounts of RAM in the from of LUTs. IT is easy to assume that the number of system gates in an FPGA refers to the number of NAND gates and NOR gates in a particular chip. But, in reality, all combinatorial logic (ANDs, ORs, NANDs, XOR, and so on) is implemented as truth tables within LUT memory. A truth tables is a predefined list of outputs for every combination of inputs.

Here is the quick refresher from digital logic class.

The Boolean AND operation, for example, is shown in Figure 5.

Figure 5. NI LabVIEW Boolean AND Operation

The corresponding truth table for the two inputs of an AND operation is shown in Table 1.