The three inputs to the MUX on the left a, b, and c select the appropriate output value. Here the bits across the top 0 1 0 0 0 1 1 1 represents the output of the truth table for this LUT. The LUT is actually implemented using a combination of the SRAM bits and a MUX: Obviously, the number of inputs to an LUT can be far more than two. So it is not the same LUT in each case, since the LUT defines the output. By programming the multiplexer inputs to be whatever we want, we can use this LUT to. For a two-input AND gate, Address In () Outputįor your second example, only the truth table changes: Address In () OutputĪnd finally, A xor B: Address In () Output The two inputs to the LUT are the select inputs of the multiplexer. The output of a LUT is whatever you want it to be. This is done by transferring the contents of the configuration memory into the SRAM. So for example a 3 input LUT uses an 8x1 SRAM (2³=8)īecause RAM is volatile, the contents have to be initialized when the chip is powered up. The output of the LUT is whatever value is in the indexed location in it's SRAM.Īlthough we think of RAM normally being organized into 8, 16, 32 or 64-bit words, SRAM in FPGA's is 1 bit in depth. Here's a snapshot from FPGA Architecture by Altera:Ī two input LUT (lookup table) is can be represented generically like this:Ī LUT consists of a block of SRAM that is indexed by the LUT's inputs. This brings us to viewing the LUTs simply as memory, where the inputs are the address, and the corresponding outputs are the data stored in the given address. This table would be stored in RAM or ROM. For example, you could build a table for a complex mathematical function, which would work much faster than actually calculating the value by following an algorithm. You can also create your own lookup tables. So the same physical LUT can implement Y=AB and Y=AB', but the LUT-Mask is different, since the truth table is different. The way FPGAs typically implement combinatorial logic is with LUTs, and when the FPGA gets configured, it just fills in the table output values, which are called the "LUT-Mask", and is physically composed of SRAM bits. In other words, whatever behavior you get by interconnecting any number of gates (like AND, NOR, etc.), without feedback paths (to ensure it is state-less), can be implemented by a LUT. This truth table effectively defines how your combinatorial logic behaves. In the context of combinational logic, it is the truth table. A LUT, which stands for LookUp Table, in general terms is basically a table that determines what the output is for any given input(s).
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